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Seed production
Seed of pearlspot is available throughout the year along the east and south-west
coasts of India. The peak season of abundance is during the months of May-July and
November-February. It can be easily collected from both the brackishwater and freshwater
tanks and ponds. A simple method of seed collection is adopted taking advantage
of the tendency of the fish to congregate in large numbers for feeding on epiphytic
growth. In this method twigs or branches are kept submerged in the water a week
ahead of day of collection. The juveniles congregating for feeding purpose are trapped
using an encircling net or trap. Fecundity of pearl spot is low and has been estimated
to be around 3000-6000; hence a successful hatchery production of seeds is difficult.
However, Central Institute of Brackishwater Aquaculture (CIBA), Chennai using the
technique of environmental manipulation, has successfully demonstrated the hatchery
seed production of pearl spot.
Pond Preparation
Before letting in water, the ponds are drained and lime is applied at the rate of
300 kg/ha. In undrainable ponds, piscicide (Mohua oil cake @ 200-250 ppm) may be
used to eliminate the weed fishes etc. After a time gap of 10-15 days for the neutralization
of the residual effect of the piscicide, water is let in through screens to avoid
the entry of undesirable fishes. The pond is filled up to the appropriate level
(1.2 m) and cow dung applied at the rate of 1500-2000 kg/ha for promoting plankton
production.
Acclimatization and stocking of breeders
Adult Etroplus in the weight range of 50-125 g procured from the wild or
culture ponds are stocked @ 5000/ha after one week of fertilization of the pond.
Breeders collected from the wild are to be disinfected by dipping in 1% commercial
formalin and acclimatised before introducing into the pond. Since the fish is monomorphic,
sex differentiation is difficult and it has to be assumed that the sex ratio of
the stocked breeders is approximately 1:1. Additional breeders have to be added
to the existing stock from the second year onwards to compensate the natural mortality
of breeders. Breeders once stocked will be normally viable for three years.
Provision of spawning surfaces
In the natural environment the fish attaches its eggs to submerged substrata like
stones, aquatic plants etc. As a prepared pond may not have such natural spawning
surfaces, materials like palmyrah leaves tied in bunches to fixed poles, coconut
leaf petioles, coconut husks, bricks, pieces of asbestos sheets etc., have to be
provided in the ponds.
Water quality monitoring and management
Maintenance of optimum water quality is important for the successful breeding of
Etroplus. Water quality parameters like salinity (15-30 ppt), dissolved
oxygen (>3.5 ppm), pH (7-8), temperature (24-32°C), transparency (>50
cm) and ammonia (<1 ppm) have to be maintained. Optimum water level in the pond
is 1.2 m. The loss of water due to seepage and evaporation is to be compensated
by pumping in water. Exchange of pond water through sluice or any other means is
not desirable, as it will lead to escape of hatchlings and fry. Salinity should
not be allowed to go beyond 30 ppt. The fishes should not be disturbed frequently.
Feeding
Feeding of the breeders has to be initiated within 3-4 days after stocking. Artificial
feed prepared with groundnut oil cake 40%, rice bran 45% and fish meal 15%, fortified
with vitamin and mineral mix @ 2.5 kg per 100 kg feed, is to be supplied daily @
3-5% of the fish biomass, either in pelleted or in dough form. Feed can be supplied
in feeding trays kept at the bottom of the pond. The feeding trays should be examined
daily and cleaned outside the pond. The quantity of the feed can be reduced whenever
left-over feed is present in the trays, to avoid wastage and water pollution. The
presence of hatchlings indicates that the pond is to be manured with cow dung @
500 kg/ha for the production of plankton, which forms the food for the hatchlings.
Small quantities of the artificial feed (250-300 g/pond of 1000 m2) also
can be broadcast in powder form during early morning.
Fry production and harvesting
The experiment conducted in a pond of 100 m2 area gave an actual production
of 3500 fry from five sets of spawnings in a year (projected production rate is
3.5 lakhs/ha/yr) when the breeders were stocked at the rate of 6800/ha. Fry production
from a pond of 300 m2 area from 3 sets of spawnings over a period of
5 months was 9600 and this works out to more than 5 lakhs/ha/year from five sets
of spawnings, when the breeders were stocked at the rate of 5400/ha. This probably
indicates that the rate of fry production may be enhanced from a bigger pond even
at a low stocking density of breeders.
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