COLD TOLERANCE OF GENETICALLY PRODUCED ALL-MALE TILAPIA
HYBRIDS (ORECOCHROMIS AUREUS)
Eran Lahav1 and Zivi Ra’anan2
1
2
Nir David fish Breeding Farm, D.N. Gilboa, Israel
Aquaculture Production Technology (Israel) Ltd., P.O.Box 4330, Jerusalem, Israel
Abstract
The performance of all-male fry progeny of O. aureus was examined under
temperate conditions, in comparison with that of the commercially cultured O.
niloticus x O. aureus all-male hybrids.
In both cases all males progeny were
obtained genetically without use of hormonal sex reversal.
The effects of low temperature was evaluated on three production traits: (a) The
onset of spawning at Spring time; (b) The growth rate during interim seasons as well
as during summer; and (c) The survival in winter temperature, as a function of both
absolute minimal temperatures as well as duration of exposure to that temperature.
It was found that O. aureus begins to reproduce in Spring, at water temperature 3 oC
lower than the cross of O. niloticus x O. aureus, resulting in the availability of fry for
stocking 25 days prior to the appearance of the hybrid fry. When water temperature
is lower than 24oC, fry of O. aureus grow significantly faster than the hybrid fry.
However, this advantage diminishes during the warm summer season, when the
hybrids grow faster. Thus, the relative duration of the cold period determines the
extent of the O. aureus advantage in this respect. Regarding cold resistance, O.
aureus and the hybrid showed a significant advantage over O. niloticus, and only a
slight advantage of O. aureus over the hybrid, which may not be significant.
The implications of these finding to commercial production are discussed.
1
Introduction
Temperature tolerance is a key factor in tilapia production in temperate zones. Most
strains of commercially grown tilapia cease feeding at temperatures lower than 18oC.
Water temperatures below 14o or 12oC (depending on the species and duration of
exposure) can cause mortality (Tave, 1990; Tave et al., 1990).
In tropical regions,
the preferred species for commercial production is Oreochromis niloticus and its
hybrids, due to their high growth rate in tropical climate.
However, tilapia
aquaculture in temperate, and to some extent in sub-tropical zones, such as Israel
and the USA, is characterized by seasonal changes in water temperature. During
winter, water temperature may drop to levels that cause severe growth inhibition and
sometimes, mortality. The gradual increase and decrease of temperature during the
interim seasons of Autumn and Spring, affect the physiology of the fish. Under
these conditions, O. niloticus, which is not very tolerant to cold temperatures (Khater
and Smitherman, 1988) is at a disadvantage, and a species or a hybrid with greater
cold tolerance may significantly improve the profitability of the industry.
Oreochromis aureus is the most cold-resistant species among commercially
produced tilapias in Israel, as well as in other countries (Saris, 1969, Behrends &
Smitherman, 1984).
For this reason, Nir David Fish Breeding Farm, which has
been producing the all-male genetic hybrid of O. niloticus x O. aureus, decided to
examine the production potential of O. aureus. The method proposed by Shelton et
al. (1978) was adopted to established O. aureus broodstock which produces all-male
fry without the need for hormonal sex-reversal (Lahav, 1993). O. aureus males,
which are believed to be homozygous in their sex chromosomes, were sex-reversed
by feeding them with estrogen impregnated fish meal.
Sex-reversed females
(phenotypic females with a male genotype) were identified by mating them with
normal O. aureus males. Pairs which produced 100% male progeny were selected
and the females, once identified, were bred again with males from the same O.
aureus line. The all-male fry produced were treated with estrogen to produce the
female line of the commercial broodstock. Crossing these females with normal O.
aureus males resulted in the commercial production of 98-100% male populations.
The tolerance to low temperature of the O. aureus all-male fry was examined on
three production traits.
2
First, we examined the initiation of reproduction and spawning activity. This trait is
of importance since the earlier the spawning begins in Spring, the longer is the
growth period, until the onset of the following winter. Moreover, early spawning
means more efficient utilization of the spawning facilities, which, otherwise, may stay
idle during the greater part of the year.
The second production trait examined was the growth rate of the O. aureus hybrid, in
comparison with that of the commonly produced O. niloticus x O. aureus hybrids.
Although it is well established that under optimal water temperatures (24 o - 30oC), O.
niloticus and its hybrids grow faster than O. aureus (Wohlfarth and Hulata, 1981), it
was important to compare their growth during the transition period under conditions
of gradual seasonal changes in water temperature (i.e. spring and fall time), as well
as during summer.
The third production trait examined was the sensitivity of the fish to exposure to low
temperatures.
Winter kill of tilapia in temperate zone aquauclture is a serious
problem (Snodgrass, 1991).
Episodes of cold temperature shocks (where the
determining factor is the absolute minimal temperature) which result in tilapia kills
(Stauffer, 1988) are rather rare. More often, cold-tolerance is more than just a direct
function of the absolute lowest temperature tolerated by the fish.
Rather, the
resistance to low temperature is determined primarily by the combination of
temperature and time of exposure, and is affected also by the rate of decrease in
temperature, by the physical condition of the fish in terms of health and nutrition, by
water quality and possibly more.
This aspect is of utmost importance for the tilapia
industry in Israel, since, in most cases, tilapia cannot reach market-size within one
season and must be stored in open ponds over-winter. Under such conditions “winter
kills” occur quite often, causing losses to the industry.
Materials and Methods
a.
Onset of spawning activity
A comparison was made between females of O. niloticus crossed with O. aureus
males, and those of O. aureus phenotypic females and males.
Four spawning
ponds of 40 m3 were allocated for this purpose, two were stocked with 7 females and
2 males of the hybrid parents, and two were stocked with identical number of parents
3
of O. aureus.
Water flow rate was 5 m3/hour, from a nearby spring. The experiment was carried
out from March 27 to April 30, year 1992. During this period, water temperature
increased gradually from 18oC to 23oC. Temperature was measured daily in all
ponds, early in the morning.
Daily observations for early detection of fry were carried out until each pond
produced three spawning cycles (after which, it was impossible to distinguish
between old and new batches of fry).
Fish were fed according to a standard feeding table, corrected according to water
temperature.
b.
Growth rate at different temperature ranges
In this experiment, growth rate of O. aureus was compared to that of O. niloticus x O.
aureus hybrids (the common hybrid commercially produced in Israel).
Both fry
populations were all- males.
The growth trial was carried out in two stages of 40 days each: (a) from March 15 to
April 25, during which temperature increased from 20 o to 23oC degrees, representing
the period of seasonal change; and (b) from May 25 to July 5, during which
temperature increased from 26o to 30oC, representing summer.
Each trial was carried out in three replicates, in six 2 m3 tanks with central drainage.
The temperature of the stream water, pumped through the tanks, changed according
to season.
Each tank was stocked with 30 fish. Whenever a fish was found dead,
another fish of identical size and source replaced it (stand-by populations of both
lines were maintained under similar conditions, for this purpose).
Altogether, 14
fish, randomly distributed among the tanks, had died and were replaced (4% of the
total number of fish).
Fish were weighed upon stocking a, to minimize stress.
Fish were fed 30% protein pellets, according to a standard feeding table. Feed was
manually administered twice per day.
c.
Cold resistance
Cold resistance of three groups of fish was examined, including O. niloticus (males
4
only), O. aureus (all male), and all male F1 hybrids of O. niloticus x O. aureus.
Three replicates were examined for each group. Each replicate included 40 fish, at
an average weight of 100 g. In order to equalize the physical status of the fish
participating in the study, they were all grown together in a large communal tank,
under optimal conditions, for one month prior to the stocking of the trial groups.
The tanks used for this trial are similar to those described above. A recirculating
water system was used.
Water from all tanks drained into a common external
sump, from which it was pumped back into the tanks, using a submersible pump.
Fresh make-up water was added to the common sump as required.
This
arrangement ensured that all tanks were kept at exactly the same temperature.
Gradual temperature drop was allowed to occur naturally, according to the cooling
during the onset of winter. Tanks were aerated by airlifts.
The trial was carried out for 77 days, from November 15 to January 31. During this
period, water temperature naturally dropped from 20 o to 8.5oC. Temperatures were
recorded every morning at sunrise.
Tanks were fed 30% protein pellets, calculated according to 0.5% biomass.
Cleaning was done by flushing water through the central drainage.
accumulating in the common external pit was removed manually.
Sludge
When water
temperature dropped below 17oC, feeding (and cleaning) was stopped altogether.
A fish was considered dead upon loosing balance and sinking to the bottom, lying on
its side. Dead fish were collected and counted daily.
Results
a. Onset of spawning activity
Onset of spawning activity in each pond was determined by the appearance of the
first three batches of fry (each representing a single spawning).
Results are
presented in Figure 1.
It can be clearly seen that the first three batches of fry produced in the two groups of
O. aureus parents were observed earlier (between the 2 nd and 9th of April) than those
of the O. niloticus parents (between the 24th and 30th of April). Onset of spawning of
the O. aureus group occurred at lower temperature, as compared with the O.
5
niloticus x O. aureus group (20.9 and 24.8oC, respectively).
Figure 1.
Onset of spawning in 2 groups of O. aureus and 2 groups of O.
nilticus x O. aureus, as a function of increase in water temperature.
Temp oC
27
25
23
21
Temp C
aureus-1
aureus-2
hybrid-1
hybrid-2
19
17
02/05/92
28/04/92
24/04/92
20/04/92
16/04/92
12/04/92
08/04/92
04/04/92
31/03/92
27/03/92
15
Date
b.
Growth rate at different temperature ranges
In Phase I of the growth trial, fish of both groups (O. aureus and O. niloticus x O.
aureus hybrid) were stocked at an average weight of 149.0 g. Water temperature at
the beginning of the period was 20oC and at the end, 40 days later, 23 oC. All fish
6
were weighed upon harvest. Results of Phase I are summarized in Table I.
Table I. Growth rate at water temperatures ranging between 20-23oC
O. aureus
O. niloticus x O. aureus
Tank #
Av. Wt. (g)
Daily Growth (g)
Tank #
Av. Wt. (g)
Daily Growth (g)
1
191.1
1.05
4
173.1
0.60
2
185.2
0.90
5
177.8
0.72
3
181.8
0.82
6
169.4
0.51
Average
185.9
0.92
Average
173.4
0.61
Results indicate that within the water temperature range examined, O. aureus grew
by an average of 50.8% faster than the hybrid.
In phase II of the growth trial, fish averaging 135 g, were stocked for a 40-day, at
water temperatures ranging from 26 to 30oC. Fish were weighed at the end of the
trial. Results are summarized in Table II.
Table II. Growth rate at water temperatures ranging between 26-30oC
O. aureus
O. niloticus x O. aureus
Tank #
Av. Wt. (g)
Daily Growth (g)
Tank #
Av. Wt. (g)
Daily Growth (g)
1
174.4
1.01
4
179.8
1.12
2
174.2
0.98
5
185.2
1.25
3
185.1
1.25
6
187.4
1.31
Average
178.2
1.08
Average
184.1
1.23
Results indicate that at the higher water temperature range, the hybrids grew, on the
average, 13.9% faster than O. aureus.
c.
Cold resistance
7
As water temperature gradually decreased, fish from the three groups started to die.
Dead fish were counted daily. The rate of mortality is plotted against the drop in
water temperature (Figure 2).
Each curve is the summary of three groups and
represents 120 fish.
The curve shows that mortality in the O. niloticus group started on Day 36, when
water temperature was about 12-13oC. Within the next 13 days, all fish in this group
died. At the same time, water temperature further dropped to 10 oC.
Mortality in the other two groups started 10-11 days later.
Temperature was
10-11oC at the onset of mortality and dropped further to 8.5oC. Temperature was
not allowed to go further down since this is the minimal temperature observed under
Israeli winter conditions, in most areas where tilapia is cultivated. The comparison
between the hybrid and O. aureus show quite similar results, with some advantage to
O. aureus, mainly with respect to resistance period. Whereas mortality in the hybrid
group started on Day 46 and the continued for 17 days, the mortality in the O. aureus
group started on Day 49 and continued for 20 days. This is a slight advantage,
which may not be statistically significant.
8
Figure 2.
The rate of mortality of fish exposed to decreasing water
temperature
Temp oC
# of dead fish
25
20
15
Temp - C
10
Niloticus
Hybrid
Aureus
5
0
1
5
9
13
17
21
25
29
33
37
41
45
49
53
57
61
65
69
73
77
Days
Discussion
The main purpose of this work was to compare between the performance of the
commercial hybrid, O. niloticus x O. aureus, and all-male O. aureus, under temperate
climatic conditions, such as those prevailing in Israel.
Our results show that O. aureus has an advantage over the hybrid, in all aspects
examined.
Early spawning allows prolongation of the growout season, since fry are stocked in
the ponds earlier. Moreover, it increases the efficiency of spawning pond utilization,
which would otherwise stay idle for the interim period between the two main seasons.
In fact, during the last two years, a commercial broodstock capable of producing
all-male O. aureus without hormonal treatment is already used.
Results of
commercial activities demonstrate that O. aureus fry are available for stocking as
early as the beginning of May, approximately 25-30 days before the hybrid fry are
obtained (late May, beginning of June).
9
The cold tolerance which results in faster growth potential during interim periods,
when water temperature is rather low (>24oC), allows the early stocked fry to start
gaining weight and continue to do so while the growth of the hybrid is still inhibited by
the cold water.
However, since the hybrid grows faster during the period when
water temperature is higher than 24oC, the relative advantage of O. aureus depends
on the relative duration of the interim periods and the summer season. Therefore,
this aspect should be evaluated for each case separately, based on site-specific
conditions.
In Nir David commercial ponds, O. aureus fry averaging 1 g, were
stocked on March 15 (fry obtained from greenhouse covered spawning ponds) and
reached an average weight of 440g within 245 days (average daily growth of 1.8
g/day). During the same season, hybrid fry were stocked at an average weight of 1
g on April 4, and grew under similar conditions, to an average weight of 410 g, within
220 days (average daily growth of 1.86 day).
It can be clearly seen that although
the average daily growth rate of the hybrid, over the entire season, was higher, O.
aureus reached a larger size at harvest.
This is due to the prolonged growout
season. In other locations, where the cold season is longer, the relative advantage
of O. aureus may be more pronounced.
Regarding cold resistance, our results correspond with those presented in the
literature (Tave, 1990), showing that O. niloticus is the least tolerant to cold
temperatures, whereas the hybrid is between the two parental lines, closer to the O.
aureus. Although we do not yet have concrete data regarding over-wintering fish in
commercial ponds, the O. aureus stock seems to be in better shape and less
susceptible to various kinds of parasites, which infect the other fish during winter.
10
References
Behrends, and R. O. Smither, 1984. Deof cold-tolerance population of red tilapia
through introgressive hybridization. Journal World Mariculture, 15:272.
Khater, A. A. and R. O. Smitherman, 1988. Cold tolerance and growth of three
strains of Oreochromis niloticus. The 2nd International Symposium on Tilapia
in Aquaculture, Bangkok (Thailand). No. 15 pp. 215-218.
Lahav, E., 1993.
Use of sex-reversed females to produce all-male tilapia
(Oreochromis aureus) fry. The Israeli Journal of Aquaculture - Bamidgeh,
45(3):131-136.
Sarig, S., 1969. Winter storage of tilapia. FAO Fish Culture Bulletin, 2:8-9.
Shelton, W. L., Hopkins, K. D. and G. L. Jensen, 1978.
Use of hormones to
produce monosex tilapia for aquaculture. Pp. 10-33. In: R. O. Smitherman,
W. L. Shelton and J. H. Grover (eds). Culture Exotic Fishes Symp. Proc. Fish
Culture Section, Am. Fish Soc., Auburn, Alabama.
Snodgrass, J. W., 1991. Winter kills of Tilapia melanotheron in coastal Southeast
Florida, 1989. Florida Science, 54(2):85-86.
Starling, S. M., Bruckler, R. M., Strawn, R. K. and W. H. Neill, 1994. Predicting the
lethality of fluctuating low temperature to blue tilapia.
Trans-American
Fishery Society, 124(1):112-117.
Stauffer, J. R., Boltz, S. E., and J. M. Boltz, 1988. Cold shock susceptibility of blue
tilapia from the Susquehanna River, Pennsylvania. North American Journal
of Fishery Management, 8(3):329-332.
Tave, D., 1990. Cold tolerance in tilapia. Aquaculture Magazine, 4:86.
Tave D., Jayaprakas V. and R. O. Smitherman.
1990.
Effects of intraspecific
hybridization in Tilapia nilotica on survival under ambient winter temperature in
Alabama. J. World Aquaculture Society, 21:201-204.
Wohlfarth, G. W. and G. Hulata, 1981.
Applied Genetics of Tilapias.
ICLARM
Studies and Reviews 6. International Center for Living Aquatic Resources
Management, Makati, Metro Manila. 26 pp.
11