Cage culture of freshwater fishes in reservoirs
Training manual for Kattakaduwa fishing community
By
N.P.P.Liyanage S.M.Ruwanpathirana
Dr. S. C. Jayamanne
Acknowledgements
Cage culture of freshwater fish in reservoir
This booklet was produced for the benefit of the fishing community
of Kattakaduwa Fisheries Society who are participating in the
production of fish in collaboration with NARA and NAQDA under
AECI sponsored AIDA Project. The author wishes to acknowledge
the financial support given by AIDA and AECI to fulfill this
requirement.
The author also wishes to acknowledge the efforts of Hon. Felix
Perera, the Minister of Fisheries and Aquatic Resources, Mr. Javier
Gila, the Director of AIDA, Ms. Priyadarshini Jayawardena, the
consul of Spain in Sri Lanka and Mr. K.Haputantri, the Chairman of
NARA. Dr. Elmo Weerakoon Director General of NAQDA who
geared this project in to reality. Encouragement and moral support
given by Ms. Avelina Lopez, the consultant of the project and Mr.
Ventura Rodriguez, the coordinator of the project are also highly
appreciated. The author also express his deepest gratitude to Mr.
Vidath Dharmadasa, Senior Aquaculturist of NAQDA for providing
photograph for cover page.
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Cage culture of freshwater fish in reservoir
Table of contents
1. Introduction
What is Cage Culture?
Cage culture in Sri Lanka
Advantages and Disadvantages
2. Site Selection
Site criteria
3. Environmental Issues
Water Quality
Carrying Capacity
5. Culture practices
Culture species
Stocking
In food fish culture cages
In fry to fingerling raring cages
Feeding
Feeding method
6. Problems
Fish are difficult to observe in cages
Stress comes from the water body
Human errors
Bio-fouling
Accumulations of waste
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Cage culture of freshwater fish in reservoir
1. Introduction
What is Cage Culture?
Cage culture is an aquaculture production system where fish are held
in floating net pens. Cage culture of fish utilizes existing water
resources but encloses the fish in a cage or basket which allows water
to pass freely between the fish and the pond permitting water
exchange and waste removal into the surrounding water.
The origins of cage culture are a little vague. It can be assumed that
at the beginning fishermen may have used the cages as holding
structures to store the captured fish until they are sent to the market.
The first cages which were used for producing fish were developed
in Southeast Asia around the end of the 19th century. Wood or
bamboos were used to construct these ancient cages and the fish were
fed by trash fish and food scraps. In 1950s modern cage culture
began with the initiation of production of synthetic materials for cage
construction.
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Cage culture of freshwater fish in reservoir
Fish production in cages became highly popular among the small or
limited resource farmers who are looking for alternatives to
traditional agricultural crops. Cage culture is advantageous for
farmers as it offers the farmer a chance to utilize existing water
resources. Therefore, the farmers do not have to invest on
accumulating water.
Cage culture in Sri Lanka
Sri Lanka is a small continental tropical island situated southeast of
the Indian peninsula which has lot of man made reservoirs. The
density of reservoirs (230 ha for every 100 km2 of land area) in Sri
Lanka is the highest in the world. In number, there are 10,245 man
made reservoirs which covers 153,811 ha. Therefore Sri Lanka can
be regarded as most suitable place for cage culture. But in earlier
times cage culture was not popular in Sri Lanka. Cages were installed
only in few selected reservoirs such as Chandrika wewa and
Kattakaduwa wewa. Now cage culture is becoming popular in Sri
Lanka due to the involvement of NAQDA and NARA.
In Sri Lanka there are two major types of culture methods employed
in cage culture.
Food fish culture
Here the fish are reared from fingerling or advanced fingerling stage
up to marketable size.
Fry to Fingerling rearing
In this method fry are reared in cages up to fingerling stage.
Thereafter, they are transferred either to rearing cages or to reservoirs
or seasonal tanks.
Advantages and Disadvantages
As with any production scheme, cage culture of fish has advantages
and disadvantages that should be considered carefully before
choosing it as the production model.
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Cage culture of freshwater fish in reservoir
Advantages
 Cage culture has some distinctive advantages compared to other
fish culture methods which include:
 Many types of water resources can be used, including lakes,
reservoirs, ponds, streams and rivers
 Low capital cost is required (Can be installed in an existing body
of water)
 Harvesting is simple.
 Observation and sampling of fish is simple and therefore only
minimum supervision is needed
 Easier handling, inventory and harvesting of fish
 Better control of fish population
 Efficient control of fish competitors and predators
 Effective use of fish feeds
 Reduced mortality
 High stocking rate
 Total harvesting and swift or immediate return of investment
 Less manpower requirement
Disadvantages
 Stock is vulnerable to external water quality problems eg. Algal
blooms, low oxygen levels
 Growth rates are significantly influenced by ambient water
temperatures
 Feed must be nutritionally complete and kept fresh
 Low Dissolved Oxygen Syndrome (LODOS) is an ever-present
problem and may require mechanical aeration
 The incidence of disease can be high and diseases may spread
rapidly
 Vandalism or poaching is a potential problem
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Cage culture of freshwater fish in reservoir
2. Site Selection
Site selection is the most important part of the cage culture. Proper
selection of a site will reduce most of the problems arise with cage
culture and operational cost.
Site criteria
Different types of sites may be adapted to cage culture. Lakes,
reservoirs, ponds, quarries, rivers and streams can be used as
potential sites provided that the water quality is suitable and there is
adequate water depth beneath the cages to allow water movement.
The depth should be sufficient to keep the nets clear of the sediment
and allow water exchange beneath the nets. The cage units should be
built to withstand prevailing wind and wave conditions at the
selected site. Good water exchange is also important in cage culture
to replenish oxygen and flush away wastes.
Kattakaduwa , Mahawewa Tank
Before attempting cage culture in an existing water body, the
following criteria should be considered. The surface area should be at
least one half acre and preferably an acre1 or larger (but should not
include weed infested areas).
1
One acre is 4.046,9 m2
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Cage culture of freshwater fish in reservoir
Should be at least 6 feet2 deep over a considerable area.
Good water quality.
Should not have direct access by livestock or large numbers of
livestock in the watershed.
Watershed should not be a highly erodable one or one that allows the
accumulation of large amounts of organic debris.
The water level should not fluctuate greatly (2 to 3 feet)
Problems with aquatic weeds, overpopulations of wild fish, surface
scums or oxygen depletion problems should not be present.
Weather and shelter - They can impact on both the cage structure and
enclosed fish.
Should have an all-weather access road.
3. Environmental Issues
The success of cage culture depends on maintaining good water
quality around the fish cages and so it is in the farmer’s best interests
to minimize environmental impacts. Size and intensity of the process
should fit to the size of the water body and water exchange rate. It
may facilitate to overcome adverse impacts on water and sediment
quality.
Some of the environmental issues related to cage culture are:
Nutrient enrichment of waters that may lead to increased algal
growth and downstream impacts
Sediment accumulation which can lead to a deterioration in sediment
and water quality
Interactions with wild fish populations
Accumulation of uneaten feed and fish excreta under the cage can
become an environmental problem, but this can be avoided by
2
One feet is 0,3 m
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Cage culture of freshwater fish in reservoir
selecting a site with good water exchange to install the cage.
Generally, the environmental impacts of cage culture can be
minimized through:
Proper site selection
Appropriate anchoring or mooring systems
Using extruded feed or stable pellet diets to avoid leaching of
nutrients
Keeping feed wastage in low level and higher Food Conversion
Ratios
4. Water Quality
Dissolved oxygen - Dissolved Oxygen (DO) concentration and its
availability are critical to the health and survival of caged fish. In
general, warm water species such as catfish and tilapia need a
dissolved oxygen concentration of 4 mg/l DO (or ppm) or greater to
maintain good health and feed conversion. Dissolved Oxygen levels
below 3 mg/l can stress fish. If this level goes below 2 mg/l can
increase the mortality of fish.
Temperature – The most important physical factor controlling the life
of a cold-blooded animal like fish is temperature. It is critical in
growth, reproduction and sometimes survival. Each species of fish
has an optimum temperature range for growth, as well as upper and
lower lethal temperatures.
pH- Uptake and release of CO2 during photosynthesis and
respiration affect on pH in a pond and due to this, it fluctuates daily.
The lowest level of pH appears at or near dawn whilst the highest is
at mid-afternoon. The desirable range of early morning pH for fish
production is from 6.5 to 9. Acid death point of the fish is
approximately pH 4 and the alkaline death point is approximately pH
11. Slowed growth of fish, reduced reproduction, and susceptibility
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Cage culture of freshwater fish in reservoir
to disease increasing can be caused if the pH is not at the optimum
range.
Other - Turbidity, Nitrate and Phosphate levels, COD, Alkalinity,
and Salinity are also affecting on fish culture in cage.
Carrying Capacity
Carrying capacity of the water body which contains the cages is a
very important factor. If we over exploit the water body by exceeding
its carrying capacity it will affect on fish growth, survival and will
increase the pollution which impact badly on caged fish.
5. Culture practices
Cages are used to culture several types of shell fish and fin fish
species in fresh, brackish and marine waters. Cages in freshwaters
are used for food fish culture and for fry to fingerling rearing.
Placement of cages
Location of the cage in the water body is a key factor to the success.
Factors to consider for cage placement:
Water body must have at least 1/2 acre or more in surface area
There should be at least 1 feet to 2 feet of water column below the
bottom of the cage to allow waste materials to be flushed away from
the cage.
Locate cage in water body where the maximum natural circulation of
water presents to provide overall mixing and aeration. This will help
to circulate the freshwater through the cage continuously. It will
provide natural feed and remove the waste away from the cage.
It should be placed away from the coves, weed beds and over
heading trees. This can reduce wind circulation.
In a place convenient for feeding and inspection
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Cage culture of freshwater fish in reservoir
In a place with good water quality where runoff is not contaminated
with high levels of pesticides or large amounts of livestock wastes.
Away from frequent disturbance from people and/or animals. This
may increase the stress.
An appropriate distance should be kept between cages otherwise it
may reduce Dissolve Oxygen level and cause adverse effects to the
caged fish.
The arrangement of the cages is not a problem if there are only few
of these. However, 8 or more should be arranged depending upon the
direction of the wind. On the other hand its better if the longest side
of the cage is oriented perpendicular to the direction of the wind.
Culture species
Big head carp
Catla catla
Red Tilapia
Common carp
Many of the fish species are suitable for culture in cages. Most
commonly in Sri Lanka tilapia and carps are cultured in cages up to
“table fish” level and fry to fingerling rearing also become
successful. There are also some other species such as catfish, trout,
striped bass, red drum, bluegill sunfish reared in cages in southeast
Asian countries.
The desired species characteristics for cage culture are:
Fast growth rate, in regional environmental conditions,
Tolerance for crowded conditions.
Good market value.
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Cage culture of freshwater fish in reservoir
Stocking
Fish stocking in cages can be slightly different from species to
species and also depends on the preferred stocking density and the
size of the seeds stock. On the other hand high stocking density
requires feed in adequate quantity and quality to promote and
guarantee fish growth.
Before stocking it is very important to check the condition of the
seed that is going to be stocked in the cage. These seeds must be free
from diseases and should have the recommended size, according to
the cage mesh size.
Stocking in food fish culture cages
The recommendation is to stock graded 6 to 8 inch fingerlings, which
ensure best production of marketable sized fish at the end. For carps,
Tilapia, and catfish, the recommended minimum stocking density is
80 fish/m3, higher densities are possible in some cases. For
beginning farmers, number of fish that will collectively weigh 150
kg/m3 when the fish reach a predetermined harvest size (Schmittou,
1991) can be recommended as maximum stock density.
Recommended smallest fingerling size is 15 g. A 15 g fish could not
escape from a 13-mm bar mesh net. Advance fingerlings can also be
stocked into cages.
Stocking in fry to fingerling rearing cages
Recommended stocking density for cages located in tanks are 225 285 fry/ cm3 and for the cages in the ponds are 114 fry/ cm3.
Recommended size of the fry is 2.5 – 3.0 cm for tilapia and carp
species.
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Cage culture of freshwater fish in reservoir
Survival
Survival rates in well-placed and well-managed cages are typically
98 to 100 %.
Feeding
Feeding is the most important management practice that a fish farmer
does each day. Simply stated, no feed will mean no growth. Without
growth there will be no profit. On the other hand improper feeding
can be adversely affected to the culture species.
Feed should match to the feeding habit of the fish and it should
prefer the artificial feed that we provide from the outside. Quality of
the feed is very important because most of the times the fish will not
receive natural food. Therefore feed must be a nutritionally balanced
diet which has adequate protein and energy levels, is balanced in
amino acids, and in essential fatty acids, and is supplemented with a
complete array of vitamins and minerals.
There are many formulae for determining the proper amount of food
to give cage raised fish. In fact, feed may comprise over 50% of
your variable costs. Therefore, most of the times farmers prefer to go
for a low cost feed. Usually Sri Lankan farmers use low cost feed
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Cage culture of freshwater fish in reservoir
made from rice bran, oil cake and powdered dried fish. Adequate
supplies of good quality feed must be available.
Feeding methods
Fresh food.
Respective feed types are mixed with hot water and prepared as
dough. Feed dough is provided using feeding trays and the daily ratio
is adjusted depending of fish consumption.
Dry food (pellet)
Feeding fish twice per day at the adequate rate depending on water
temperature, species, size, culture and density is recommended.
Feeding ratio can be determined through the sampling that was
carried out periodically or applying standard growth tables. Dry food
consumption has also to be monitored through feeding trays and the
daily ratio has to be adjusted depending on the fish consumption.
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Cage culture of freshwater fish in reservoir
Maintenance
Algae or bryozoa, a grey jelly-like mass, may sometimes accumulate
on the cage. It should be raked off with a stick or broom periodically
to ensure adequate water circulation through the cage.
Cage floating structures may have to be adjusted or replaced during
the growing season. Also the cage door must be regularly inspected
to make sure it is tightly closed. A strong wind can blow the lid open
and allow fish to escape.
Cleaning cages
6. Problems
Fish are difficult to observe in cages. Sampling to observe may stress
the fish and led to secondary infections. Therefore, the observation
during the feeding, when the fish come up to eat at the
water surface, is critical. This day to day observation is essential to
keep the healthy fish culture and increase the harvest.
Stress comes from the water body. Accumulation of the feed, organic
materials, livestock waste and pesticides in the water body may be
harmful to the fish. These factors change the optimum water
condition and affect on the caged fish. These things can be avoided
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Cage culture of freshwater fish in reservoir
by doing an appropriate site selection and a proper maintenance of
the cage.
Human errors. Poor cage construction, stocking poor quality
fingerlings, high or low density stocking, poor quality of feed,
improper feeding methods, ignoring regular monitoring of fish, poor
handling of the fish, improper site selection and inadequate cage
maintenance can be considered as human errors.
Bio-fouling. It is very common in cage culture. Biofouling is caused
by organisms that attach themselves to the structure or to the net cage
and restrict water exchange. Biofouling can be reduced by cleaning
cages at the right time and applying anti bio-fouling paint on the
cage.
Accumulations of waste – This can happen mainly due to the uneaten
feed. Waste accumulation can control by the following practices:
To facilitate water exchange, use a mesh size as large as possible and
select the areas where gentle breeze action can circulate water
through the cage.
Ideal exchange rate is one cage volume per every 30 to 60 seconds.
Place the broad side of a cage into the prevailing wind to aid water
exchange
Bottom of the cage should keep at least 50 cm above from the bottom
of the water body.
Feed only as much as the fish will consume within 15 minutes. If fish
do not consume the feed in 15 minutes or stop feeding, reduce or stop
feeding
until
fish
respond
willingly
to
the
feed.
GOOD CULTURE PRACTICES
Stock fry or fingerlings in a cage with appropriate size for the mesh
to avoid fish escapes
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Cage culture of freshwater fish in reservoir
Clean the nets regularly
Review the cage structure regularly
Monitor health status of the fish visually and send samples to
laboratory if a problem is detected
Adjust the quantity of feed according to consumption after
monitoring through feeding trays
Be aware of the, salinity, pH and temperature changes of the
waterbody
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Cage culture of freshwater fish in reservoir
NARA-AIDA Project
2006-2009
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