AN ABSTRACT OF THE DISSERTATION OF
Felicien Rwangano for the degree of Doctor of Philosophy in Fisheries Science
presented on April 15, 1998. Title: Growth and Reproduction of Oreochromis
niloticus (L.) in Tropical Aquatic Microcosms at Fluctuating Temperature
Regimes
Abstract approved:
Redacted for Privacy
Richard A. Tubb
The effects of fluctuating temperatures on the growth and reproduction
of Oreochromis niloticus were examined under controlled conditions in two sets
of experiments. In each experiment, two mean temperatures and three
temperature fluctuation levels were duplicated in twelve 0.7-m3 (1.4-m2)
experimental tanks and five fish were stocked per tank at a 2:3 male:female
ratio. Fresh chicken manure was applied daily at a rate of 500 kgDW/ha/week
in each tank and water quality was monitored. For the growth experiment, fish
averaging 16.9+0.24 g were randomly stocked at 19 deg.0 ( ±1, ±3 and +6
deg.C), and at 25 deg.0 (±1, ±3 and +6 deg.C) for 122 days under a 12-h
photoperiod simulating tropical conditions. Higher growth rates were obtained in
treatments with larger temperature fluctuations at 19 and 25 deg.0 and were
highest at 25+6 deg.C. Reproduction was insignificant and occurred only at
25+6 deg.0 during the last month. Results indicated a positive linear
relationship between cumulative degree-days and body weight (r2=0.76) and
net fish yield (r2=0.65). The reproduction and growth experiment was conducted
for 153 days at 22 deg.0 (±1, ±3 and ±6 deg.C), and at 28 deg.0 (±1, ±3 and
+6 deg.C) on 0. niloticus averaging 19.81+5.13 g at stocking. Reproduction
occurred during the first month of experiment in all tanks at 28 deg.0 but was
delayed until after 90 days in treatments at 22 deg.C. Relative fecundity was
highest and comparable at 28+3 and 28+6 deg.C. Larger diel thermocycles
induced higher growth and reproductive effects at both 22 and 28 deg.C, but
growth performance and yields were better at fluctuating 28 deg.0 than at 22
deg.C. Body weight and net yield were positively correlated with cumulative
degree-days (r2=0.80 and 0.53, respectively).
© Copyright by Felicien Rwangano
April 15, 1998
All Rights Reserved
Growth and Reproduction of Oreochromis niloticus (L.) in
Tropical Aquatic Microcosms at Fluctuating Temperature Regimes
by
Felicien Rwangano
A DISSERTATION
submitted to
Oregon State University
in partial fulfillment of
the requirements for the
degree of
Doctor of Philosophy
Completed April 15, 1998
Commencement June 1998
Doctor of Philosophy dissertation of Felicien Rwangano presented on
April 15, 1998
APPROVED:
Redacted for Privacy
Major Professor, representing Fisheries Science
Redacted for Privacy
Head of Departmen o Fish
s and Wildlife
Redacted for Privacy
I understand that my dissertation will become part of the permanent collection
of Oregon State University libraries. My signature below authorizes release of
my dissertation to any reader upon request.
Redacted for Privacy
Felicien Rwangano, Author
ACKNOWLEDGEMENTS
I would like to thank Dr. Richard A. Tubb, my major professor, for his
unparalleled support, patience and encouragement. I gratefully acknowledge
the assistance, suggestions and constructive comments that Wayne K. Seim
provided during the planning and execution of the experiments. I also wish to
thank other members of my Graduate Committee: Drs. Frederick W. Obermiller,
C. David Maclntire, William J. Liss, and Robert L. Jarvis. I am indebted to Drs.
Cliff Pereira, Robert Anthony, and John Van Sickle for their advice on
statistical analyses; to Dominic De Maio, Froduald Harelimana, David Garland,
Emeritha Uwimana, and Wayne Seim for their help in data collection; to Rose
Hanrahan and Kristin Ellis for computer data input; to Mark Keller of the Oregon
State University (OSU) Department of Animal Science Lab and to the
employees at the OSU Poultry Farm for their help and kindness.
I am grateful to the OSU Office of International Education, the OSU
Department of Fisheries and Wildlife, the Collaborative Research Support
Program in Pond Dynamics/Aquaculture (PD/A CRSP), and the African-
American Institute for financial assistance during my graduate studies at
Oregon State University.
Last but not least, I express heartfelt gratitude to my loving and enduring
family; and to many friends for their precious moral support. My sincere thanks
to my colleagues involved in CRSP research and to Dr. Hillary Egna, Director of
the PD/A CRSP, for various contributions and assistance.
This research was funded in part by the United States Agency for
International Development under Grant: DAN-4023-G-00-0031-00 and by the
OSU Department of Fisheries and Wildlife.
TABLE OF CONTENTS
Page
CHAPTER 1. Introduction
1
Geographical distribution of Oreochromis niloticus (L.)
1
Temperature in aquaculture
3
Scope of the study
5
Literature cited
6
CHAPTER 2. Experimental Protocol for Oreochromis niloticus Growth and
Reproduction Experiments in Thermocyclic Tropical Aquatic Microcosms ....
Physical setup
9
9
Temperature treatments
13
Organic fertilization of the tank water
16
Fish preparation, stocking, and sampling
16
Water quality analyses and measurements
18
Computations and data analyses
20
Literature cited
22
CHAPTER 3. Growth and Net Yield of Oreochromis niloticus (L.)
at Fluctuating Cool Temperature Regimes
26
Abstract
27
Materials and Methods
28
Results
28
Discussion
45
Literature cited
55
Appendix
60
TABLE OF CONTENTS (Continued)
Page
CHAPTER 4. Effects of Thermocycles in Warmwater Tanks on the
Growth, Reproduction, and Net Yield of Oreochromis niloticus (L)
63
Abstract
64
Materials and Methods
65
Results
65
Discussion
85
Literature cited
94
CHAPTER 5. Conclusions and Management Strategies
100
BIBLIOGRAPHY
103
LIST OF FIGURES
Figure
Page
(a) Layout of the 12 experimental tanks in groups of 4; (b) High
efficiency air blower simulated nightly wind; (c) Oreochromis
niloticus eggs collected from a female's mouth
15
(a): Mean and standard deviation of water temperature (deg. C)
in experimental fish tanks; (b) Diel temperature patterns in fish
tank water during a 122-d experiment
30
Diel temperature profiles in heated tanks at 19+1deg.0 (a);
19+3 deg.0 (b); and 19+6 deg. C (c)
31
3.3
Diel temperature profiles in heated tanks at 25+1 deg.0 (d);
25+3 deg. C (e); and 25+6 deg.0 (f)
32
3.4
Diel pH (a) and dissolved oxygen (b) patterns in tanks
at fluctuating cool temperatures
33
3.5
Oreochromis niloticus mean weight (a) and specific growth rate (b)
at fluctuating cool water temperatures during a 122-d experiment.
38
3.6
(a) Mean and standard deviation for the cumulative number of
degree-days per treatment and the corresponding mean and
standard deviation for 0. niloticus wet body weight (g) at harvest.
(b) Linear model of the average weight (g) and exponential model
of the specific growth rate (g/day) as functions of the cumulative
number of degree-days
41
(a): Oreochromis niloticus average weight (linear graph) at harvest
and forecasted net fish yield (bar graph) from each experimental
treatment. (b): Fitted linear curve predicting changes in fish net
yield as a function of the cumulative number of degree-days
44
4.1.
Mean diel temperature (±SD) and corresponding cumulative
degree-days (±SD) per treatment during the 153-d experiment
66
4.2
Changes in 0. niloticus weight (a) and total length (b) over time
during the 153-d experiment
71
2.1
3.1
3.2
3.7
LIST OF FIGURES (Continued)
Figure
Page
4.3
Cumulative number of fry harvested from experimental tanks
during the 153-d experiment
72
4.4
Cumulative number of eggs and sac-fry harvested from
experimental tanks during the 153-d experiment
73
4.5
Contribution of progeny and adults in the total fish biomass
77
4.6
Oreochromis niloticus net yield from the 153-d experiment
83
4.7
Linear relationships between cumulative degree-days and
0. niloticus body weight (W) and net yield (Y)
84
LIST OF TABLES
TABLE
Page
LOW and HIGH mean diel temperatures with three fluctuation
levels
14
Means of water chemistry variables by treatment during
the 122-d experimental period
35
3.2
Average weight, total length and condition factor (K) of males
And females at stocking and at harvest (culture period = 122-d)
37
3.3
Absolute values of the slopes of the linear regression models
for Oreochromis niloticus wet weight (g) on the duration
(122 days) of the culture period
39
Constant and slope from the regression of fish weight as a
function of total length data from monthly measurements
(n=60 for each period)
39
4.1
Means of water chemistry variables by treatment during the
153-d experimental period
68
4.2
Contribution of progeny and adults in the total biomass
of 0. niloticus in tanks at 22 and 28 deg.0 mean fluctuating
water temperatures at harvest (153-d)
76
Monthly average fish weight (g ± SD), cumulative number
of degree-days (± SD), and male/female weight ratio per
thermal treatment
78
Comparison of male and female average body weight
(g ± SD), dry body weight (g), gonad weight (mg ± SD), and
the gonado-somatic index (GSI) at harvest
79
Mean growth rate (%/day) and average adult body weight (g)
of Oreochromis niloticus in tanks at 22 and 28 deg.0 mean
fluctuating water temperatures (153-d)
82
2.1
3.1
3.4
4.3
4.4
4.5
DEDICATION
This dissertation is dedicated
To my wife and to our children,
For their love and understanding.
-- To the everlasting memory of the people of Rwanda --
Growth and Reproduction of Oreochromis niloticus (L.) in
Tropical Aquatic Microcosms at Fluctuating Temperature Regimes
CHAPTER 1
Introduction
Geographical distribution of Oreochromis niloticus (L.)
Oreochromis niloticus (L.) is one of the most widely cultured fish
species in the world. The species is native to Africa over the range from the
Senegal and Gambia river systems of West Africa to the Volta River and Niger
system and the lake Chad basin; and from the shallower parts of Lakes
Tanganyika and Kivu in Central Africa to the northern regions through the Nile
River system and its delta lakes, and the Yarkon River near Tel Aviv.
However, before the early 1900's 0. niloticus was absent in some waters such
as the western rivers of Gabon, the Zaire River and southwards, Lake Victoria,
and all the eastward-flowing rivers of Africa (Balarin, 1979; Trewavas, 1983).
Trewavas (1983) described seven subspecies of 0. niloticus distributed
over this natural geographical range: 0. n. niloticus, 0. n. eduardianus, 0. n.
vulcani, 0. n. cancellatus, 0. n. filoa, 0. n. sugutae, and 0. n. baringoensis.
0. n. niloticus occurred in West African waters and in the Nile River system; 0.
n. eduardianus was distributed in Central African regions; and the other
subspecies were rather localized in East Africa.
0. niloticus was introduced early in this century into lakes and ponds of
various regions in Africa, Asia, America and Europe. Like many tilapia
2
species, the wide distribution of 0. niloticus indicates a high degree of thermal
tolerance. Culture of the species is greatest in the low elevations of tropical
countries, but it is also cultured in higher elevations as well as higher latitudes
in temperate regions where it can be overwintered in greenhouses, in heated
recirculating systems, or in geothermally heated waters.
Despite the wide culture of the species, available literature lacks a
quantitative description of the effects of high and low temperature fluctuations
on growth and reproduction of 0. niloticus. Quantitative estimations of the
effects of fluctuating temperatures on growth and reproduction of 0. niloticus
are necessary for developing appropriate fish culture strategies for high
altitude regions of tropical countries and for cooler regions at the margins of its
range. High altitude tropical regions (2000-2800 m) have cold nights and warm
days, particularly during the dry season. In Rwanda, high altitude regions also
have serious soil erosion problems resulting from deforestation from attempts
to grow beans and other subsistence crops on steep slopes in areas of high
rainfall (> 100-150 cm/year). Using abundant water supply for the culture of
0. niloticus in these regions where other resources such as land and
necessary inputs are available could be a more appropriate means of
providing food.
3
Temperature in aquaculture
The effect of temperature on growth and production of 0. niloticus is of
particular concern in the high altitude regions of the equatorial and tropical
regions in Africa. Temperature regimes impact fish growth and reproduction
but the limiting effects of temperature have not been well defined. The
production and expected reproduction under fluctuating thermal conditions
needs to be established for a better understanding of relationships between
temperature and 0. niloticus growth and reproduction performance, allowing
the development of appropriate pond management strategies and decisionmaking regarding pond stocking and sex ratios.
Temperature is often referred to as mean or optimum values, and
extremes are solely considered as critical or lethal values. However, as diel
fluctuations in water temperature affect fish physiology, considering means
only appears inadequate. Boyd (1990) reported that sudden temperature
changes of 3-4 deg.0 may cause thermal shock and even death. A gradual
temperature change not exceeding 0.2 deg.C/min can usually be tolerated,
provided the total change in temperature does not exceed a few degrees.
0. niloticus experimentally raised at fluctuating temperatures (28 + 4
deg.0 and 30 + 4 deg.C) showed faster increases in daily weight and higher
energy assimilation rates and tissue growth rates than fish at the same
constant temperatures (Gui et al., 1989). However, tissue growth rate was
higher at 28 + 4 deg.0 than at 30 + 4 deg.C.
4
Geraldes (1980) investigated the interactions between light and
temperature variations and their effects on reproduction of Tilapia aurea and
Tilapia nilotica. He observed that the temperature required for reproduction
was 23 deg.0 with a 13-hour daily photoperiod, while reproduction occurred at
2 deg.0 higher with a 10-hour photoperiod. In Rwanda, tilapia culture has
been practiced at elevations ranging from 1300 m to 2500 m (Hanson et al.,
1988). Temperature is an important factor controlling and limiting the structure
and function of aquatic communities in ponds, thus contributing to observed
variability in 0. niloticus reproduction and total production between regions of
high and low elevation, and between seasons. Temperature fluctuations may
have a more significant effect on fish physiology, behavior and performance
than mean temperature. In low altitude regions, mean temperatures are higher
and amplitudes of variations lower than at higher elevations. At 1350-1700 m,
reproduction accounted from 20 to 50% of the total tilapia production in
Rwandan ponds; whereas little or no reproduction occurred in ponds at higher
elevations above 1900 m (Rurangwa et al., 1992; Veverica and Rurangwa,
1991; Hanson et al., 1988). From field observations, Hanson et al. (1988)
reported that cooler environments contributed to delays in the age at first
reproduction and lowered the number of fingerlings per surface area. 0.
niloticus reproduction in Rwandan ponds also decreased with colder water
temperatures during the dry season. At a given elevation fish growth rate and
reproductive activity varied greatly with seasons (dry vs wet) and water
exchange. Both factors affect the range and amplitude of thermal variation.
5
Scope of the study
The influence of fluctuating temperature regimes on pond productivity,
production and reproductive performance of 0. nfloticus needs to be defined in
quantitative terms. The purpose of this research was to test thermal effects on
0. niloticus growth, reproduction and total production through a series of
laboratory experiments that simulated the fluctuating thermal conditions found
in Rwanda and other high altitude tropical regions.
The experiments were designed to investigate: (1) whether there are
any significant differences in fish growth rates and yields as a function of
thermal treatments; (2) whether the average number of fingerlings per female,
average gonad weight, and the total production at low average temperatures
are the same as those at higher temperatures; (3) whether the average
number of fingerlings per female, average gonad weight, and the total
production are significantly affected by the amplitude of temperature variation;
and (4) whether the fish performance at fluctuating temperature varies as a
function of the mean temperature levels.
6
Literature cited
Balarin, J., D., and R. D. Haller. 1982. The intensive culture of tilapia in tanks,
raceways, and cages. In Muir, J. F. and R. J. Roberts (Editors), Recent
Advances in Aquaculture, Vol. 1, Croom Helm Ltd., London, 265-355.
Balarin, J. D., and J. D. Hatton. 1979. Tilapia: a guide to their biology and
culture in Africa. Unit of Aquatic Pathobiology, Stirling University, 174p.
Bardach, J. E., J. H. Ryther, and W. D. McLarney. 1972. Aquaculture: the
farming and husbandry of freshwater and marine organisms. WileyInterscience, New York, N. Y., 868p.
Bishai, H. M. 1965. Resistance of Tilapia nilotica (L.) to high temperatures.
Hydrobiologia, 25:473-88.
Boyd, C.E. 1979. Water quality in warmwater ponds. Alabama Agricultural
Experiment Station, Auburn University, Auburn, Alabama, 359p.
Boyd, C. E. 1990. Water quality in ponds for aquaculture. Alabama Agricultural
Experiment Station, Auburn University, Auburn, Alabama. Birmingham
Publishing Co., Birmingham, Alabama, 482p.
Gui, Y., Z. Wang, Y. Chen, W. Zheng, and F. Li. 1989. Use of fluctuating
temperature to promote growth of Tilapia nilotica. J. Fisheries of China,
13(4):326-332.
Hanson, B. J., J. F. Jr. Moehl, K. L. Veverica, F. Rwangano, and M. Van
Speybroeck. 1988. Pond Culture of tilapia in Rwanda, a high altitude
equatorial African country. In Pullin, R. S. V., T. Bhukaswan, K.
Tonguthai, and J. L. Maclean (Editors), The Second International
Symposium on Tilapia in Aquaculture, ICLARM Conference
Proceedings 15, Department of Fisheries, Bangkok, and the
International Center for Living Aquatic Resources Management, Manila,
Philippines, 553-9.
Hishamunda, N., and J. F. Moehl. 1989. Rwanda National Fish Culture
Project. Research and Development Series No. 34, Alabama
Agricultural Experiment Station, Auburn University, Alabama.
Hodgkiss, J., and H. S. H. Man. 1978. Reproductive biology of Sarotherodon
mossambicus (Cichlidae) in Plover Cove reservoir, Hong-Kong. Env.
Biol. Fish. 3(3):287-92.
7
Hopkins, K. D. 1992. Reporting fish growth: a review of the basics. J. World
Mariculture Soc., 23, 173-9.
Huet, M. 1968. Methodes biologiques d'accroissement de la production
piscicole en Europe et en Afrique (Biological methods of increasing
productivity in European and African fish ponds). Proceedings of the
World Symposium on Warm Water Pond Fish Culture. FAO Fish. Rep.
No. 44(4), FRi/R44.4:289-327.
Jalabert, B., and Y. Zohar. 1982. Reproductive physiology in cichlid fishes,
with particular reference to Tilapia and Sarotherodon. In Pullin, R. S. V.,
and R. H. Lowe-McConnell (Editors), The Biology and Culture of
Tilapias, ICLARM Conference Proceedings 7, International Center for
Living Aquatic Resources Management, Manila, Philippines, 129-40.
Lowe-McConnell, R. H. 1987. Ecological studies in tropical fish communities.
Cambridge University Press, 382p..
Moehl, J. F. Jr., K. L. Veverica, B. J. Hanson, and N. Hishamunda. 1988.
Development of appropriate pond management techniques for use by
rural Rwandan farmers. In Pullin, R. S. V., T. Bhukaswan, K. Tonguthai,
and J. L. Maclean (Editors), The Second International Symposium on
Tilapia in Aquaculture. ICLARM Conference Proceedings 15,
Department of Fisheries, Bangkok, and the International Center for
Living Aquatic Resources Management, Manila, Philippines, 561-8.
Peters, H. M. 1983. Fecundity, egg weight and oocyte development in tilapias
(Cichlidae, Teleostei). ICLARM Translations 2, International Center for
Living Aquatic Resources Management, Manila, Philippines, 28p.
Philippart, J.-CI., and J. -CI. Ruwet. 1982. Ecology and distribution of tilapias.
In Pullin, R. S. V. and R. H. Lowe-McConnell (Editors), The Biology and
Culture of Tilapias, ICLARM Conference Proceedings 7, International
Center for Living Aquatic Resources Management, Manila, Philippines,
15-59.
Rana, J. Krishen. 1988. Reproductive biology and the hatchery rearing of
tilapia eggs and fry. In Muir, J. F. and R. J. Roberts (Editors), Recent
Advances in Aquaculture, Vol. 3, Croom Helm Ltd., London, 343-407.
Rurangwa, E., K. L. Veverica, W. K. Seim, and T.J. Popma. 1992. On-farm
production of mixed sex Oreochromis niloticus at different elevations
(1370 to 2230 m). In Egna, H. S., M. McNamara, and N. Weider
(Editors), Ninth Annual Administrative Report, Pond
Dynamics/Aquaculture CRSP, 1991, Office of International Research
and Development, Oregon State University, Corvallis, Oregon, 35-40.
8
Rwangano, F., M. Van Speybroeck, E. Rurangwa, K. L. Veverica, and B. J.
Hanson. 1989. Fingerling production of Oreochromis niloticus at the
Rwasave Fish Culture Station at the National University of Rwanda. In
H. S. Egna and H. F. Horton (Editors), Sixth Annual Administrative
Report, Pond Dynamics/Aquaculture CRSP, 1987-88, Office of
International Research and Development, Oregon State University,
Corvallis, Oregon.
Soderberg, R. W. 1990. Temperature effects on the growth of blue tilapia in
intensive aquaculture. The Progressive Fish-Culturist 52:155-57.
Trewavas, E. 1983. Tilapiine fishes of the genera Sarotherodon, Oreochromis,
and Danakilia. British Museum (Natural History), Cromwell Road,
London, 583p.
Verheust, L., F. 011evier, K. L Veverica, T. Popma, A. Gatera, and W. Seim.
1994. High elevation monoculture and polyculture of Oreochromis
niloticus and Clatias gariepinus in Rwandan ponds. In Egna, H. S., J.
Bowman, B. Goetze, and N. Weider (Editors), Eleventh Annual
Administrative Report, Pond Dynamics/Aquaculture CRSP, 1993, Office
of International Research and Development, Oregon State University,
Corvallis, Oregon, 163-170.
Veverica, K. L., and E. Rurangwa. 1991. Rwanda rural pond survey. In Egna,
H. S., J. Bowman, M. McNamara (Editors), Eighth Annual
Administrative Report, Pond Dynarnics/Aquaculture CRSP, 1989-90,
Office of International Research and Development, Oregon State
University, Corvallis, Oregon, 40-42.
Weatherly, A. H., and H. S. Gill. 1987. The biology of fish growth. Academic
Press, Orlando, Florida, USA.
9
CHAPTER 2
Experimental Protocol for Oreochromis niloticus Growth and
Reproduction Experiments in Thermocyclic Tropical Aquatic
Microcosms
Physical setup
Two controlled experiments simulating tropical conditions were
conducted at the Oak Creek Laboratory of Biology of Oregon State University,
Corvallis (USA). Twelve insulated fiberglass tanks were set up to simulate
seasonal and altitudinal temperature variations in tropical aquaculture earthen
ponds. Each tank measured 1.25 m x 1.12 m x 0.60 m and was filled with a 5
cm thick layer of a clay loam soil (approximately 28.5% clay, 28.5% silt and
43% sand) as a bottom substratum. Oak Creek stream water (alkalinity of 150
mg CaCO3) was added to fill each tank at 0.70 m3 capacity. Evaporation
losses were replaced once a week before water dropped 5 cm below normal
level.
The twelve experimental tanks were arranged in two rows by groups of
4 (Figure 2.1a). Over each pair of tanks, a 1000-watt metal halide lamp was
suspended along the central line of the tanks at approximately 1.80 m above
the water surface to provide a light intensity of 13.5 ( ±6) Ergs/m2/day,
equivalent to 3.6 (±1.4) cal/m2/sec). This photosynthetic photon flux density
was high for laboratory system but actually less than the 17.5-38.0 Ergs/
m2/day solar radiation recorded in 1987 at the Rwasave fish culture and
10
research station of the National University of Rwanda in Butare, Rwanda. The
light intensity was measured with a Quantum/Radiometer /Photometer Model
LI-185B (LI-COR Inc./LI-COR Ltd., Nebraska). Electric timers controlled the
metal halide lamps to allow a 12-hour photoperiod (starting at 08:00) during
both experiments.
The daily maximum water temperature was achieved by the use of
submersible 300-watt VISITHERM water heaters with sensor (± 1 deg.C),
calibrated and set at the desired highest temperature. One to three heaters
were used per tank depending on the target temperature. In order to establish
fluctuating daily temperatures, heaters installed in each set of four
experimental tanks were set for a 10-hour ON daily pattern controlled by one
electric timer switching on at 08:00.
The daily minimum water temperature was controlled by one or two
250-watt water heaters per tank, calibrated and set to achieve the lowest
temperature for each particular thermal treatment. These heaters were
activated by their built-in thermal sensors (±1 deg.C). The water in the two
tanks set for the LT-HV treatment (LT + 6 deg.C, Table 2.1) was cooled by
heat exchange with chilled water circulating through a 1.00 cm inside diameter
tygon tubing (15 m long per tank) in a tightly closed system. One end of the
tubing was connected to a water pump submersed in a separate water tank
(whose water was chilled using Blue-M constant flow chiller Model
PCC24SSA-1 (Blue-M Electric Company, Blue Island, Illinois) with a Microtol
controlling thermostat set at 4 deg.C); the other end was attached to a hose
11
bringing the water back to the chilled water tank. The water pump in this tank
was connected to an electric timer switching on at 18:00 for a 14-hour ON
daily pattern.
The water in two other tanks receiving the LT-LV treatment (LT ± 3
deg.C, Table 2.1) was also cooled by heat exchange with the cool stream
water circulating in experimental tanks through a 1.00-cm inside diameter
tygon tubing (15-m long per tank). The flow in the tubing was regulated by a
Richdel solenoid valve model R213 (Richdel Cpy., Carson City, Nevada)
connected to one end of the tubing. The solenoid valve was controlled by an
electric timer set on a 14-hour ON and 10-hour OFF daily pattern (switching on
at 18:00). The outlet of the tubing was directly open to a drain to discharge
stream water after the heat exchange process.
The experimental constant temperature tanks were heated by
submersible 300-watt Visitherm heaters with thermal sensors ( ±1 deg.C). One
heater was installed for the low, constant temperature (LT-CT), and two
heaters were used for the high, constant temperature treatment (HT-CT)
(Table 2.1).
Gas exchange at the air-water interface was maintained by wind
simulation using two high efficiency air blowers (free air delivery: 1930 cubic
feet/minute; 1/3-1/4 hp and 1725 rpm motor) installed at both ends of the
longitudinal tank line and about 2.20 m from the nearest tank. The blowers
were controlled by an electric timer set for a 14-hour ON daily pattern starting
at 18:00 for an overnight ventilation and water mixing. Six small exhaust fans
12
(three built-in the northern lab wall for inward blowing and three others
installed in the southern wall for outward blowing) were activated according to
the same timing as for the blowers to increase nightly gas and heat exchange.
A direction-adjustable plexiglass was vertically installed in the middle of each
of the two blowers'outlets to homogenize air distribution over the experimental
tanks. Wind speed at 20-25 cm above the water surface was measured using
a sensitive anemometer Model 3034/AC (C. F. Casella & Co. Ltd, London).
Compressed air bubbling regulated by a timer-controlled solenoid valve
was provided occasionally during the growth experiment and throughout the
growth and reproduction experiment anytime the DO level dropped below 1.0
mg/L due to dense algal die-off. This artificial aeration was supplied on a daily
14-hour period starting at 18:00.
Experimental tanks were maintained filled with water but without fish for
3 months to allow colonization and natural planktonic blooms. Tanks were
then heated according to temperature treatments for an additional three-week
period for temperature control and adjustment prior to the start of each
experiment. Fish preparation, calibration of heating, adjustment of light
conditions and ventilation, and input source assessment were accomplished
during this preliminary period.
13
Temperature treatments
Two mean diel temperature levels, low temperature (LT) and high
temperature (HT), and three levels of temperature variation within each mean
temperature levels established 6 temperature regimens (Table 2.1). Low and
high temperatures simulated mean diel water temperatures in Rwandan fish
ponds at high (2200-2500 m) and lower (1700 m) elevations, respectively; or
mean minimum and maximum water temperatures, respectively. Low and
high temperature fluctuation levels represented seasonal temperature
variations about the means. Constant temperatures (CT+1 deg.C) were used
as controls. Two consecutive experiments were conducted with six
temperature regimens applied in duplicates and assigned to twelve
experimental tanks in a completely randomized design. Experiment 1
investigated the effects of fluctuating temperature regimes (LT=19 deg.0 and
HT=25 deg.0 combined with each variation level, Table 2.1) on the growth of
0. niloticus in chicken manure-loaded ponds for 122 days from February
through June 1992.
Experiment 2 was run for 153 days (July to December 1992) to
evaluate the growth and reproduction performances of 0. niloticus in response
to fluctuating temperature regimes, but at 3 deg.0 higher mean diel
temperature than in the first experiment (LT=22 deg.0 and HT=28 deg.0
combined with each variation level, Table 2.1).
14
Table 2.1 LOW and HIGH mean diel temperatures with three fluctuation levels
LOW mean diel
HIGH mean diel
temperature (LT)
temperature (HT)
19 + 1 deg.0
25 + 1 deg.0
Low variation, LV
19 + 3 deg.0
25 + 3 deg.0
High variation, HV
19 + 6 deg.0
25 + 6 deg.0
22 + 1 deg.0
28 + 1 deg.0
Low variation, LV
22 + 3 deg.0
28 + 3 deg.0
High variation, HV
22 + 6 deg.0
28 + 6 deg.0
Treatments
Experiment 1
Constant, CT
Experiment 2
Constant, CT
15
(b)
(a)
(c)
Figure 2.1 (a) Layout of the 12 experimental tanks in groups of 4; overhead
1000-watt halide lamps are not visible in this photo; (b) High efficiency air
blowers (1/3 hp; 1725 rpm; and 1930 cubic feet of free air delivered per
minute) simulated nightly wind; (c) Oreochromis niloticus eggs collected
from females' mouth treatment F.
16
Organic fertilization of the tank water
Fresh chicken droppings (CD) without litter were collected every day
from the OSU chicken farm and were applied to each experimental tank for
input of nutrients that enhance the growth of food organisms suitable for 0.
niloticus natural diet. At the measured average 26% dry weight, the daily CD
input of 500 kgDW/ha/week was adjusted as 38 g fresh manure/tank-d,
equivalent to 27.1 g/m2/day. Manure samples were analyzed monthly at the
OSU Soil Testing Laboratory for organic carbon, total nitrogen, and total and
available phosphorus. Total nitrogen and phosphorus contents of the fresh CD
were 3.9% and 2.3%, respectively. Manure application started one day before
fish stocking for initial monitoring of dissolved oxygen and ammonia levels.
Fish preparation, stocking and sampling
0. niloticus (Ivory Coast strain) obtained from Auburn University,
Alabama, were used in this study. Young fish kept in aquaria were grouped by
length and sex. They were then individually marked with color-coded and
numbered T-bar anchor tags using a "Monarch marking pistol 3000" (Pitney
Bowes Cpy.). A same color was used for each sex and size group. Tags were
inserted leaning toward the posterior in the thick dorso-lateral muscle above
the operculum but between the lateral line and the base of the dorsal fins
(Nielsen, 1992). After tagging, fish were held for 28 days in tanks with running
17
water and thermostat-controlled temperature (23-26 deg.C) for recovery, and
were fed ad libitum a 35%-protein trout floating-pellet.
Prior to stocking, individual fish were weighed and the total length was
measured. Fish samples were also taken at the beginning of the experiment 2
for dry weight (DIN) determination. Fish selected for stocking were in good
condition and averaged 16.8 ± 0.24 g and 19.7 ± 5.13 g for the first and
second experiments, respectively. Fish were randomly distributed by size and
sex at a 2:3 male to female sex ratio into twelve buckets representing the
number of experimental tanks. Each 5-fish group was then stocked into one
tank, resulting in approximately 3.6 fish/m2. Monthly seinings were carried out
to monitor changes in fish weight and total length. Eggs, sac-fry and fry (or
alevins) collected during the monthly sampling and the fry harvested when
seen swimming near the water surface were counted and removed from the
tank. The total weight of the fry was also recorded, and the average individual
weight of the fry was computed. At the end of each experiment, all fish were
weighed individually; total length was measured; and any reproduction data
was recorded. Adults were dissected and the gonads extracted and weighed
wet. Gonads and fish carcasses were dried for 4 days at 70 deg.0 and cooled
in a dessicator before dry weights were taken. The final dry weight (DW) of
individual whole-fish was computed as the sum of gonad DW and the DW of
the fish without gonads. Gonad weights, relative fecundity, age and length at
first spawning, as well as the number of eggs, sac-fry and juveniles per female
are reported for each thermal treatment. An average condition factor K was
18
calculated at each sampling period for each temperature treatment. Average
weight, mean total length, daily specific growth rate, and fish yield per
temperature regimen were calculated.
Water quality analyses and measurements
Daily water temperatures were continuously recorded at 25 cm below
the surface in six tanks receiving the different thermal treatments, using
Part low temperature recorders (Part low Corporation, New Hartford, NY) with
7-day pressure sensitive charts (Model PS R112C or PS R112F).
Temperatures at 10, 25, 40 and 50 cm below water surface were measured
with Cooper digital thermometer model IT670A ( ±2 deg. F or 2% reading
accuracy) during the monthly diel measurements performed at 4-hour intervals
starting at 07:00.
Dissolved oxygen (DO) and pH at 10 cm below the surface were part of
the monthly diel measurements, and were also monitored weekly along with
temperature at 07:00-08:00 before lights and water heaters were turned on
and at 17:30-18:30 at peak temperatures (heaters automatically turned off at
18:00). DO concentration was measured using the oxygen meter YSI model
58, ± 0.03 mg/L (Yellow Spring Instruments Co., Inc., Ohio). The water pH
was measured with a Schott Gerate pH meter model CG837 ( ±0.02 pH
reading accuracy) carefully calibrated with pH 7.0 and 10.0 buffer solutions.
Water samples were taken once a month for water quality monitoring.
Analytical procedures followed Standard Methods (APHA/ AVVWANVPCF,
19
1989), or the methods described by Boyd (1979). Total ammonia nitrogen
concentration was determined by the electrode method using a digital ionizer
Model 601 with a CORNING ammonia-selective electrode. Un-ionized
ammonia was estimated using a conversion table based on percentage unionized ammonia in aqueous solution as a function of water pH and
temperature (Boyd, 1990). The titration method was used to determine total
alkalinity from morning (07:00-08:00) and afternoon (15:00-16:00) water
samples taken on the same day as chlorophyll a analysis. Since no
phenolphthalein alkalinity was detected in these samples, total alkalinity was
determined as equal to bicarbonate alkalinity. Bicarbonate concentration
(mg/L) was computed as equal to bicarbonate alkalinity x 1.22 (Boyd, 1979).
Total nitrogen determination followed the semi-micro Kjeldahl digestion
method. An automated distillation/titration unit with Blichi 342/322 distillator
and Brinkman E526 titrator was used. Well mixed tank water samples were
taken monthly for total phosphorus, orthophosphate and chlorophyll-a
determination. Soluble orthophosphate was determined spectrophotometrically following filtration, and addition of ammonium molybdate and
stannous chloride; total phosphorus was determined after a persulfate
digestion and using the stannous chloride methods (Boyd, 1979 and
APHA/AVVWANVPCF, 1989).
The pigments contained in the phytoplankton were extracted in 90%
acetone, and chlorophyll-a concentrations were detected using a B&L
Spectrophotometer model 20.
20
Computations and data analyses
The amount of heat accumulated in the water after "t" days of the
experimental period was expressed in degree-days. Degree-days were
computed as a sum of the observed mean daily water temperatures minus the
threshold temperature of 15 deg.0 (Soderberg, 1990; Bardach et al., 1972).
Fish parameters were computed as follows: (1) condition factor
K=WiTL3x102, where W and TL are individual weight (g) and total length (cm),
respectively; (2) specific growth rate SGR=((lnWrInW0)/(t-to))x102, where: the
SGR is expressed as % increase in body weight per day (%/day); InWt and
InWo (g) are natural logarithms of the final and initial individual weights,
respectively; and t-to = time interval in days; and (3) net production (g) per tank
(1.4 m2) at the end of each 30-day time period (Hopkins, 1992; Weatherly and
Gill 1987; Warren, 1971).
Net fish production (NFP) was calculated as a cumulative value:
NFPtank=(Bt-B0)+(mBt-m130)+113, where, all after t days: Bo and Bt represented
the total fish biomass at stocking and after t days, respectively; (Bt-Bo) = total
change in biomass of living fish (n=5 per tank); (mBt-mBo) = total change in
biomass of all dead fish, if any; and rB = total reproduction biomass,
if
any.
Extrapolated net fish yield was expressed as kg/are/year and was
calculated as (365/14t)x(NFPtank,
1 The coefficient (365/14t) was obtained from
the conversion of 1 kg=103g, 1 are=102 m2=10-2 ha; and one year=365 days;
t=duration of the experiment, in days.
21
At the end of the second experiment, the gonosomatic index (GSI) was
calculated as relative fecundity = gonad weight (absolute fecundity)/body
weight (DeVlaming et al., 1982; Weatherly and Gill, 1987).
Statistical analyses investigated differences between replicate tanks.
Differences were tested on fish performance parameters and water quality
data by analysis of variance and multiple range analysis at each sampling time
period. When no significant differences between fish tanks were detected at
the 0.05 probability level, data were pooled by treatment and subsequent
comparisons were conducted to determine differences between temperature
regimens. Regression analysis was conducted to establish relationships
between fish growth parameters and the cumulative number of degree-days.
Data sets were handled using Quattro Pro 6.0/8.0 for computations,
Statgraphics 6.0 for statistical analyses, and Quattro Pro 6.0/8.0 and
Slide Write Plus version 3.0 for generating graphs and linear models.
22
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examination of water and wastewater, 17th edition, APHA-A1MNAWPCF, Washington D. C., USA.
Balarin, J., D., and R. D. Haller. 1982. The intensive culture of tilapia in tanks,
raceways, and cages. In Muir, J. F. and R. J. Roberts (Editors), Recent
Advances in Aquaculture, Vol. 1, Croom Helm Ltd., London, 265-355.
Balarin, J. D., and J. D. Hatton. 1979. Tilapia: a guide to their biology and
culture in Africa. Unit of Aquatic Pathobiology, Stirling University, 174p.
Bardach, J. E., J. H. Ryther, and W. D. McLarney. 1972. Aquaculture: the
farming and husbandry of freshwater and marine organisms. WileyInterscience, New York, N. Y., 868p.
Bishai, H. M. 1965. Resistance of Tilapia nilotica (L.) to high temperatures.
Hydrobiologia, 25:473-88.
Boyd, C.E. 1979. Water quality in warmwater ponds. Alabama Agricultural
Experiment Station, Auburn University, Auburn, Alabama, 359p.
Boyd, C. E. 1990. Water quality in ponds for aquaculture. Alabama Agricultural
Experiment Station, Auburn University, Auburn, Alabama. Birmingham
Publishing Co., Birmingham, Alabama, 482p.
DeVlaming, V. L., G. Grossman, and F. Chapman. 1982. On the use of the
gonosomatic index. Comp. Biochem. Physiol. 73A:31-39.
Gui, Y., Z. Wang, Y. Chen, W. Zheng, and F. Li. 1989. Use of fluctuating
temperature to promote growth of Tilapia nilotica. J. Fisheries of China,
13(4):326-332.
Guy, C. S., H. L. Blankenship, and L. A. Nielsen. 1996. Tagging and marking.
In Murphy, B. R., and D. W. Willis (Editors), Fisheries Techniques, 2'
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Hanson, B. J., J. F. Jr. Moehl, K. L. Veverica, F. Rwangano, and M. Van
Speybroeck. 1988. Pond Culture of tilapia in Rwanda, a high altitude
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Tonguthai, and J. L. Maclean (Editors), The Second International
Symposium on Tilapia in Aquaculture, ICLARM Conference
Proceedings 15, Department of Fisheries, Bangkok, and the
International Center for Living Aquatic Resources Management, Manila,
Philippines, 553-9.
Hishamunda, N., and J. F. Moehl. 1989. Rwanda National Fish Culture
Project. Research and Development Series No. 34, Alabama
Agricultural Experiment Station, Auburn University, Alabama.
Hopkins, K. D. 1992. Reporting fish growth: a review of the basics. J. World
Mariculture Soc., 23, 173-9.
Huet, M. 1968. Methodes biologiques d'accroissement de la production
piscicole en Europe et en Afrique (Biological methods of increasing
productivity in European and African fish ponds). Proceedings of the
World Symposium on Warm Water Pond Fish Culture. FAO Fish. Rep.
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with particular reference to Tilapia and Sarotherodon. In Pullin, R. S. V.,
and R. H. Lowe-McConnell (Editors), The Biology and Culture of
Tilapias, ICLARM Conference Proceedings 7, International Center for
Living Aquatic Resources Management, Manila, Philippines, 129-40.
Lowe-McConnell, R. H. 1987. Ecological studies in tropical fish communities.
Cambridge University Press, 382p.
Moehl, J. F., Jr., K. L. Veverica, B. J. Hanson, and N. Hishamunda. 1988.
Development of appropriate pond management techniques for use by
rural Rwandan farmers. In Pullin, R. S. V., T. Bhukaswan, K.Tonguthai,
and J. L. Maclean (Editors), The Second International Symposium on
Tilapia in Aquaculture. ICLARM Conference Proceedings 15,
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models. Richard D. Irwin, Inc, 2nd edition, 667p.
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24
Peters, H. M. 1983. Fecundity, egg weight and oocyte development in tilapias
(Cichlidae, Teleostei). ICLARM Translations 2, International Center for
Living Aquatic Resources Management, Manila, Philippines, 28p.
Philippart, J.-CI., and J.-Cl. Ruwet. 1982. Ecology and distribution of tilapias.
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Center for Living Aquatic Resources Management, Manila, Philippines,
15-59.
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tilapia eggs and fry. In Muir, J. F. and R. J. Roberts (Editors), Recent
Advances in Aquaculture, Vol. 3, Croom Helm Ltd., London, 343-407.
Rwangano, F., M. Van Speybroeck, E. Rurangwa, K. L. Veverica, and B. J.
Hanson. 1989. Fingerling production of Oreochromis niloticus at the
Rwasave Fish Culture Station at the National University of Rwanda. In
H. S. Egna and H. F. Horton (Editors), Sixth Annual Administrative
Report, Pond Dynamics/Aquaculture CRSP, 1987-88, Office of
International Research and Development, Oregon State University,
Corvallis, Oregon.
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Soderberg, R. W. 1990. Temperature effects on the growth of blue tilapia in
intensive aquaculture. The Progressive Fish-Culturist 52:155-57.
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25
Weatherly, A. H. 1972. Growth and ecology of fish populations. Academic
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26
CHAPTER 3
Growth and Net Yield of Oreochromis niloticus (L.) at
Fluctuating Cool Temperature Regimes
Felicien Rwangano
27
Abstract
A 122-day experiment was conducted in thermally controlled aquatic
microcosms to evaluate the growth and reproduction of Oreochromis niloticus
in cool fluctuating diel temperatures. Fish averaging 16.9+0.24 g were
randomly stocked at 19 deg.0 (±1, ±3 and ±6 deg.C), and at 25 deg.0 ( ±1, ±3
and ±6 deg.C) in twelve 0.7-m3 experimental tanks under a 12-h photoperiod
simulating tropical conditions. Fresh chicken manure was applied daily at a
rate of 500 kgDW/ha/week in each 0.7 m3 (1.4 m2 ) tank and water quality was
regularly monitored. Five fish were individually tagged and stocked per tank at
a 2:3 male:female ratio. Body weight, total length, and reproduction data were
collected monthly. Reproduction was insignificant and occurred only at 25+6
deg.0 during the last month of the experiment. Higher growth rates were
obtained in treatments with larger temperature fluctuations at 19 and 25
deg.C, but were highest at 25+6 deg.0 Results indicated a positive correlation
between cumulative degree-days and body weight (r2=0.76) and net fish yield
(r2=0.65).
28
Materials and Methods
The experiment on the growth and net yield of Oreochromis niloticus
(L.) at fluctuating cool temperature regimes was conducted over a 122-day
period following the experimental protocol described in Chapter 2.
Temperature treatments were A=19+1 deg.C; B=19+3 deg.C; C=19+6 deg.C;
D=25+1 deg.C; E=25+3 deg.0 and F=25+6 deg.C.
Results
Water quality variables
Diel temperature, pH, and dissolved oxygen fluctuations
Daily temperatures were generally maintained within the target range of
the thermal treatments (Figure 3.1a). Maximum surface water temperatures
were reached at 1500-1900 and minimum temperatures were recorded at
0700 (Figure 3.1b). However, a few large variations were recorded on some
days because of extreme fluctuations of the ambient air temperature. Based
on mean diel temperature profiles, all experimental tanks were stratified from
1100 through 0300 and mixed at about 0700 (Figures 3.2 and 3.3).
Cumulative degree-days in each treatment were: 597.80 ± 17.25 at 19+1
deg.0 (A), 585.60 ± 60.39 at 19+3 deg.0 (B), 564.25 + 4.31 at 19+6 deg.0
(C), 1232.20 ± 34.51 at 25+1 deg.0 (D), 1235.25 ± 38.82 at 25+3 deg.0 (E)
and 1338.95 ± 64.70 at 25+6 deg.0 (F) (Appendix).
29
Mean diel pH varied between 7.7 and 8.7 in all tanks and treatments
(Figure 3.4a). Changes in diel pH followed the usual photoperiodic pattern
with minimum in the morning at about 0700 and maximum at 1500-1900.
There was no significant differences between tanks and between treatments
(p>0.05) with regard to pH values. Dissolved oxygen (DO) also followed the
usual photoperiodic pattern (Figure 3.4b), and daily fluctuations increased with
greater algal production in experimental tanks. Mean DO was highest and
constantly above 3.5 mg/L in tanks at 19+1 deg.0 and lowest in tanks at 19+6
deg.0 (from less than 2 to 0.5 mg/L). Overall, DO declined as the experiment
progressed because of the accumulation of chicken manure and the
biodegradation of phytoplankton that accumulated after each cyclic die-off.
The accumulation of organic materials increased the biochemical oxygen
demand and oxygen consumption by respiration.
Other measured variables.
All water quality variables were measured at the start of the experiment
and at each 30-d interval. Their concentrations increased throughout the 122day experimental period. For each time period, however, there were no
significant differences (p>0.05) between individual tanks and between
treatments for morning total alkalinity, afternoon total alkalinity, organic
nitrogen, ammonia nitrogen, total phosphorus and filterable orthophosphate.
30
32
30
28
26
24
22
20
18
16
14
12
A BCD
E
F
Treatments
33
30
27
24
21
18
15
12
0700 11'00 1500 19100 2300 0300 0700
A -.1- B
Time (hours)
C
D
E -A- F
Figure 3.1 (a) Mean and standard deviation of water temperature (deg.C)
in experimental fish tanks; (b) Diel temperature patterns in fish tank water
during a 122-d experiment. Nominal temperature treatments were:
31
10
25
40
50
E
0.
'42
(a)
10
25
40
(b)
10
25
so
_
12
_EEEF 0700
14
16
1100
18
1600
20
1900
24
22
2300
28
26
_ED_ 0300
_AL_ 0700
Temperature (deg. C)
Figure 3.2 Diel temperature profiles in heated tanks at 19+1 deg.0 (a);
19+3 deg.0 (b); and 19+6 deg.0 (c). The legend indicatesthe sampling
time.
32
10
25
40 _
(d)
50
E
a.
CD
m
10
25
40
50
10
25
_
40
_
50
12
14
0700
16
Ago_ 1100
18
1500
20
_><__ 1900
22
_3E, 2300
24
26
0300
Temperature (deg. C)
Figure 3.3 Diel temperature profiles in heated tanks at 25+1 deg.0 (d);
25+3 deg.0 (e); and 25+6 deg.0 (f). The legend indicates the sampling
time.
28
0700
33
8.9
(a)
8.7
8.5
Xa.
8.3
8.1
--
7.9
7.7
7.5
1b
14 =_____(b)
132 12
E
au
CD
10
8
06
07100 11'00 15100 1900 2300 0300 07017
Time (hours)
A -a- B C -9- D
E
F
Figure 3.4 Diel pH (a) and dissolved oxygen (b) patterns in tanks at
fluctuating cool temperatures. Nominal temperature treatments were:
A=19+1 deg.C; B=19+3 deg.C; C=19+6 deg.C; D=25+1 deg.C; E=25+3
deg.0 and F=25+6 deg.0
34
Total alkalinity averaged from 154.0 to 156.75 mg CaCO3/L in the
morning, and from 322.30 to 341.00 mg CaCO3/L in the afternoon (Table 3.1).
At pH concentrations of about 8-9 and with the levels of alkalinity present,
there were large supplies of inorganic carbon in the bicarbonate form that
sustained high algal blooms in all microcosms.
The daily additions of chicken manure increased the nitrogen and
phosphorus concentrations an order of magnitude over the course of the
experiment. Total nitrogen concentrations averaged 0.02 mg/L to 7.83 mg/L
throughout the experiment, while ammonia nitrogen varied from 0.006 to 0.33
mg/L. Mean total phosphorus increased from 0.32 mg PO4-P/L to 2.09 mg
PO4-P/L throughout the experiment, while the soluble orthophosphate varied
from 0.23 mg PO4-P/L to 1.74 mg PO4-P/L.
There were no significant differences in chlorophyll a concentrations
between temperature treatments at the start of the experiment. Chlorophyll a
concentrations averaged 70.63 + 2.60 mg/m3 at 19 ± 1 deg.C; 61.03 ± 8.95
mg/m3 at 19+3 deg.C; 76.49 + 1.63 mg/m3 at 19+6 deg.C; 63.26 ± 9.15 mg/m3
at 25+1 deg.C; 70.38 + 2.03 mg/m3 at 25+3 deg.C; and 82.07 ± 7.84 mg/m3 at
25+6 deg.C. Concentrations increased substantially during the 122-d trial
and averaged 973.81 + 73.44 mg/m3 at 19+1 deg.0 (A); 1050.04 ± 46.40
mg/m3 at 19+3 deg.0 (B); 967.80 + 40.30 mg/m3 at 19+6 deg.0 (C); 907.32 ±
31.52 mg/m3 at 25+1 deg.0 (D); 850.20 ± 84.86 mg/m3 at 25+3 deg.0 (E); and
777.85 ± 17.53 mg/m3 at 25+6 deg.0 (F) at the end of the experiment.
Table 3.1 Means of water chemistry variables by treatment during the 122-d experimental periods.
Thermal treatments
25 deg.0
19 deg.0
Mean temperature
+1 deg.0
+3 deg.0
+6 deg.0
+1 deg.0
Morning alkalinity (mg CaCO3/L)
154.00
156.75
155.65
155.10
Afternoon alkalinity (mg CaCO3/L)
332.75
326.15
Ammonia (mg NH3-N/L)
0.19
(0.03)
0.15
(0.03)
Organic nitrogen (mg/L)
4.46
(0.82)
Total phosphorus (mg PO4-P/L)
+3 deg.0
+6 deg.0
152.90
153.45
341.00
339.35
338.25
0.16
(0.04)
0.12
(0.03)
0.14
(0.04)
0.12
(0.02)
3.50
(0.79)
3.86
(0.85)
2.72
(0.69)
3.38
(0.89)
2.72
(0.59)
1.56
(0.21)
1.48
(0.20)
1.39
(0.17)
1.37
(0.23)
1.52
(0.20)
1.45
(0.23)
Filtrable orthophosphate
(mg PO4-P/L)
1.32
(0.18)
1.24
(0.17)
1.18
(0.15)
1.15
(0.19)
1.27
(0.17)
1.20
(0.18)
Secchi disk visibility (cm)
4.38
(0.38)
4.00*
(0.50)
4.75
(0.25)
8.13
(0.63)
7.00
(1.50)
24.75*
(1.75)
598.98
(114.06)
(133.50)
558.18-
542.01
(115.35)
520.45
(106.19)
489.35
(98.50)
433.67-
Nominal fluctuation level
Treatment designation
Chlorophyl a (mg/m3)
A
B
C
322.30
aNumbers in parentheses are standard deviations.
*Significantly different (p<0.05).
-Treatments significantly different during the last month of experiment (p<0.05).
D
E
F
(77.05)
36
Overall, chlorophyll a concentrations averaged between 433.67 mg/m3
(F) and 598.98 mg/m3 (A); and significant differences were observed between
treatments F and B at the end of the experiment (p<0.05). Secchi disc
visibility ranged between 4.00 (B) and 24.75 cm (F), and generally decreased
with chlorophyll a concentrations due to algal turbidity (Table 3.1).
Somatic growth and production of Oreochromis niloticus (L.)
Changes in weight and growth rates
Fish weights were not significantly different between tanks at stocking
(16.9 ± 0.24 g), but fish weights at harvest were significantly different among
tanks at 25 ± 6 deg.0 (47.89 + 3.19 g) compared to tanks at 19 + 3 deg.0
water temperature (32.13 ± 4.72 g) (Table 3.2). Final average weights were
not significantly different between the other four treatments. Overall, average
fish weight at harvest was consistently higher in tanks at 25+6 deg.0
compared to fish from other treatments (Figure 3.5a).
Significant differences (p<0.05) were detected between male and
female initial weights in treatments A and B, although total lengths were not
significantly different between sexes. However, no significant differences were
detected among treatments for both male and female weights and total length
(p>0.05). The condition factor was also comparable between males and
females and among treatments (K = 2.13-2.35%) (Table 3.2).
Table 3.2 Average weight, total length and condition factor (K) of males and females at stocking(' and at harvest
(culture period = 122d).
Males
Females
At stocking
'20.41+1.30 b14.43±1.06
A
'22.25+1.44 b13.42±1.17
B
18.71+1.77 15.91+1.44
C
17.95+1.26 15.69+1.26
D
16.89+1.97 17.06+1.60
E
17.14+2.17 16.92+1.77
F
At harvest
A
B
C
D
E
F
Average total length (cm)
Mean weight (g+SE)
Treatment
`50.64+1.91 025.48+1.56
d42.02+2.36 025.54+1.93
50.15+5.10 35.18+4.17
55.05+2.82 34.03+2.82
`50.60+4.28 35.17+3.50
c55.92+2.69 d42.54+2.20
Males
K (%)
Females
K (%)
16.83
16.95
17.03
16.82
16.99
10.78
10.90
10.33
10.22
10.20
10.13
2.21
9.53
9.40
9.85
9.68
9.87
10.05
2.22
2.14
2.22
2.32
2.35
2.22
35.54
32.13'
41.17
44.54
41.34
14.55
13.78
14.58
15.18
14.80
15.30
3.12
3.04
3.06
11.52
11.45
12.80
12.76
12.85
13.80
3.00
3.00
Grand mean
17.01
47.899
2.33
2.28
2.26
2.13
2.19
3.01
2.97
3.01
3.01
3.01
3.01
3.02
(*) Different superscripts in a row denote significant difference between males and females (p<0.05). Different
superscripts in a column denote significant difference between treatments (p<0.05). Nominal temperature
treatments were: A=19+1 deg.C; B=19+3 deg.C; C=19+6 deg.C; D=25+1 deg.C; E=25+3 deg.C; F=25+6 deg.C.
38
1.80
1.60
1.40
1.20
1.00
0.80
0.60
0.40
0.20
0.00
30
0
60
120
90
150
Days
V
A
B° C+ D
A
E
F
Figure 3.5. Oreochromis niloticus mean weight (a) and specific growth rate
(b) at fluctuating cool water temperatures during a 122-d experiment.
Nominal temperature treatments were: A=19+1 deg.C; B=19+3 deg.C;
C=19+6 deg.C; D=25+1 deg.C; E=25+3 deg.0 and F=25+6 deg.C.
39
Table 3.3 Absolute values of the slopes(*) of the linear regression models
for Oreochromis niloticus wet weight (g) on the duration (122 days) of the
culture period
Treatment
A
B
C
D
E
F
Cumulative
degree-days
Mean body weight (g, n=60)
Intercept
Slope
F-statistic
17.490
17.330
16.760
17.480
19.330
18.540
0.153
66.14
36.53
28.36
62.67
18.96
55.59
0.124'
0.198
0.227
0.200
0.253b
597.80
585.60
564.25
1232.20
1235.25
1338.95
(1Different superscripts in a column denote significant differences between
slopes among treatments (p<0.05). Nominal temperature treatments were:
A=19+1 deg.C; B=19+3 deg.C; C=19+6 deg.C; D=25+1 deg.C; E=25+3
deg.0 and F=25+6 deg.C. Values of the slopes of the regression represent
the fish daily growth rates in g/d.
Table 3.4. Constant and slope from the regression of fish weight as a
function of total length data from monthly measurements (n=60 for each
period). The function Wt= a(TL)b was transformed into the linear form
Log Wt= Log(a) +bLog(TL).
Culture period
(days)
0
31
61
91
122
Constant
Log(a)
Slope
-1.647
-1.617
-1.573
-1.683
-1.519
2.87
2.85
b
2.81
2.91
2.76
Condition factor (%)
K=10'9(a) x100
2.25
2.42
2.68
2.07
3.03
r2
0.939
0.912
0.975
0.947
0.980
40
The growth rate decreased through time and was 0.4-0.5%/day in all
treatments at the end of the experiment (Figure 3.5b). The comparison of the
slopes of the linear regressions of the fish wet body weight (g) on the duration
of the experiment (d) showed that fish raised at higher average temperatures
and higher temperature fluctuations grew faster and were heavier than those
in other treatments. The absolute values of these slopes represented the daily
growth rate (g/day) and were much higher at 25+6 deg.0 compared to 19+1
deg.0 or 19+3 deg.0 (p<0.05) (Table 3.3).
At harvest, females from treatments A (19+1 deg.C) and B (19+3
deg.C) were considerably smaller than females from other treatments; females
from treatment F (25+6 deg.C) were the largest. Males were generally 1.2 to
2 times heavier than females except for treatment F where females were
bigger and for treatment B where males were noticeably smaller in size (Table
3.2). The 95% Duncan range test showed that the average male body weight
from treatment B was significantly lower than that of males from treatments D
and F. No differences in total length were detected between males and
females or among treatments at harvest (p>0.05). The condition factor (K)
computed from weight and total length data at harvest was about 3.0, similar
among treatments and between males and females. A regression function of
fish weight on total length expressed as Wt= a(TL)b showed that the slope b
varies from 2.81 - 2.91 and that K=Wt/(TL)bx100 varied from 2.5 at the
beginning of the experiment to 3.03 at harvest (Table 3.4). There was a strong
relationship between fish body weight and the total length (r2>0.91).
60
41
i
(a)
F
50
C
40
I
30
A
1+1
B
20
520
585
I---1-1
D
E
1
v.,
650 1180 1310 1440
Degree-days
60
2.00
50
1.60
40
1.20
S:
v
co
.:='
30
0.80
20
0.40
10
300
0
A
600
900
1200
%MO
cc
0
cn
0.00
1500
Degree-days
Weight
SGR
Figure 3.6 (a) Mean and standard deviation for the cumulative number of
degree-days per treatment and the corresponding mean and standard deviation
for 0. niloticus wet body weight (g) at harvest. Nominal temperature treatments
were: A=19+1 deg.C; B=19+3 deg.C; C=19+6 deg.C; D=25+1 deg.C; E=25+3
deg.0 and F=25+6 deg.C; (b) Linear model of the average weight (g) and
exponential model of the specific growth rate (%/day) as functions of the
cumulative number of degree-days. I: Predicted average weight (g) = 19.645 +
0.022 (DDYS); the coefficient of determination (r2) and r2 adjusted for the
degree of freedom = 0.76 (n=72); II: Predicted SGR (%/d) = 0.407+0.798*exp((DDYS)/448.975)); r2=0.22 (n=72), where DDYS represents the cumulative
number of degree-days and SGR the specific growth rate.
42
Relationships between fish body weight and the cumulative number of
degree-days
At harvest, the average number of cumulative degree-days in
treatments at higher average temperatures was more than twice the average
number of cumulative degree-days in treatments at lower average temperature
(Figure 3.6a; Appendix). The corresponding average weights varied in direct
relation with the number of degree-days. Further regression analysis showed a
linear relationship and a strong correlation (r=0.87) between fish average wet
weight and the cumulative number of degree-days (Figure 3.6b; n=60). The
linear model was in the form:
=19.645+0.022(DDYS); where if = predicted
fish weight (g) and DDYS = cumulative number of degree-days. In this model,
the coefficient of determination (r2) and r2 adjusted for the degrees of freedom
was equal to 0.76, meaning that changes in the cumulative number of degreedays explained 76% of the variation in fish weight. These results, along with
the observed differences in fish body weight and growth rates between
treatments, indicated that higher temperatures and larger temperature
fluctuations are more conducive to better growth performances for 0. niloticus.
However, based on the physiological responses of 0. niloticus to high
temperatures, 38-40 deg.0 constitutes the upper tolerance limit and water
temperatures above 40-41 deg.0 are lethal (Balarin, 1979).
Net yield of Oreochromis niloticus
0. niloticus net yield was 19.99+1.91 kg/are/year for treatment A;
16.22+5.33 kg/are/year for treatment B; 25.79+9.43 kg/are/year for treatment
43
C; 29.62+7.63 kg/are/year for treatment D; 26.03+8.47 kg/are/year for
treatment E; and 33.01+3.70 kg/are/year for treatment F. In general, higher
yields were obtained from treatments at higher average temperature and from
the treatment at lower average temperature with larger temperature
fluctuations (treatments C, D, E, and F; Figure 3.7a). Yields from treatments A
and B were significantly lower. The fitted linear regression model predicting
changes in fish net yield as a function of the cumulative number of degreedays was: ?=12.954+0.013(DDYS) and r2 was 0.65 (r2 adjusted for the degree
of freedom = 0.56), where if = predicted net fish yield in kg/are/year and
DDYS = cumulative number of degree-days (Figure 3.7b).
There was a very minor contribution of reproduction to the total yield
since 0. niloticus did not reproduce sufficiently probably due to the cool water
temperatures and small size of the fish. At harvest, however, there were nests
in tanks at higher average temperatures and some females had eggs or fry in
their mouth. There were a few fry (about two-week old) in these tanks, only 5
from treatment D, 6 from treatment E, and 16 from treatment F. Fry body
weights averaged 9 mg.
44
60
40
50
35
30
40
22
as
25
30
20
20
15
10
10
A
1
B
C
D
E
F
z
Treatments
r"4 Net yield IP Weight
40
35
(b)
D
30
25
20
15
10
500
A
-o
680
860
1040
1220
1400
Degree-days
Figure 3.7 (a): Oreochromis :factious average weight (linear graph) at harvest and
forecasted net fish yield (bar graph) from each experimental treatment. The up-down
error bars represent the standard deviations for the mean yield calculated from the
fish data from two tanks per treatment (n=12). Nominal temperature treatments
were: A=19+1 deg.C; B=19+3 deg.C; C=1946 deg.C; D=25+1 deg.C; E=25+3 deg.0
and F=25+6 deg.C; (b): Fitted linear curve predicting changes in fish net yield as a
function of the number of degree-days: Net yield = 12.954+0.013(DDYS);r2=0.65 and
r2 adjusted for the degrees of freedom = 0.56, where DDYS = cumulative number of
degree-days.
45
Discussion
Water quality variables
Temporal variability in temperature comprises two components,
seasonal and diel variability. Both are cyclic, and diet variation is nested within
the seasonal variability. However, in the tropics, diel temperature variations
are more important than seasonal variability. At the Rwandan Ndorwa fish
culture station located at 2200 m, mean monthly minimum pond water
temperature recorded in the morning at 10 cm below the surface was 15.6
deg.C; mean maximum temperature was 25.6 deg.0 in the afternoon (Hanson
et al. 1988). The station's temperature fluctuations about the mean were only
2.2 deg.0 in the morning and 3.2 deg.0 in the afternoon, and the mean
difference between extremes was 10 deg.C. At the Rwasave fish culture and
research station (1700 m) in Butare, Rwanda, mean minimum and maximum
temperatures measured at 25 cm below water surface during the dry season
(June-August) ranged from 17-20 and 24-27 deg.C, respectively (Lin et al.,
1988, Hanson et al., 1988). In the tropics, considerable temperature
differences exist between low and high altitudes, as well as rainy and dry
seasons (Lin et al., 1997; Balarin, 1988; and Hanson et al., 1988). In addition,
intense solar radiation commonly causes diurnal thermal stratification in deep
tropical ponds (1.5 m or deeper) or in shallower ponds characterized by high
turbidity or high algal blooms (Boyd, 1979; Lin, 1997).
46
This study simulated thermocycles based on temperature data reported
for the dry season in equatorial Rwandan ponds (Figure 3.1a and b) and
reflected diel fluctuations of water temperatures in tropical ponds at elevations
as high as 2200 m. Daily minimum and maximum temperatures as well as diel
temperature variations recorded during this 122-d experiment varied within the
range of thermal treatment. Just as in tropical pond systems, stratification
occurred in all experimental tanks from 1500 to 1900 and was marked in tanks
with the largest temperature variations (Figures 3.2 and 3.3).
Pond stratification prevents effective water mixing and results in
nutrient-limitation in surface water during the day, particularly in ponds where
blue-green algae are concentrated on the water surface. However, such
nutrient limitation is minimal in shallow ponds due to the nocturnal mixing
(Szyper and Lin, 1990). In this investigation, all experimental tanks were
completely or nearly mixed in the morning from 0300 to 0700 (Figures 3.2 and
3.3). Higher mean chlorophyll a concentrations and higher algal turbidity were
recorded in colder tanks, particularly in treatments B (19+3 deg.C) (Table 3.1),
where the water surface was covered by a thick algal scum at the time of
maximum photosynthetic activity. The algal scum may have resulted from the
decreased fish feeding activity in the colder tanks.
The mean concentration of un-ionized ammonia ranged from 0.12 to
0.19 mg/L and was 10 to 25% higher in colder tanks compared to warmer
tanks (Table 3.1). Such ammonia levels may have growth dampening effects
and sublethal effects since acutely toxic levels of NH3 for short-term exposure
47
lies between 0.6 and 2.0 mg/L (Boyd, 1979) and long-term exposure to unionized ammonia reduces growth (Soderberg, 1997), particularly when DO
concentrations are low (Boyd, 1979). The maximum un-ionized ammonia
concentration that 0. niloticus tolerates without reducing its growth is 0.08
mg/L (Abdalla et al., 1992), and 0. niloticus growth ceases at 1.5 mg/L at 28
deg.0 and 1.7 mg/L at 33 deg.0 (Abdalla, 1990). Abdalla (1990) reported that,
at 28 deg.C, the 96-h LC50 for 3.4 g 0. niloticus was 1.4 mg/L un-ionized
ammonia and 2.8 mg/L un-ionized ammonia for a 45.2 g fish (Abdalla 1990).
Characteristically, ammonia exposure causes gill lesions; thus retarding
respiration and adversely affecting fish growth (Soderberg, 1997; Boyd, 1979).
In this experiment, un-ionized ammonia levels were higher in colder tanks
although all tanks received the same level of chicken manure inputs. These
higher concentrations were caused by the generally higher pH recorded in
colder tanks (Figure 3.4a) and to higher primary productivity (Table 3.1). Unionized ammonia concentration increases with pH and temperature, and high
concentrations occur following phytoplankton die-offs (Boyd, 1979).
Recorded mean pH values (7.7- 8.7) and mean alkalinity levels (Table
3.1) fell within the suitable range for fish production (Boyd, 1979). As
expected, dissolved oxygen (DO) concentration in experimental tanks
fluctuated throughout the diurnal photosynthetic cycle and was low from 23000300 in all tanks except treatment A (19+1 deg.C). DO levels reached 0.2-1.2
mg/L (Figure 3.4b) due to the high density of the phytoplankton. Oxygen
concentrations below 3 mg/L during early morning are generally undesirable in
48
fish ponds (Boyd, 1990), but tilapias are very tolerant to hypoxic conditions
(Fernandes and Rantin, 1994; Balarin, 1979). However, artificial aeration is
recommended to minimize the impact of low DO concentration on survival and
growth. Comparable growth rates of 0. niloticus were achieved with aeration
at 10% of saturation (0.8 mg/L at 26 deg.C) and at 30% of saturation (2.4 mg/L
at 26 deg.C) (Teichert-Coddington and Green, 1993).
No artificial aeration of the tanks was provided for the experimental
tanks, but surface water movement was activated by two air blowers installed
in the experimental room to simulate wind and increase turbulence and
oxygen diffusion at the air-water interface. DO levels were generally low in
warmer tanks (less than 3.5 mg/L mean DO throughout the experiment) but no
fish mortality was recorded.
Somatic growth and production of Oreochromis niloticus (L.)
0. niloticus growth rates were generally low and ranged from 0.124
g/day in treatment B to 0.253 g/day in treatment F (Table 3.3). Relatively low
temperatures, low dissolved oxygen concentration, high fish density, and
exposure to ammonia contributed to the slow growth obtained from this
experiment. Similar poor growth performance (0.293 g/day) was reported for
0. niloticus in Rwandan ponds where comparable temperatures were
recorded (Soderberg, 1997; Chang, 1989). In addition, the stocking density
(3.6 fish/m2) in experimental tanks was much higher than the usual 0.5-2
fish/m2 stocked in low intensity grow-out or experimental ponds (Soderberg,
49
1997; Seim et al., 1994; Verheust et al. 1994; Hishamunda and Moehl, 1989;
Moehl et al., 1988; Hanson et al., 1988).
When the same mean temperatures are considered, comparatively
higher growth rates and larger fish were obtained from tanks where
temperature fluctuations were the largest ( ±6 deg.C). 0. niloticus performance
in tanks where water temperature was nearly constant (±1 deg.C) was
intermediate; and the +3 deg.0 treatment resulted in lowest fish performance
(Table 3.3, Figure 3.5a and b). Fish growth rate was directly related to the
amplitude of temperature fluctuation and inversely related to the cumulative
number of degree-days at low mean temperature (19 deg.C), but growth rates
were directly related to both the amplitude of temperature fluctuation and the
cumulative number of degree-days at high mean temperatures (25 deg.C).
The results suggested that fish benefited more from a larger temporary
rise in temperature at mean temperatures below 20 deg.0 than at relatively
higher mean water temperature or at relatively constant water temperature.
However, the cumulative number of degree-days is reduced at low mean
temperature associated with a large variation because the rise in temperature
is counterbalanced by a fall of an equal magnitude and because of the 15
deg.0 threshold used in calculating degree-days. Studies comparing tilapia in
response to cool cyclic temperatures have seldom been conducted in the
laboratory. However, field experiments in tropical ponds have consistently
demonstrated that tilapia growth rates are reduced at high altitudes or during
the dry season where water temperatures are lower. In contrast, a better
50
growth performance is obtained at low altitudes during the rainy seasons when
the pond water is warmer (Seim et al., 1994; Balarin, 1988; Hanson et al.,
1988; Moehl et al., 1988; Lowe-McConnell, 1987; Therezien, 1966). Although
tilapias can tolerate temperatures as low as 8-10 deg.0 for short periods (3-4
h) (Chimits, 1957; Sarig, 1969; Yashouv, 1960), growth and reproduction
performance are severely impaired at temperatures below 20 deg.C, mostly as
a result of reduced feeding rate (Soderberg, 1997; Balarin and Haller, 1982;
Platt and Hauser, 1978; Balarin, 1979; Bishai, 1965; Fryer and Iles, 1972).
Soderberg (1990) demonstrated that growth rate (mm/day) in Oreochromis
aureus increased linearly with temperature within the range 20-30 deg.0 and
the intercept for the theoretical zero growth corresponded to 17.8 deg.C.
However, Soderberg (1992) noted that growth is proportional to temperature
only when fish are fed maximum rations. Growth rates can then be accurately
predicted from the linear equation at temperatures between the intercept of the
temperature vs. growth regression and the optimum temperature for growth.
The latter varies with the species of tilapias but ranges between 20-35 deg.0
(Soderberg, 1992; Balarin and Haller, 1982; Balarin, 1979). 0. niloticus
survives prolonged periods at 15 deg.0 but does not feed or grow (Balarin and
Haller, 1982). Therefore, the threshold temperature of 15 deg.0 was used to
calculate the degree-days for tilapias (Green and Teichert-Coddington, 1993;
Soderberg, 1990, Bardach et al., 1972).
In this experiment, 0. niloticus growth relied on the natural productivity
of the experimental tanks fertilized with fresh chicken manure at the rate of
51
54.3 g/m3 (27.1 g/m2, equivalent to 500 kgDW/ha/week) and depended
primarily on primary production, which was not limiting given the high
chlorophyll a concentration in all treatments (Table 3.1). The linear model
based on this experimental data allowed the prediction of the expected
average fish weight as a function of cumulative degree-days. The predicted
average fish weight was 19.65 g at 15 deg.0 ("zero" degree-days) and
increased linearly with temperature, up to the upper thermal tolerance limit.
The linear model predicting net yield as a function of the cumulative number of
degree-days indicated that, under comparable experimental conditions,
expected net yield at 15 deg.0 was 12.95 kg/are/year. In this experiment, 0.
niloticus average yield was 38.5% higher at 25 deg.0 average temperature
treatments compared to 19 deg.0 treatments. Average fish yield was 18.9%
lower at 19+3 deg. and 29% higher compared to that at 19+1 deg.C. The
pattern at 25 deg.0 was similar; fish yield was 12.1% lower at 25+3 and 11.5%
higher at 25+6 deg.0 compared to 25+1 deg.C. This pattern reflected the
beneficial effect of a larger temperature variation as long as the maximum
temperature reaches the optimum range and that the minimum is above the
lower 15 deg.0 lower activity threshold for tilapias.
Yields from these experiments were comparable to those obtained in
the field in similar conditions. At the Rwasave station (1625 m), Rwanda, 0.
niloticus in ponds fertilized with chicken manure at 500 kgDW/ha/week
reached a productivity exceeding 30 kg/are/year; while the average yield from
compost or bran-fed ponds at the stations of National Fishculture Project
52
(PPN) ranged from 8.5-20 kg/are/year. Stocking rate in Rwandan ponds was
0.6-2 fish/m2 and growth rate was 0.3-0.8 g/day (Soderberg, 1997; Seim et al.,
1994; Moehl et al., 1988; Boyd et al., 1988). Yields from tilapia ponds in
Madagascar at elevations above 1500 m and in Malawi at 1000-1500 m
ranged approximately from 4.0 to 20 kg/are/year (Balarin, 1988).
In this experiment, 0. niloticus did not initiate reproduction until just
before harvest (122-d) when a few eggs and fry were observed in the mouth of
females from tanks at 25 deg.C. No signs of reproduction were observed in
tanks at 19 deg.C. At harvest, fish were approximately 5 months old. This
result is consistent with the dynamics of tilapia reproduction in natural waters;
and similar delays in reproduction were observed in tropical pond systems
where cool water temperatures prevail due to high elevation or to seasonal
variations. Persistent low water temperatures encountered at high elevations
in Rwandan ponds prolong the time required for tilapia reproduction. At
elevations of approximately 1700 m in Rwanda, 0. niloticus first spawning
occurred when fish were 6-9 months old and spawning activity increased 2-3
months later. At elevations higher than 2000 m, onset of reproduction was
delayed until fish were 10 to 11 months old (Hanson et al., 1988). Similarly,
Balarin (1988) reported a 3- to 6-month period of no growth or breeding in
tilapias cultured above 1,000 m in Madagascar and Malawi. 0. niloticus
spawns naturally at 21-35 deg.C, and optimal spawning temperatures range
from 25 to 35 deg.0 (Balarin, 1988; Rothbard and Pruginin, 1975; Philippart
and Ruwet, 1982;). When water temperatures decrease during the cooler
53
season, reductions or delays in tilapia spawning occur (Green and TeichertCoddington, 1993; Maluwa and Costa-Pierce, 1993; Hanson et al. 1988;
Moehl et al., 1988; Galman et al., 1988; Guerrero, 1986). Little or no
recruitment was reported in Rwandan ponds stocked with mixed-sex tilapia at
elevations higher than 2000 m (Hanson et al., 1988; Rurangwa et al., 1992;
Hishamunda and Moehl, 1989). Few fry were harvested from these ponds and
females were observed with eggs but not with sac-fry in their mouth.
Incidences of eggs failing to hatch have also occurred in Rwanda at 1625 m
when water temperatures decreased below 18 deg.0 for several hours in egg
incubation jars (Green et al., 1997). In addition, prolonged exposure of 0.
niloticus to temperatures below 20 deg.0 resulted in a failure of immatures to
mature and in a gonadal reabsorption in mature fish (Balarin and Haller,
1982).
The effect of cooler temperature on delaying reproduction was probably
combined with the small size and young age of the fish. 0. niloticus female
average weight at harvest was 28.7 g and at an average total length of 11.9
cm in tanks at 19 deg.C; and 37.3 g at an average total length of 13.1 cm in
tanks at 25 deg.C. The corresponding average length was 11.9 cm and 13.1
cm. In ponds, tilapias can spawn at under six months of age after stocking at a
size >10 cm, but tilapias can also mature at a larger size depending on
environmental conditions such as food availability, temperature, condition
factor, intraspecific social interactions, and genetic traits (Brummett, 1995;
Jalabert and Zohart, 1982; Balarin and Haller, 1982). Fish from this
54
experiment were in good condition and food (phytoplankton) was not in short
supply, and temperature was the main factor limiting growth and reproduction.
The response of the experimental fish confirmed the field observations
in Rwandan ponds. At the Ndorwa station situated at 2,200 m, the 69-g 0.
niloticus stocked in fertile ponds at 1 fish/m2 reached 231 g after a 6-month
culture period but did not reproduce (Hanson et al., 1988). In general, little
reproduction occurred at 1900 m and no reproduction was reported for ponds
at elevations above 2200 m (Veverica and Rurangwa, 1991).
55
Literature cited
Abdalla, A. A. F. 1990. The effect of ammonia on Oreochromis niloticus (Nile
tilapia) and its dynamics in fertilized tropical fish ponds (abstract). In
Egna, H. S., J. Bowman, and M. McNamara (Editors), Seventh Annual
Administrative Report, Pond Dynamics/Aquaculture CRSP, 1989,
Oregon State University, Corvallis, Oregon, 52.
Abdalla, A. A. F., McNabb, C., Knud-Hansen, C., and Batterson, T. 1992.
Growth of Oreochromis niloticus in the presence of un-ionized
ammonia. In Egna, H. S., M. McNamara, and N. Weigner (Editors),
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60
APPENDIX
Appendix. Minimum, maximum and mean temperatures, mean cumulative degree-days, and average weight of
Oreochromis niloticus under fluctuating cool water temperatures.
Treatment
A
A
A
A
A
B
B
B
B
B
C
C
C
C
C
D
D
D
D
D
Number
of daysMinimum
0
31
61
91
122
0
31
61
91
122
0
31
61
91
122
0
31
61
91
122
Average
weight (g)
Temperature (deg.C)
Cumulative
Maximum
Mean Delta degree-days SD
SD
18.80
17.60
17.35
18.70
18.75
21.75
21.20
20.45
20.75
21.05
20.28 1.47
19.40 1.80
18.90 1.55
19.73 1.03
19.90 1.15
0.00
136.40
237.90
429.98
597.80
0.00
15.34
47.45
22.52
17.25
16.83
23.59
25.49
30.88
35.54
0.54
2.23
16.15
15.95
16.15
16.20
15.85
23.35
23.85
22.60
22.95
23.75
19.75 3.60
19.90 3.95
19.38 3.23
19.58 3.38
19.80 3.95
0.00
151.90
266.88
416.33
585.60
0.00
8.77
2.16
9.65
60.39
16.95
21.72
24.60
28.65
32.13
0.27
1.23
4.40
4.07
4.72
13.20
13.65
13.10
12.90
13.20
26.45
26.70
25.50
26.05
26.05
19.83 6.63
20.18 6.53
19.30 6.20
19.48 6.58
19.63 6.43
0.00
160.43
262.30
407.23
564.25
0.00
9.86
0.19
2.29
4.31
17.03
22.48
29.11
36.36
41.17
21.30
22.90
22.25
23.40
23.55
25.25
26.30
25.20
27.10
26.65
23.28 1.97
24.60 1.70
23.73 1.47
25.25 1.85
25.10 1.55
0.00
297.60
532.23
932.75
1232.20
0.00
17.54
49.60
128.69
34.51
16.82
24.87
31.94
38.26
44.54
0.12
1.05
3.04
7.64
7.03
4.31
9.65
0.45
0.85
3.41
6.01
9.57
9.01
Appendix. Minimum, maximum and mean temperatures, mean cumulative degree-days, and average weight of
Oreochromis niloticus under fluctuating cool water temperatures.
Treatment
A
A
A
A
A
B
B
B
B
B
C
C
C
C
C
D
D
D
D
D
Number
of daysMinimum
0
31
61
91
122
0
31
61
91
122
0
31
61
91
122
0
31
61
91
122
Temperature (deg.C)
Cumulative
Maximum
Mean Delta degree-days SD
Average
weight (g)
SD
18.80
17.60
17.35
18.70
18.75
21.75
21.20
20.45
20.75
21.05
20.28
19.40
18.90
19.73
19.90
1.47
1.80
1.55
1.03
1.15
0.00
136.40
237.90
429.98
597.80
0.00
15.34
47.45
22.52
17.25
16.83
23,59
25.49
30.88
35.54
0.45
0.85
16.15
15.95
16.15
16.20
15.85
23.35
23.85
22.60
22.95
23.75
19.75
19.90
19.38
19.58
19.80
3.60
3.95
3.23
3.38
3.95
0.00
151.90
266.88
416.33
585.60
0.00
8.77
2.16
9.65
60.39
16.95
21.72
24.60
28.65
32.13
0.27
1.23
4.40
4.07
4.72
13.20
13.65
13.10
12.90
13.20
26.45
26.70
25.50
26.05
26.05
19.83
20.18
19.30
19.48
19.63
6.63
6.53
6.20
6.58
6.43
0.00
160.43
262.30
407.23
564.25
0.00
9.86
4.31
9.65
4.31
17.03
22.48
29.11
36.36
41.17
0.19
2.29
21.30
22.90
22.25
23.40
23.55
25.25
26.30
25.20
27.10
26.65
23.28
24.60
23.73
25.25
25.10
1.97
1.70
1.47
1.85
1.55
0.00
297.60
532.23
932.75
1232.20
0.00
17.54
49.60
128.69
16.82
24.87
31.94
38.26
44.54
0.12
1.05
3.04
7.64
7.03
34.51
3.41
0.54
2.23
6.01
9.57
9.01
63
CHAPTER 4
Effects of Diel Thermocycles in Warmwater Tanks on the Growth,
Reproduction, and Net Yield of Oreochromis niloticus (L)
Felicien Rwangano
64
Abstract
The effects of diel temperature variations ( ±1, ±3, and ±6 deg.C) at 22
and 28 deg.0 mean temperatures on 0. niloticus were examined under
controlled laboratory conditions. Two male and three female averaging
19.81+5.13 g initial weight were randomly assigned to each of the twelve 0.7m3 (1.4-m2) fiberglass. Each experimental tank water was fertilized daily with
fresh chicken manure at a rate of 500 kgDW/ha/week. Water quality and fish
growth and reproduction were monitored over a 153-day period. Reproduction
occurred 30 days after stocking in all tanks at 28 deg.0 and was delayed until
after 90 days in treatments at 22 deg.C. Relative fecundity was highest and
similar at 28+3 and 28+6 deg.C. Results indicated a positive growth and
reproductive effect of larger diel thermocycles at both 22 and 28 deg.C; but
growth performance and yields were better at a fluctuating 28 deg.0 than at 22
deg.C. Oreochromis niloticus body weight and net yield were positively
correlated with cumulative degree-days (r2=0.80 and 0.53, respectively).
65
Materials and Methods
The effects of thermocycles on the growth, reproduction, and net yield
of Oreochromis niloticus (L.) were investigated during a 153-day laboratory
experiment conducted according to the methods described in Chapter 2.
Temperature treatments were A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C;
D=28+1 deg.C; E=28+3 deg.0 and F=28+6 deg.C.
Results
Water quality variables
Diel temperature, dissolved oxygen, and pH fluctuations
Surface temperatures in experimental tanks stayed within the expected
mean values, with a +1 deg.0 margin, and temperature fluctuations were
excessively larger in treatments A and D. Temperature values were: 23.3 ± 3
deg.0 for A, 22.8 + 4 deg.0 for B, 23.2 + 6 deg.0 for C, 26.7 ± 3 deg.0 for D,
27.3 + 4 deg.0 for E, and 29.1 ± 5 deg.0 for treatment F. Cumulative number
of degree-days were: 1190 + 59 for treatment A., 1098 + 73 for B, 849 ± 0 for
C, 1733 + 50 for D, 1729 ± 8 for E, and 2111 ± 31 for F (Figure 4.1).
The diel temperature profile followed the expected pattern with
minimum values at 0700 and maximum at 1500-1900. Diel dissolved oxygen
ranged from 2.8-4.9 at 0300-0700 to 4.4-7.0 mg/L at 1500-1900 in all
treatments. Diel pH values ranged from 7.8-8.8.
66
35
33
31
29
27
25
23
21
19
17
15
700
1000
1300
1600
1900
2200
Cumulative Degree-days
Figure 4.1 Mean diel temperature (-±SD) and corresponding cumulative
degree-days (±SD) per treatment during the 153-d experiment. Nominal
temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C;
D=28+1 deg.C; E=28+3 deg.C; and F =28 ±6 deg.C.
67
Water chemistry variations throughout the experiment.
Water chemistry variables were measured monthly over the 153-day
experimental period. No significant differences (p>0.05) in morning total
alkalinity and in afternoon total alkalinity were detected between tanks and
between treatments at each monthly sampling. Mean morning total alkalinity
in all tanks and treatments ranged from 207.63 to 211.20 mg CaCO3/L. Mean
afternoon total alkalinity averaged from 332.20 to 376.20 mg CaCO3/L (Table
4.1).
Organic nitrogen concentrations averaged 4.06-5.20 mg/L to 5.54-9.48
mg/L throughout the experiment, while ammonia nitrogen varied from 0.17-
0.22 to 0.23-0.40 mg/L. From the third month to the end of the experiment,
treatments B and C had significantly higher concentrations of organic nitrogen
and ammonia than treatments F. Overall, organic nitrogen and ammonia
nitrogen concentrations were significantly different between temperature
treatments C and F. Organic nitrogen content was 7.03 ± 0.62 mg/L for
treatment C and 4.78 ± 0.19 mg/L for treatment F; ammonia concentrations
were 0.30 mg NH3-N/L for treatment C and 0.20 mg NH3-N/L for treatment F
(Table 4.1).
Mean total phosphorus increased from 0.45-0.78 mg PO4-P/L to 1.161.27 mg PO4-P/L throughout the experiment, while the soluble orthophosphate
varied from 0.60-0.76 mg PO4-P/L to 0.97-1.06 mg PO4-P/L. Overall, there
were no significant differences in total phosphorus and orthophosphate
concentrations between treatments (p>0.05).
Table 4.1. Means of water chemistry variables by treatment during the 153-d experimental perioda.
Thermal treatments
28 deg.0
22 deg.0
Mean temperature
+1 deg.0
A
+3 deg.0
±6 deg.0
+1 deg.0
+3 deg.0
±6 deg.0
Morning alkalinity (mg CaCO3/L)
207.90
(31.13)
211.20
(31.44)
210.65
(31.84)
210.10
(31.77)
207.63
(31.62)
208.73
(31.93)
Afternoon alkalinity
(mg CaCO3/L)
348.15
(19.33)
332.20
(13.22)
334.68
(7.88)
355.85
(15.02)
376.20
(21.49)
367.13
(20.30)
Ammonia (mg NH3-N/L)
0.27
(0.01)
0.29
(0.03)
0.30
(0.03)
0.24
(0.02)
0.26
(0.02)
0.20.
(0.01)
Organic nitrogen (mg/L)
6.45
(0.32)
6.88
(0.63)
(0.62)
7.03-
5.65
(0.48)
6.20
(0.46)
(0.19)
Total phosphorus (mg PO4-P/L)
1.04
(0.07)
0.96
(0.11)
0.93
(0.08)
0.93
(0.09)
1.05
(0.08)
0.98
(0.07)
Filtrable orthophosphate
(mg PO4-P/L)
0.86
(0.05)
0.79
(0.09)
0.78
(0.07)
0.78
(0.07)
0.87
(0.06)
0.82
(0.06)
862.88
(134.78)
679.45
(109.71)
646.32
(94.60)
658.71
(112.68)
747.72
(154.08)
849.26
(158.22)
Nominal fluctuation level
Treatment designation
Chlorophyl a (mg/m3)
aNumbers in parentheses are standard deviations.
*Mean NH3-N concentrations significantly different between C and F (p<0.05).
-Mean organic nitrogen concentrations significantly different between C and F (p<0.05).
4.78-
69
There were no significant differences (p>0.05) in chlorophyll a
concentrations between temperature treatments at the start of the experiment.
Chlorophyll a concentrations averaged 159.05 mg/m3 at 22 + 1 deg.C; 168.52
mg/m3 at 22 ± 3 deg.C; 160.68 mg/m3 at 22 + 6 deg.C; 156.36 mg/m3 at 28 +
1 deg.C; 138.70 mg/m3 at 28 + 3 deg.C; and 153.27 mg /m3 at 28 ± 6 deg.0 at
stocking. As expected, chlorophyll a concentrations increased substantially
during the 153-d trial and averaged 1232.95 mg/m3 at 22+1 deg. (A); 843.00
mg/m3 at 22+3 deg.0 (B); 842.67 mg/m3 at 22+6 deg.0 (C); 884.75 mg/m3 at
28+1 deg.0 (D); 972.09 mg/m3 at 28+3 deg.0 (E); and 926.93 mg/m3 at 28+6
deg.0 (F) at the end of the experiment. Overall, chlorophyll a concentrations
averaged between 646.32 mg/m3 (C) and 862.88 mg/m3 (A) (Table 4.1).
Significant differences (p<0.05) in chlorophyll a concentrations were observed
between temperature treatments F and A during the third month, F and C
during the fourth month, and A and C during the last month of experiment.
Reproduction
Time and size at first spawning
Fish raised in tanks at 28 deg.0 water temperature started spawning
earlier than fish in tanks at 22 deg.C. Fry were first observed in all 28 deg.0
tanks by the first 30 days of the experiment (Figure 4.2a) and the number of
fry increased with the amplitude of temperature fluctuation (Figure 4.3). At the
end of the first month of the experiment, female average weights were 19.6 g
(A), 19.6 g (B), 19.5 g (C), 18.7 g (D), 20.9 g (E), 19.4 g (F) and the cumulative
70
number of degree-days averaged 281+31 (A), 205+12 (B), 164+9 (C), 384+24
(D), 408+5 (E), and 446+4 (F).
In tanks with water temperature at 22 + 6 deg.0 (treatment C), fry were
observed for the first time at the end of the third month of the experiment
(Figure 4.2). No fry were produced in tanks at 22 + 3 deg.0 (treatment B)
during this experiment and only one fry was harvested from tanks at 22 ± 1
deg.0 (treatment A) at the end of the experiment (Figure 4.3a and b).
However, some eggs and sac-fry were produced in these tanks from the third
monthly fish sampling through the end of the experiment (Figure 4.4). At the
end of the fourth month, female average weight was: 28.8 g (A), 28.9 g (B),
25.1 g (C), 28.0 g (D), 33.8 g (E), 38.5 g (F). The corresponding cumulative
number of degree-days averaged 1095+98 (A), 901+9 (B), 935+12 (C),
1488+6 (D), 1577+3 (E), and 1728+25 (F).
Egg, sac-fry and fry production per thermal treatment
Eggs and sac-fry were collected and counted when observed during the
monthly fish sampling. Fry were harvested regularly from the edges of the
experimental tanks or were collected at the monthly fish sampling. Based on
the cumulative number of eggs, sac-fry and fry collected during the
experimental period, the average egg and sac-fry production per kg female per
day was 25.4 in treatment A, 6.8 in treatment B, 27.0 in treatment C, 11.0 in
treatment D, 40.3 in treatment E, and 27.9 in treatment F (Figure 4.4).
71
55
45
(a)
First reproduction for treatments D,
E, and F. Average weight = 19.7g
35
25
First reproduction for treatment C
Average weight = 27.6 g
15
160
E
.c
0H
ea
150
140
122
31
163
(b)
First reproduction for treatments
D, E, and F
130
120
110
100
First reproduction for treatment C
31
62
Days
92
122
163
Figure 4.2 Changes in 0. niloticus weight (a) and total length (b) over
time during the 153-d experiment.
72
1200_
A (0.0;0.0)
900_
600
300
1200
900
_
_
0
600
300
0
I
I
1
2
I
3
I
4
5
I
1200
a
900
900
600
600
300
300
0
0
-CI
1200
900
600
300
C (2.5;0.17)
-
0-
E (29.8;2.32)
B (0.0;0.0)
-
0
..
.4-
0
900
600
300
0
3
4
5
D (11.8;0.78)
1200_
1200
2
1
-
_
0
-
2
3
4
5
F (29.8;2.57)
I
MIMI
2
3
4
I
5
Months
Figure 4.3 Cumulative number of fry harvested from experimental tanks
during the 153-d experiment. Number in parentheses represent the number
of fry/kg female/day and the number of fry produced per m2/day. Nominal
temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6
deg.C; D=28+1deg.C; E=28+3 deg.C; and F=28+6 deg.C.
73
A (25.4;1.81)
1500
1500
1000
1000
500
500
0
0
1
2
3
4
5
C (27.0;1.86)
1500
1500
1000
1000
500
500
0
1000 500
0
-
0
D (11.0;0.73)
1
E (40.3;3.15)
1500
B (6.8;0.47)
1500
III I I
1
2
3
4
1
F (27.9;2.40)
1000
500
5
Months
Figure 4.4 Cumulative number of eggs and sac-fry harvested from
experimental tanks during the 153-d experiment. Number in parentheses
represent the number of eggs and sac-fry/kg female/day and the number of
eggs and sac-fry produced per m2 per day. Nominal temperature treatments
were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C; D=28+1deg.C; E=28+3
deg.C; and F=28+6 deg.C.
74
The average number of fry produced per kg female per day was 0 in
treatment A, 0 in treatment B, 2.5 in treatment C, 11.8 in treatment D, 29.8 in
treatment E, and 29.8 in treatment F (Figure 4.3). The total number of seeds
(eggs, sac-fry, and fry) produced per m2 per day was 1.82 in treatment A, 0.47
in treatment B, 2.03 in treatment C, 1.51 in treatment D, 5.47 in treatment E,
and 4.96 in treatment F. The number of seeds/kg female/day was positively
correlated to the cumulative number of degree-days (r2=0.82) and to water
temperature (r2=0.66). There was no relationship between the number of
seeds/kg female/day and the male:female ratio.
The percentage contribution of the reproduction weight to the total fish
biomass was highest for higher temperature regimens with larger thermal
variations. Conversely, the percentage contribution of adult weight to total fish
biomass was highest in cooler water tanks where reproduction weight was the
lowest. The biomass of juveniles accounted for 4.7 and 3.6% of the total
biomass at 28 + 3 and 28 + 6 deg.C, respectively. These ratios were
significantly different from those obtained from other treatments. The
contribution of the reproduction weight to the total fish biomass was lowest at
22 + 3 deg.0 (treatment B) and averaged 0.3% of the total biomass (Table
4.2; Figure 4.5). In general, higher juvenile production and greater average
adult body weight were obtained in warm water experimental tanks,
particularly at larger temperature variations, resulting in a greater total
biomass (adults and fry combined) than in cooler tanks (Table 4.2).
75
In practice, fish farmers interested in the production of commercial size
fish for consumption are only concerned with the adult production and seek to
minimize reproduction. A high recruitment rate in ponds can result in the
stunting of fish as a result of competition for food and space. Some of the field
methods used to prevent this competition include (a) monosex (male only)
culture, (b) polyculture with a predator, or (c) regular seining to harvest the
recruits (if any).
Gonosomatic index
In general, there was no significant difference in male-to-female weight
ratio determined at each monthly fish sampling (Table 4.3). The average
weight of male was 1.10 to 2.0 times greater than that of females. At harvest,
within male body weight comparison showed that the male average weight of
40.45+3.03 g obtained from treatment A (22 +1 deg.C) was significantly lower
than the male average weight obtained from all tanks at 28 deg.C. Male-tofemale comparison showed that the male average weight obtained at 22 ± 1
deg.0 was not significantly different from the female average weight harvested
from tanks at 28 + 3 deg.0 (treatment E) and 28 ± 6 deg.0 (treatment F).
Compared to other treatments, the gonosomatic index (GSI) was significantly
lower in treatment E and F (p<0.05) for both males and females because of
reproduction. The GSI was particularly high for fish from treatments at 22
deg.0 compared to those raised in tanks at fluctuating 28 deg.0 (Table 4.4).
Table 4.2 Contribution of progeny and adults in the total biomass of Oreochromis niloticus in tanks at 22 and 28
deg. C mean fluctuating water temperatures at harvest (153-d).
Treatments
Cumulative
of degree-days
Fish biomass at harvest (g/m3)
Adults
849b
258.4
238.2
250.5
2111d
315.3
367.0
A
1190a
B
1 098ab
C
D
E
F
1733'
1729'
310.1
0/0
99.1
99.7
98.8
99.0
95.3
96.4
Progeny
2.5
0.7
3.1
3.3
15.6
13.8
0.9
0.3
1.2
1.0
4.7(1
3.6(1
Indicates a significant difference with other treatments (p<0.05).
"Ad Values with different superscripts within the same column are significantly different (p<0.05).
Nominal temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C; D=28+1 deg.C;
E=28+3 deg.C; and F=28+6 deg.C.
77
100--
IN1111,
co
o 80
cco
E
25 60
cti
V,
c
40
a)
E.' 20
a)
0
0.3
0.9
A
B
1.2
C
1.0
D
Treatments
4.7*
E
3.6*
F
Ei Adults II Progeny
Figure 4.5 Contribution of progeny and adults in the total fish biomass.
*:Biomass of fry significantly higher in treatments E and F (p<0.05).
Nominal temperature treatments were: A=22+1 deg.C; B=22+3 deg.C;
C=22+6 deg.C; D=28+1deg.C; E=28+3 deg.0 and F=28+6 deg.C.
Table 4.3 Monthly average fish weight (g + SD), cumulative number of degree-days (+ SD), and male/female weight ratio
per thermal treatment
Experimental periods
Treatments
A
B
C
E
F
0
1
2
3
4
5
Duration (days)
0
31
62
92
122
153
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
19.81 + 4.76
0
1.51
25.25 + 8.22
281 + 31
26.50 + 7.53
477 + 68
27.23 + 7.48
715 + 90
33.49 + 6.98
1095 + 98
36.18 ± 5.52
1190 + 59
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
0
1.61
22.76 + 6.46
205 + 12
26.45 + 7.79
490 + 3
28.04 ± 7.72
660 + 44
33.92 + 8.07
901 +9
37.05 ± 6.35
1098 + 73
26.99 + 9.67
164 + 9
26.24 + 6.86
383 + 29
511 +0
27.98 + 6.96
32.68 + 10.14
735 +
38.97 + 6.22
849 + 0
25.20 + 9.23
384 + 24
28.06 + 9.97
747 + 22
1.79
31.03 + 9.78
1042 + 30
1.64
37.39 ± 12.69
1488 + 6
43.41 ± 7.64
1733 + 50
24.40 + 5.93
408 + 5
1.44
31.72 + 10.65
789 + 2
35.33 + 10.10
1040 + 5
41.20 + 10.01
44.15 + 7.65
1729 + 8
24.83 + 7.01
446 + 4
1.70
28.87 + 7.48
866 + 5
30.92 ± 10.87
1270 + 18
45.11 + 9.08
1728 + 25
19.80 + 5.02
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
0
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
0
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
0
Average weight (g)
Cumulative degree-days
Male/Female weight ratio
0
18.77 + 4.68
1.52
19.74 + 4.92
1.54
19.76 + 4.77
1.53
19.70 + 4.69
1.48
1.86
1.65
1.97
1.86
1.70
1.65
1.63
1.69
1.60
1.64
1.57
1.62
1.62
1.10
1.40
1.52
1.76
1.84
1577 +3
1.55
1.43
1.21
1.48
1.47
2.01
1.53
51.38 + 9.35
2111 + 31
1.70
Nominal temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C; D=28+1 deg.C; E=28+3 deg.C; and F=28+6
deg.C.
Table 4.4 Comparison of male and female average body weight (g+SD), dry body weight (g), gonad weight
(mg+SD), and the gonado-somatic index (GSI) at harvest.
Treatment
Males
A
B
C
D
E
F
Females
A
B
C
D
E
F
Testes / ovary
dry weight (mg)
Average body
wet weight (g)
Average body
dry weight (g)
40.45 + 3.03(a)
47.19 + 3.28(ab)
47.44 + 3.87(ab)
68.24 + 4.64(0
12.78
19.08
19.60
32.99
26.74
40.92
25 + 13
44 + 34
70 + 57
49 + 39
22 + 14
18 + 10
33.34 + 4.95(d)
31.98 + 6.97(d)
32.19 + 2.51(d)
30.93 + 7.87(d)
36.46 + 3.78(a)
40.14 + 3.61(a)
15.55
15.02
14.65
12.75
17.38
21.54
926 + 371
966 + 502
927 + 534
607 + 384
274 + 166
458 + 232
62.13 + 3.91(0
55.68 + 6.37(b)
GSI
(%)
0.20
0.23
0.36
0.15
0.08(*)
0.04(1
5.95
6.43
6.33
4.76
1.58(")
2.13r1
)Sex specific GSI significantly lower than in other treatments (p<0.05)
(a'b'c'd')Values with different superscripts within the column are significantly different (p<0.05).
Nominal temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C; D=28+1 deg.C;
E=28+3 deg.C; and F=28+6 deg.C.
(
80
Somatic growth and net yield of Oreochromis niloticus (L.)
Body weight and growth rate
Individual fish weights were not significantly different at stocking
(p>0.05) and averaged 19.81+5.13 g. At harvest, the overall mean fish weight
was 41.99+12.63 g. Average harvest weight was significantly higher for fish
raised in tanks at 28 + 6 deg.0 (51.38+14.35 g) compared to fish from other
treatments. Fish raised in tanks at 22 + 1 deg.0 were the smallest (36.18+5.52
g). However, there were no significant differences in average fish weights and
total length (p>0.05) between the three treatments at 22 deg.0 (Figure 4.2;
Table 4.5). As expected, the daily growth rate decreased through time. On
average, it was highest in tanks at 28+6 deg.0 (0.63%/day) and lowest in
tanks with water temperature at 22+1 deg.0 (0.39%/day) (Table 4.5). At 22
deg.C, the average relative growth rate ranged from 0.39 to 0.48%/day. The
mean growth rate for treatment C (22+6 deg.C) was significantly different from
the mean growth rates obtained at the same average temperature with lower
amplitude of the thermocycle (p<0.05). Similarly, at 28 deg.C, the relative
growth rate for treatment F (28+6 deg.C) differed significantly (p<0.05) from
the mean growth rates obtained at the same average temperature with lower
amplitude of the thermocycle. The relative growth rate at 28 deg.0 treatments
ranged from 0.52 to 0.63%/day. Although absolute values of the growth rates
at 28 deg.0 were higher than those obtained at 22 deg.0 (p<0.05), growth rate
from treatment C (22 + 6) was not significantly different from growth rates
obtained at 28 deg.0 with low fluctuations (Table 4.5).
81
Relationships between the cumulative number of degree-days and 0. niloticus
weight and net yield.
0. niloticus from treatment F (28 + 6 deg.C) were significantly larger
than those from other treatments; fish from tanks at 28 ± 1 and 28 + 3 deg.0
had comparable body weights (Table 4.5). 0. niloticus yield was highest in
warm water tanks with greater amplitude of temperature variation. It reached
57.29 kg/are/year at 28 + 6 deg.C) but was lowest in tanks at 22 deg.0 where
it ranged from 26.61 to 30.18 kg/are/year (Figure 4.6).
Generally, there was an increase in growth rate with increasing number
of degree-days (Table 4.5). Regression analysis indicated a strong correlation
between the number of degree-days and the fish average weight (r2 = 0.80)
and net yield (r2 = 0.53). Predictive linear models for fish weight and net yield
as functions of the cumulative number of degree-days were, respectively:
W=20.89+0.015(DDYS); r2 = 0.80; n = 66, where: W = average fish weight (g)
and DDYS = cumulative number of degree-days, with 15 deg.0 used as the
zero growth threshold; and Y=7.80+0.020(DDYS); r2 = 0.53; n = 66 where:
Y = net fish yield (kg/are/year) and DDYS = cumulative number of degreedays (Figure 4.7). The regression of total length as a function of cumulative
degree-days was Y=107.7+0.02(DDYS); r2 = 0.95; n = 36, where Y = total
length (mm).
Table 4.5 Mean growth rate (%/day) and average adult body weight (g) of Oreochromis niloticus in tanks at 22
and 28 deg.0 mean fluctuating water temperatures (153-d)
Treatments
A
B
C
D
E
F
Average body weight (g+SD)
At stocking
At harvest
19.81 + 4.76
19.80 + 5.02
18.77 + 4.68
19.74 + 4.92
19.76 + 4.77
19.70 + 4.69
a36.18 + 5.52
a37.05 + 6.35
a38.97 + 6.22
b43.41 + 7.64
b44.15 + 7.65
c51.38 + 9.35
Growth rate (%/d)
Cumulative degree-days (±SD)
0.39a
0.41a
0.48b
0.52b
0.53b
0.63c
a1190 + 59
ab1098 + 73
b 849 + 0
c1733 + 50
c1729 + 8
d2111 + 31
c. Values with different superscripts in the same column are significantly different (p<0.005).
Nominal temperature treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C; D=28+1 deg.C;
E=28+3 deg.C; and F=28+6 deg.C.
a.
83
70
co
57.29
60
sg, 50
41.11
a)
0.40
40.
a)
%" -- 3
-a
0
45.27
28.49 26.61 30.18
5%. 20
ri)
z10
0
A
BCDE F
Temperature regimens
Figure 4.6 Oreochromis niloticus net yield from the 153-d experiment.
ABCDE F*: 0. niloticus net yield from treatment F was significantly
higher than yields from other treatments (p<0.05). Nominal temperature
treatments were: A=22+1 deg.C; B=22+3 deg.C; C=22+6 deg.C;
D=28+1deg.C; E=28+3 deg.C; and F=28+6 deg.C.
84
70
W=20.89+0.015(DDYS) r2=0.80
Y=7.80+0.02(DDYS) r2=0.53
60
70
60
50
50
40
40
30
30
20
20
10
10
0
500
0
A
1000
1500
2000
0
2500
Degree-days
Weight
Net yield
Figure 4.7 Linear relationships between cumulative degree-days and
Oreochromis niloticus body weight (W) and net yield (Y).
85
Discussion
Reproduction
Previous field observations reported that water temperature affected
tilapia reproduction, and therefore, fingerling production (Green, 1997;
Brummett, 1995; Rana, 1982; Jalabert and Zohar, 1982; Philippart and Ruwet,
1982; Balarin and Haller, 1982; Balarin and Hatton, 1979). Tilapias reach
sexual maturity and normally spawn at temperatures above 20-23 deg.0
(Balarin and Haller, 1982; Balarin and Hatton,1979).
In our experiments, reproductive activity was stimulated by an increase
in temperature. In treatments D, E, and F, 0. niloticus fry were observed one
month after stocking; and reproduction in these tanks occurred 2 months
earlier than in treatments A, B, and C (Figure 4.3). At 30-d after stocking, the
female average weight in treatments D, E, and F was 19.7 g and the average
cumulative number of degree-days was 413. At this age, the average
cumulative number of degree-days in treatments A, B, and C was 47.5% lower
than in treatments D, E, and F. However, the average weight of females in
treatments A, B, and C was 19.5 g and not significantly different from that of
fish in treatment D, E, and F. Since other water quality variables were not
significantly different between treatments during the first month of the
experimental period, water temperature level and the resulting number of
degree-days were the only factors that prevented fish in treatments A, B, and
C from reproducing early.
86
In treatments A, B and C, eggs and sac-fry were observed for the first
time 90 days after stocking. At this age, the extrapolated average number of
eggs and sac-fry harvested from these treatments was 57/kg female/day,
compared to 94/kg female/day from treatments D, E, and F. Differences in
temperature fluctuations and in the cumulative number of degree-days
explained the difference in reproduction performance. Experimental tanks
receiving temperature treatments A, B, and C accumulated 628 degree-days,
representing 43.8% less than the number of degree-days recorded in tanks
receiving treatments D, E, and F. The average weight of females from
treatments A, B, and C was 22.8 g, 17.4% smaller than the 27.6-g average
weight of females from treatments D, E, and F. The difference in female
weight reflected the effect of temperature and the corresponding degree-days
on fish growth. The delayed reproduction in treatments A, B, and C was
consistent with field observations and indicated that 0. niloticus size at time of
first reproduction in cooler environments is larger than the fish size at the
onset of reproduction in warmer waters.
At harvest, the average number of seeds (eggs, sac-fry and fry)
produced per kg female per day at 22 deg.0 was 59.1% less than that
produced at 28 deg.C. Within the 22 deg.0 treatments, larger thermal variation
(+ 6 deg.C) and relatively constant temperature (± 1 deg.C) resulted in 285.7%
more seeds/kg female/day compared to the ± 3 deg.0 treatment. This result
indicates that, at 22 deg.0 average temperature, the larger thermal
fluctuations above 20 deg.0 promotes a better compensatory reproductive
87
effect than narrow temperature variations. At 28 deg.0 treatments, ± 3 and ±
6 deg.0 resulted in 204% and 152% more seeds/kg female/day than the + 1
deg.0 treatment, respectively, indicating that thermocycles are more beneficial
than static temperatures. However, narrow variations are better than larger
variations because the water temperature does not rise to the upper tolerance
limit for 0. niloticus or increase further the dampening effect of lower DO
concentrations at higher temperatures.
Various studies indicated that changes in temperature trigger spawning
behavior in teleosts and that water warming stimulates vitellogenesis and the
onset of spawning (Stacey, 1984). Balarin and Haller (1982) reported a
beneficial effect of temperature manipulation by lowering temperatures from
25 to 18 deg.0 for two weeks and then raising it back again. Such temperature
changes resulted in inducing spawning in over 50% of female 0. niloticus. The
experimental results were consistent with the field observations that the
spawning of tilapias in their natural environment depends particularly on
temperature (Macintosh and Little, 1995; Rana, 1988; Jalabert and Zohar,
1982; Philippart and Ruwet, 1982; Balarin and Haller, 1982; Balarin and
Hatton, 1979), and lower temperatures result in reductions or delays in tilapia
spawning (Guerrero, 1986; Galman et al., 1988; Green and TeichertCoddington, 1993; Green and Teichert-Coddington, 1991; Balarin and Haller,
1982). For 0. niloticus, optimal spawning temperatures are 25 to 30 deg.0
(Rana, 1988; Philippart and Ruwet, 1982; Rothbard and Pruginin, 1975).
Since tilapias are multiple spawners, the number of spawnings and of fry
88
yields are temperature-dependent and vary consequently with latitude and
altitude (Philippart and Ruwet, 1982; Hodgkiss and Man, 1978; Siddiqui,
1977).
The number of fry (0.7/female/day) or of total seeds (1.6/female/day)
produced at 28 deg.0 was comparable to the highest range reported in
Rwandan ponds at 1600-1700 m; but fish spawned 3-7 months earlier than in
Rwandan ponds (Rwangano et al., 1989; Hanson et al., 1988). Early
reproduction was attributable to warmer temperatures. Rwangano et al. (1989)
indicated that fingerling production in broodstock ponds at Rwasave ranged
from 0.3-1 fingerling/female/day. Nevertheless, the reproductive performance
in our experiment was low even at 28 deg.0 apparently due to the small size
of the fish. Fish were relatively small and their diet was entirely based on
primary production, which was not limiting given the high chlorophyll a
concentration in all treatments (Table 4.1). Santiago et al. (1985) indicated that
the supply of supplemental dietary protein to 0. niloticus increases seed
production compared to a diet of natural foods only. 0. niloticus relative
fecundity within an individual year class can increase with increasing female
size (Rana,1986), absolute productivity per spawn is greater in older, larger
females (Rana, 1988); and older, larger females are more likely to spawn
during each spawning cycle (Siraj et al., 1983).
Popma and Green (1990) based recommendations for duration of fry
production cycles on water temperature. Green and Teichert-Coddington
(1993) recommended the harvest 0. niloticus fry between 195 to 220
89
cumulative degree-days, corresponding to 14 to 20-d after stocking. The
duration was shorter for periods of warmer water temperatures. The
experimental data set also indicated that the cumulative number of degreedays was an important factor that controls reproduction and that, under the
experimental conditions, the first spawning occurred between 217 and 413
degree-days.
Results indicated that the cumulative number of degree-days can be
used to predict 0. niloticus fry production and that this factor must be
considered when planning fry production. The 38.3-g average weight of
females from treatments E and F was 19.3% higher than that from other
treatments. This partially explained the higher relative fecundity (expressed as
number of seeds per kg female) of females from treatments E and F since the
number of eggs spawned by 0. niloticus brooders of similar age increases
with their size (Rana, 1986), but the higher weight gain was temperaturedependent.
Because of increased reproduction in tanks receiving treatments E and
F, the biomass of 0. niloticus juveniles in these treatments was significantly
greater than that from other treatments. The findings indicated that commercial
production of 0. niloticus fry for the distribution to "grow-out" farmers can only
be efficient in warmer low elevation regions. Under these conditions, the timing
of reproduction in broodstock ponds can be estimated based on temperature
in terms of degree-days. Low temperatures recorded in cooler environments
retard 0. niloticus spawning and prevent overpopulation in grow-out ponds.
90
In the experimental tanks, 0. niloticus gonosomatic indices (GSI) were
significantly lower in treatments E and F because of the high reproductive
activity of fish from these tanks. This relatively high activity resulted in reduced
weight of spent gonads and a lower GSI (Table 4.4).
Somatic growth and production of Oreochromis niloticus (L.)
Spigarelli et al. (1982) and Gui et al. (1989) reported positive effects of
the diel thermocycle on fish growth. In brown trout, thermocyclic regimes (12.5
+ 4.6 deg.C) resulted in significantly greater food consumption and growth rate
compared to fish raised at constant 13 deg.0 or at naturally arrhythmic
temperatures (4-11 deg.C) (Spigarelli et al., 1982). Gui et al. (1989) observed
that energy assimilation and tissue growth rates were significantly higher in 0.
niloticus raised at fluctuating 28 and 30 + 4 deg.0 than in fish at constant
temperatures, but the best performance was recorded at 28 + 4 deg.C.
However, Popma et al. (1994) noted that daily temperature fluctuations (± 4
deg.0 vs. constant) at 26 and 22 deg.0 did not affect growth rate; but appetite
and food conversion efficiency were positively related to average temperatures
and feeding rates. At feeding rates above 1.9% body weight, fish converted
more efficiently and grew faster at 26 deg.0 than at 22 deg.C; but at feeding
rates below 1.9% body weight, 0. niloticus grew faster at 22 deg.C. Popma et
al. (1994) concluded that temperature-related differences in growth rates were
greatest at high feeding rates. In a review of fish responses under fluctuating
and constant temperature at the mean fluctuation, Spigarelli et al. (1982)
91
reported that positive or negative effects of thermocyclic conditions depended
on ration or on the position of the mean of the thermocycle relative to the
optimum metabolic temperature for the species. He indicated that the
enhancement effect of temperature variation may not occur under conditions
of limited food supply or when the central tendency of the fluctuation exceeds
the optimum metabolic temperature. Studies comparing thermocycles with
constant temperatures indicated a greater growth advantage of fluctuating
temperatures with adult brown trout than with juvenile fishes; and showed that
metabolic responses of juvenile fish held under thermocycles are equivalent to
those under constant temperatures falling between the mean and maximum
cycle temperatures (Spigarelli et al., 1982).
Results from this experiment indicated a positive growth effect of a
larger diel thermocycle at both 22 and 28 deg.C; but growth performance and
yields were better at a fluctuating 28 deg.0 compared to 22 deg.C. At 28
deg.C, 0. niloticus was within its optimum thermal range and benefited from
the higher number of degree-days. At a lower average water temperature (22
deg.C), there was a confounding effect between the amplitude of temperature
variation and the number of degree-days. Data from this study indicated that
larger temperature fluctuations (± 6 deg.C) at 22 deg.0 resulted in lower
cumulative number of degree-days but slightly higher fish growth rate, body
weight, and net yield compared to tanks with smaller thermocycle within the
same treatment (Table 4.5; Figure 4.6).
92
Cumulative number of degree-days can be effectively used for
predicting total length (mm) (r=0.98), body weight (g) (r=0.96), and net yield
(kg/are/year) (r=0.94). However, regression analysis of the data set from this
experiment indicated that the mean temperature itself accounted only for 4%
of the total variation in total length or in weight (r2 = 0.04) and 6% of the total
variation in net yield (r2 = 0.058). Therefore, use of cumulative number of
degree-days instead of absolute mean temperature values is more appropriate
for predicting fish total length, body weight, or yield.
Daily growth rates from this experiment ranged from 0.11-0.21 g/day. At
the temperature levels used in this experiment, 0. niloticus could have
reached a better growth performance. High stocking density (3.6 fish/m2) and
lack of supplemental feeding may have limited 0. niloticus performance in the
experimental microcosms where fish relied solely on phytoplankton
production. In addition, ammonia levels ranging from 0.20-0.30 mg/L,
associated with short-term exposure to nocturnal low oxygen concentrations
may have a negative effect on fish growth (Soderberg, 1997; Abdalla et al.,
1992; Abdalla, 1990; Boyd, 1979).
Tilapia growth rates in low intensity tropical ponds stocked at 1 fish/m2
and receiving fertilizers or feeds ranged from 0.293-1.7 g/day and reached
2.26 g/day with reduced stocking density (Soderberg, 1997; Seim et al., 1994;
Rana, 1988; Balarin and Haller, 1979). The net yields attained at 28 deg.0
are comparatively higher than those from Rwandan ponds stocked at 1.5
fish/m2 and fed with rice bran at 10% of body weight per day (Rurangwa et al.,
93
1989). This finding indicated not only the differential positive effect of
temperature, but also a better enhancement of productivity by fertilization with
chicken manure. The net yield at 22 deg.0 was comparable to fish yields in
low intensity but well managed tropical ponds at elevations below 1700 m
(Soderberg, 1997; Seim et al., 1994; Rurangwa et al., 1989).
Growth and production data from this study contradict those from
Popma et al. (1994), but are consistent with the findings from other studies
(Gui et al., 1989; Spigarelli et al. 1982). It appears that differences in the
dynamics of fish response to temperature and to thermocycles are associated
with other factors such as feeding rates, age and size of the fish, stocking
density, and water quality, but too little is known about these interactions,
particularly in tilapias. Under these conditions, comparisons between specific
performance parameters from different studies cannot be justified unless some
size-related adjustments are made or further experiments are conducted.
Future controlled experiments will require a detailed investigation of
temperature-related effects in combination with different feeding rates, fish
size, and stocking densities.
94
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100
CHAPTER 5
Conclusions and Management Strategies
The growth and production performance of Oreochromis niloticus
reared in experimental tanks under conditions of cool fluctuating temperatures
confirmed observations in tropical pond systems at higher elevations or during
the dry season. Inputs of fresh chicken droppings at a rate of 500 kg
DW/kg/ha/week resulted in high phytoplankton production and an abundant
algal mass was observed in tanks, particularly at lower temperatures where
fish feeding activity was limited. Poor growth rates ranging from 0.12 to 0.25
g/day, no reproduction, and net fish yields ranging from 16.2 to 33.0
kg/are/year are consistent with results obtained from tropical ponds subject to
similar environmental conditions.
Large temperature variations contributed to higher growth rates and
bigger fish compared to more thermally stable waters. At cooler temperatures,
large thermal fluctuations benefit fish growth, but water temperatures should
stay above 15 deg.C.
Results indicated that linear relationships exist between the cumulative
number of degree-days and 0. niloticus body weight and net yield. Heat loss
from ponds decreases body weights and yields of 0. niloticus. In cool natural
environments, fish culturists can maintain pond temperatures within the
optimum range for the cultured fish species by appropriate site selection and
101
adequate pond management strategies that minimize heat loss and make
ponds more effective thermal sinks. Management strategies for decreasing
heat loss include: minimizing water losses by seepage or flow-through
drainage because the replacement water from the inlet supply is usually
several degrees colder than pond water, and the construction of shallower
ponds with a 1-2% slope so that the minimum and maximum depth are 0.400.50 m and 1 m, respectively.
Growth and reproductive advantages were also demonstrated under
fluctuating temperatures in warmwater tanks. 0. niloticus grew better near the
optimum temperature (28 deg.C) than near the minimum temperature at
fluctuating 22 deg.C. 0. niloticus reproduction was shown to be related to
water temperature, thermal fluctuations, and cumulative degree-days. The
onset of reproduction is inversely related to mean temperature and directly
related to the cumulative number of degree-days. The first reproduction
occurred in 3-month old females (19.7 g) in warmer tanks at 216-413 degreedays and in 5-month old females (22.8 g) in cooler tanks at 450- 628 degreedays. The relative fecundity also increased with the number of cumulative
degree-days and with the amplitude of temperature variation.
Average reproduction at 28 deg.0 was highest and reached 23.8 fry/kg
female/day (1.89/m2/day) and a total of 50.2 seeds/kg female/day (3.98
seeds/m2/day) while it reached 0.8 fry/kg female/day (0.06/m2/day) and 20.3
seeds/kg female/day (1.44 seeds/m2/day) at 22 deg.C.
102
Extrapolated average fish yield was highest in tanks at 28 deg.0 with
the largest fluctuation. It reached 57.29 kg/are/year at 28 + 6 deg.0 and 45.27
kg/are/year at 28 + 3 deg.C; and reproduction accounted for 3.6% and 4.7% of
the total fish biomass, respectively. Management of production tilapia ponds in
warmer environments will require the control of reproduction or the use of
monosex males to prevent crowding and competition for available resources.
Increased reproduction in warm water ponds leads to the harvest of small size
fish. Therefore, a good management strategy for the control of 0. niloticus
reproduction will contribute to the production of a fish size more for the market.
The experimental results demonstrated that it is possible to simulate
dynamics of tropical fish ponds in laboratory microcosms and evaluate the
effect of variables such as fluctuating temperatures on fish performance.
Simulated pond experiments are expensive, but the controlled experiments
generate data that can be manipulated and analyzed for a better
understanding of pond dynamics and for practical applications in the field. In
areas where temperatures are favorable and where the distribution of good
quality fingerlings to farmers is appropriate, seeds can be produced in properly
managed reproduction tanks.
103
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