Production of Pearlspot

The pearlspot ( Etroplus suratensis) is a high valued food fish endemic to peninsular India and Sri Lanka. It has an elevated laterally compressed body and a small cleft mouth. In the natural habitat, the fish is light green in colour with eight vertical bands. Scales are slightly ctenoid. Most of the scales above the lateral line have a pearly white spot and some irregular black spots are seen on the abdominal scales. The species was declared as the 'State fish of Kerala' in the year 2010. It is considered as one of the potential candidate species for aquaculture, because of its high market demand, hardy nature, non-predatory habits and ability to breed naturally in confined waters.

Pearlspot is a euryhaline fish, thriving well in brackish waters and has the ability to live both in fresh and saline waters. It is an omnivorous detritus feeder which feeds mainly on plankton, small worms, prawns and algae. Periphyton dominated with spirogyra is its favourite food. Pearlspot usually attains sexual maturity during first year. Sexes are separate, but can accurately be identified only during the breeding season. It has an asynchronous type of ovary which indicates its continuous spawning habit. It breeds throughout the year with two peaks during the monsoons.

Seed Production

Breeding behaviour of pearlspot involves pairing, courtship, chasing, nest making and parental care. Induced breeding through hormonal manipulation has not been successful, because of its complexity. The breeding protocol has been developed by RARS Kumarakam and the commercial production was first achieved at state Government hatchery at Azhikode.

Broodstock management

Usually earthen pond is used for developing broodstock; which is prepared by strengthening of bunds, eradicating aquatic plants and unwanted organisms and liming as explained for the major carps. The pond is manured with cow dung at 1250 kg/ha for plankton production and a water transparency of 40-50 cm is maintained which plays a major role in pair formation. The pond is stocked with adult fish at a density of 2500 no./ha and fed with a formulated feed consisting of 45% rice bran, 40% groundnut oil cake, 15% fish meal fortified with vitamin E and mineral mix. Daily feeding is done at 3-5% of the body weight either in pellet or in dough form in feeding trays as two rations (morning and evening). The trays are taken out and cleaned every day. Excess feeding should be avoided to prevent water pollution.

The male and female fish with specific attributes get closer to form a spawning pair. The most prominent indication of a pre-mating pair formation is the clear intensification and darkening of bands in male. The male broodfish is also characterized by a bluish-green irridescence and sparkling pearly white spots. The rayed/pointed portions of the dorsal and anal fins turn a little red. The female is generally small with black spots on the ventral side between pelvic and anal fin. The genital papilla of female is oval and swollen with a blunt tip whereas in male, it is thin. The pairing of broodfish happens within 30 days. A success rate of 60% can be expected in forming breeding pairs.

Spawning

Either the same earthen broodstock pond or separate tank (cement tank of 4 x 2 x 1 m or FRP tank of 1 t capacity) is used for spawning. If tank is used, one third area of the tank bottom is filled with sand after demarcation with a single line of bricks. Then, egg attaching substrates like tiles, bamboo pieces, PVC pipes and cement bricks are placed in the sandy area of the tank/pond. In the tank/pond, the brood fish weighing 150-200 g in the sex ratio of 1:1 are stocked for spawning at a density of one pair per 1-2 m². The fish is fed at 2-3% of the body weight. In tank, excess feed and organic waste are siphoned-out daily and a partial water exchange of 20-30% is done twice in a week.

The breeding pair swims along the sides in search of substrates for the attachment of eggs. When they find an appropriate substratum, both male and female partners engage actively in nest preparation. They clean-off the substrates by browsing the algal growth by rhythmic jaw movements. Selection of the nesting site and cleaning of spawning surface are often completed within a week and the paired fish are found to form an isolated territory.

The male fish begins to excite the female by hitting on the vent and nibbling on the abdomen and the pair swims around the chosen substratum. During ovulation, the female lays flat on the spawning site and gently moves from end to end and begins to attach eggs carefully with the help of its ovipositor. The female fish attaches its sticky eggs by pressing closely on the nest surface, one by one in a single layer, supported by its pelvic fins. After releasing eggs, the male fish which follows close behind the female, sprays milt over the eggs in a quick movement and fertilise them instantly. The process of egg laying and fertilisation is continued several times and usually completed in 1-2 hours. The fertilised eggs are yellowish in colour and oblong in shape with 2 mm diameter. They are placed closely in a chain, not touching each other. When the embryo develops, the yolk sac become pigmented and turns brownish. Fecundity is 500-1500 no./fish.

Incubation

The female remains always close to the eggs while the male guards the territory to prevent the entry of intruders. Simultaneously, both of them together make pits of 5-10 cm diameter and 2-4 cm depth near the substratum. The eggs hatch out in 72-80 hours. The hatchling has only limited power of locomotion and are immediately picked up by the female with its mouth and placed in the pits. Both the parents guard the pits together. The hatchlings remain in pits for 5-7 days and on development of fins, they start moving-out of the nest under the watchful eyes of the parents. The young-ones of one pair move in a group and do not mix-up with other. The pairs will be breeding at regular intervals and many groups of young-ones can be seen at a time. Under natural condition, the parental care lasts till the young ones attain about 2-3 cm size.

Nursery rearing

When young-ones show active movement in tank (usually after 10- 20 days of hatching), they are collected using 300 um harvest net, while in earthen pond the young-ones are allowed to be with parents till it attains 2 cm size. The collected young-ones are stocked at a density of 1000 no./m² in cement rearing tank (8 t) or FRP tank (1 t). The young- ones are initially fed twice daily with small zooplankton or freshly hatched Artemia nauplii at 20-30 no./larva. Spirulina at 1g/tank is added to enhance the production of Brachionus, Moina and Daphnia. Shrimp larval feed with 100 qm size is also used for feeding. After 15 days of hatching, the fry accepts feed of 200 um size and is subsequently weaned on 500 pm feed in 25 dph.

It grows to a size of 2 cm in 30 dph. The fry with 2 cm size is thinned to a density of 500 no./m² and fed with micro particulate diet of 500-800 qm having 40% protein. The seed attains 4-6 cm size in 60 days and is collected using net trap, scoop net or dip net without disturbing the pond bottom.

Packing & transportation

Pearlspot is euryhaline, and can be easily acclimatised to the salinity of the waterbody to be stocked. Acclimatisation time of 30 minutes is required for adjusting salinity difference of each 5 ppt. The other aspects of packing and transportation are similar to that of the major carps.

Conditioning of seed

Farming in Cage

Site selection

Cage farming can be done in open brackish waters such as estuary, backwaters and lakes. The site should have moderate tidal water flow (0.25 m/s) and protected from strong wind and rough weather. The site should be free from navigation channel, dredging, rotting of coconut husk, fouling of aquatic weeds, pollution from industries, domestic waste etc. Bottom should be sandy or sandy clayey. The site should have a minimum water depth of 2 m for fixed cages and 4 m for floating cages during low tide. Transparency of water body shall be more than 30 cm with an optimum pH level in between 7.5 and 8.5.

Cage design

In deep brackishwater area having minimum 4 m depth, floating cages can be established having 460 x 360 cm outer frame and 400 x 300 cm inner frame with salt resistant GI pipes of l-1.5 inch diameter and 1.5 mm thickness and held together with connecting rods and clamps. A hand rail of 75 cm height is provided with the inner frame and one more connecting rod at the middle parallel to lengthwise sides. One outer frame can hold two outer HDPE cages having 360 x 230 x 280 cm size and two inner HDPE cage netting of 300 x 200 x 325 cm size are hanged from the top of the handrail and the connecting rod in between at the middle of lengthwise side. Eventhough the depth of inner cage netting is 3.25 m, effective water depth is limited to 2.5 m. The other details on cage fabrication and mooring system are same as that explained for the Nile tilapia. Ballasting with GI rod of 280 x 180 cm having 1.5-2 cm thickness is done to ensure cuboid shape of the cage.

In shallow brackish water area having less than 4 m depth and locations close to shore, fixed square PVC cage having a size of 2 x 2 x 1.5 m size is made by PVC pipes of 1.5-2 inch diameter held together by PVC joints. The PVC frame at the top is hollow and that at the bottom is kept open. The sides and bottom of the cage is wrapped around with inner HDPE netting. Top of the cage is also wrapped with the same netting material with special provision to facilitate opening and closing of the cage. The inner cage net is then covered with an outer HDPE net. The cage unit is fixed conveniently in the pre-determined location with good quality bamboo poles having 3-4 inch diameter by using synthetic ropes. In between the cages, additional ramp is provided to facilitate easy access for handling, feeding, grading, cleaning and monitoring. In areas having the attack of animals like otter and rat, an extra protection is given for the entire cage unit by using bamboo poles and net. Care should be taken to ensure that the cage has a minimum clearance of 50 cm from the bottom of water body.

Floating GI cages

Fixed PVC Cages

Stocking

Pearlspot seeds of 4-6 cm size can be stocked at a density of 80-200 no./m³ after proper acclimatisation. Initially, fish seed is stocked in inner rearing cage having 10 mm mesh size with 0.5 mm twine thickness. As the fish attains the size of 10 cm, it is transferred to grow-out cage having 18 mm mesh size with l mm twine thickness. The outer cage of 18 mm mesh size with 1 mm twine thickness is used initially and followed by 36 mm mesh size with 2 mm twine thickness.

Feeding

Quality feed in required quantity is very important for sustainable cage farming. Water stable floating pelletted feeds having 38-42% protein and 6-8% fat is given at 2-10% of the body weight. The quantity, size and protein content of feed are determined based on Table- 15.1. Initially, feeding is done four times daily and is reduced to twice daily (morning and evening) after 2 months. Floating PVC feeding ring made of closed net having 15 cm height can be used to prevent the loss of feed due to strong tidal current.

Feed requirement

Care and maintenance

Eventhough pearlspot grazes on periphyton attached to the cage net, it shall be cleaned once in a week using brush (made of natural fibres) to ensure proper water flow and to remove any fouling organism. Regular sampling of fish at fortnightly interval shall be done for evaluating the growth and health. Thinning can be done once in a month as per the size grade, if necessary.

Growth rate

Harvesting

The fish grows to a harvestable size of 250 g within 8-10 months with an expected survival rate of 80%. Based on market demand, fish can be marketed in live condition to ensure premium price. Feeding should be stopped one day prior to harvest. Pearlspot has market demand starting from 200 g size onwards; hence partial harvesting is preferred from this size onwards using scoop net. The anticipated production from a well maintained cage is 24-40 kg/m³.

Farming in Pond

Serious efforts are required for the productive utilization of the available brackish water ponds for both monoculture and polyculture of pearlspot with milkfish and/or mullets.

Site selection & pond construction

Rectangular pond having minimum 0.2 ha area and 1-2 m depth is ideal for the farming of pearlspot. It also thrives in small irrigation tank and backyard pond. Pearlspot farming in earthen pond is more sustainable as it breeds naturally and also utilises the naturally available feed. The pond is covered with bird net of 100 mm size for controlling predatory birds. The side of the pond is fenced with stiff net of 26 mm to prevent the entry of predatory animals. Other aspects of site selection and pond construction are same as that of the major carps.

Pond preparation

The pond preparation steps such as de-siltation, bund strengthening, installation of water inlet/ outlet, draining, drying, tilling, eradication of aquatic weeds and predators and liming are similar to that of the major carps. After filling 25-30 cm water column in the pond, poultry dropping at 500 kg/ha or cow dung at 2000 kg/ha is applied to boost natural feed production in the culture pond. As the plankton production is enhanced within 7 days, the water level is slowly raised to 100 cm. Bamboo poles and coconut leaves are placed along the sides of the pond with 2 m interval for facilitating the growth of periphyton. It will also act as egg attaching substrates during breeding.

Water quality requirements

• Temperature  : 25- 32°C
• Salinity           : 0-30 ppt
• pH                  : 7.0- 8.5
• Transparency  : 25-40 cm
• DO                 : >4.5 ppm
• Alkalinity        : 200-300 ppm

Stocking

After the development of plankton and raising the water level to 1 m, properly acclimatised seeds having a size of 4-5 cm are stocked in cage, happa or pen installed in the same pond to ensure maximum survival. As the growth rate is relatively poor in pond, seeds are stocked @ 30,000/ha for monoculture. If it is polyculture, stocking density is reduced @ 15,000/ha along with milkfish or mullet at 5000/ha.

Feeding

The fish is fed in the morning and evening with a formulated floating pelletted feed or conventional feed having groundnut oil cake, rice bran and fish meal with vitamin-mineral mix as ingredients. The feed contains 24-34% protein and 5% fat.

Feed requirement

Care and maintenance

Water is added intermittently to maintain a water depth of 1-2 m. Moreover, 20-30% of water is replaced fortnightly. Chances for drop in pH can be avoided by the application of agricultural lime on the inner bund @ 250 kg/ha. Subsequent doses of cow dung are added at 250-500 kg/ha for every 7 days in accordance with the availability of plankton. Paddle-wheel-aerators at 2 hp/ha are installed for a production of more than 6 t/ha. Growth is monitored by periodic sampling using cast net.

Harvesting

The fish attains marketable size of 250 g over a period of 10-12 months with a survival rate of 80%. A production of 6 t/ha can be expected.

Source : Department of Fisheries, Government of Kerala