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| Fisheries :: Culture Fisheries |  |
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Pearl Spot
The Pearl spot, Etroplus suratensis commonly known as “Karimeen” in Kerala is an indigenous fish extensively found along the east and south-west coasts of Peninsular India. It is an important candidate species for aquaculture in ponds in both brackishwater and freshwater environments. It is cultured in traditional ponds in Kerala where it is considered a delicacy fetching a high price up to Rs. 150/ kg. Though growth is slow, at a high stocking density table-size fish can be harvested in 9-12 months culture period.
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. The eggs are oblong in shape, about 1 to 2 mm in diameter, attached at one end by means of a short stalk to the nesting object. The newly laid eggs are yellowish in colour and as the embryo develops, the colour becomes brownish and the yolk sac becomes pigmented. The incubation period lasts from 82 to 100 hours. During hatching, the egg membrane bursts first over the head of the larvae, which is at the free end, and this continues along the upper side by the waving of the tail. The early larval stage lasts for 7 days during when the larvae develops into a free-swimming individual. During late larval stagethe larvae, though free swimming, are quite different from the adult. The tail remains long and the caudal fin is continuous with dorsal and anal. After a fortnight, the primary chromatophores on the back disappear and permanent colour bands begin to appear. The larvae assume adult form within a month after hatching and measure about 18mm. The young ones feed almost exclusively on zooplankton, the advanced fry on aquatic insect larvae, filamentous algae and other vegetable matter, while the adult fish subsists mainly on filamentous algae, aquatic macrovegetation and planktonic organisms. Worms, shrimps and insect larvae also form part of its food. Adult pearl spot can be fed with pelleted fish feeds. The pearlspot is suitable for culture in confined, fresh and brackishwaters. The fish is cultured on a small scale mainly in the state of Kerala. It is cultured in the traditional manner, in the 'Pokkali' fields (paddy fields). An annual yield of 3 to 5 tonnes is obtained from these fields, of which, prawns constitute 80%, while the mullets and pearlspot form 20%. In mixed-culture operations along with prawns and other fishes ranged from 768.2 kg/ha/3 months at a stocking density of 25,200/ha (24000 prawns + 1200 fish) to 845.4 kg/ha/110 days at a stocking density of 20,300/ha (20,000 prawns + 300 fish) have been reported. The culture of pearlspot is more economical under polyculture system especially with milkfish and mullets than under monoculture. The fish can attain a marketable size of 120-150 g over a period of 8-10 months. Though growth rate is relatively slow, high stocking density with low input management can yield optimum production. Under monoculture at stocking densities ranging from 20,000 to 30,000 / ha, an average production of 1,000 kg/ha/year can be obtained in brackishwater ponds. The fish can also be reared in the backyard ponds and tanks in the rural areas. Being a herbivorous fish it is suitable for polyculture. Pearlspot farming could be adopted to any scale integrating with other occupations like poultry farming. The poultry droppings form good manure for natural food production in the culture ponds. Adult fish in the weight range of 50-125 g are stocked in ponds @ 5,000 nos./ha. The fish are fed with supplementary feed @ 3.5% of the body weight (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 of feed). The feed is supplied in pellet or dough form. The hydrographical parameters desirable for the breeding and seed production of pearl spot are: water temperature 24-32°C, salinity 15-30 ppt, dissolved oxygen > 3.5 ppm, pH 7.0 to 8.0 and transparency > 50 cm. To facilitate egg attachment, the pond is provided with substrate materials such as palmyrah leaves, coconut leaf petioles, coconut husks, wooden twigs, bricks etc. Breeding occurs within 30-40 days of introduction of the brooders. A production of upto 6 lakhs fry/ha/year can be achieved. Harvesting is usually undertaken by draining the water from the ponds and operating a seine net, cast net or a drag net for capturing the fish. Pearl spot is prone to many diseases mainly caused by wide fluctuations in environmental parameters. The most common disease causing agents are bacteria including Pseudomonas, Alcaligenes, Flavobacteria, Moraxella, Vibrio and gram-positive Micrococci, Athrobacter and Bacillus spp. Bacterial diseases of E. suratensis
(Source:www.vuatkerala.org) |
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