Production of Asian Seabass

The Asian seabass, Lates calcarifer has an excellent demand in both domestic and international markets. It is euryhaline; hence it can be cultured in freshwater, brackish water and seawater. Juveniles inhabit backwaters and estuaries, while adults are marine. Usually, seabass shows protandrous hermaphroditism. The fish in size range of 2-4 kg matures as male and transforms to female when it attains the size of more than 4 kg. It is carnivorous, feeds mainly on small fish, shrimps, etc. It has an elongated and compressed body with a large, slightly oblique mouth. Upper jaw extended behind the eye, the lower edge of pre-operculum serrated with a strong spine. The eyes are bright pink and glow at night.

Seed Production

Broodstock management

The collection of sub-adult and quarantine protocol is the same as that explained for the cobia. After quarantine, the fish weighing above 3 kg is transferred and stocked in a tank (100 t capacity) having 2 m depth with aeration and re-circulation facility. The fish is stocked at a density of 2 kg/m³ and fed with trash fish at 5% of the body weight initially and two months before spawning it is reduced to 1%.

Water quality management

It is practiced in a similar way to that explained for the milkfish.

Spawning

Seabass spawns round the year. As it is a protracted spawner, eggs are released in batches. Hence, the same female fish is induced to breed at an interval of 15 days. Morphological identification of sex is difficult; therefore catheterisation is done to determine the sex. Female fish having eggs with a size of more than 400 qm is selected for induced breeding. Both male and female fish are injected with LHRH-a at 25-35 mg/kg bodyweight for induced spawning and released to FRP tank (20 t capacity). The eggs are released by the female after 34-36 hours of injection. Simultaneously, the male oozes the milt, and fertilisation takes place externally in water. The fertilised eggs are floating and transparent with a size of 0.8 mm diameter. The unfertilised eggs are opaque and sink to the bottom. The fish spawns in 3 consecutive nights. The fecundity is 1.5-3 lakh/kg of the body weight. Optimum temperature and salinity of spawning are 27-32⁰C and 30-32 ppt.

Incubation

The fertilised eggs are collected in the morning by scoop net with 300-400 pm mesh size and incubated in cylindro-conical FRP tank of 500 1 at a density of 500-1200 no./l with mild aeration. The eggs hatch out within 17-18 hours, and the newly hatched larva has a size of 1.5

Live feed production

The method is similar to that explained for the milkfish.

Rearing of hatchling

The hatchlings at the density of 30 no./l are initially transferred to indoor FRP tank of 5 t capacity containing filtered seawater for the first 10 days and then transferred at the density of 5 no./l to indoor FRP tank of 10-30 t capacity. The newly hatched larva does not feed until 48 hours after hatching. After 36 hours of hatching, rotifers, Brachionus plicatilis and Brachionus rotundiformis are added in the rearing tank at a density of 15-20 no./ml per day until 12 dph. The microalgae such as marine Chlorella, Nannochloropsis, Isochrysis, etc. are maintained for feeding rotifers at a density of 1-2 lakh cell/ml. From 8 dph onwards, the larva is also fed with Artemia nauplii at a density of 1 no./ml/day. On 15th dph, the density of Artemia nauplii is increased to a rate of 5-10 no./ml/day. Artemia biomass at a rate of 1 no./ml/day is provided from 23^rd dph and then weaned on artificial diet. The young one attains a size of 1-1.5 cm in 30 dph.

Rearing of fry

Fry having 1.5 cm size is reared to the size of 7 cm in indoor/ outdoor tank, hapa or cage. The tank (1-10 t capacity) having minimum 1 m water depth with continuous aeration and daily 100% water exchange is used for nursery rearing. The fry is fed with a formulated diet 3-4 times daily at 20% of the body weight. Initially, the feed size is 0.4 mm, which is gradually increased to 1.2 mm for nursery rearing. Grading is done at an interval of 7-10 days to avoid cannibalism. During grading, the stocking density is regulated according to the size of fish as given in below Table. Nursery rearing period is 40 days and the expected survival is 60%.

Stocking density for rearing fry

Packing and transportation

The method is similar to that of the cobia.

Cage Farming

As seabass exhibits a high rate of cannibalism in the early stages, cage farming is better than pond culture.

Site selection

Being highly euryhaline fish, it can be cultured round the year in freshwater, brackishwater and seawater. Other aspects are similar to those of the pearlspot.

Water quality requirements:

• Temperature : 27-30°C
• pH : 7.3-8.5
• Transparency : 30-50cm
• Salinity  0-35ppt
• DO : > 4ppm

Cage design

Regarding sea cage farming, the design should be the same as that specified for cobia, and in case of brackish water for cage culture it can be done as specified for pearlspot. The mesh size required for cage net is provided in below Table.

Stocking

After acclimatisation, the hatchery-produced seeds having a uniform size of 7 cm (5 g) are stocked at a density of 20 no./m³. Subsequently the cage net is replaced with higher mesh size as given below.

Mesh size of cage net

Feeding

The growth rate of sea bass and the details of feeding ration of dry feed are given in below Table. Other aspects are same as those for cobia.

Feeding requirement

Care and maintenance

The growth rate of sea bass is given in above Table. As the fish shows differential growth rate and cannibalism, it is periodically graded by size, thinned-out and reared in separate cages. Grading is done using nets or grader. Initially, the fish is graded once in 7 days and later on, once in 30 days. Other aspects are similar to that explained for cobia.

Size grader

Harvesting

The fish attains 1 kg within a culture period of 10-12 months. It has both domestic and export market. A production of 16 kg/m³ can be expected from one crop. Survival rate in cage farming is about 80-85%. Other aspects of harvesting are same as that of cobia.

Grow out Culture in Pond

It is advisable to rear the seed in nylon hapa or indoor/outdoor tanks.

Site selection & pond construction

Freshwater, brackish water as well as seawater ponds having not less than 0.2 ha area with 1-2 m depth is ideal for the farming of seabass. Sluice gates should be provided to regulate the inflow and outflow of water. Predatory birds are controlled by covering the pond with bird net of 100 mm size. The side of the pond is fenced with stiff net of 26 mm to prevent the entry of predatory animals or escape of the fish. Area prone to flooding should be avoided. Other aspects of site selection and pond construction are the same as that of the major carps.

Pond preparation

Before stocking, all the pond preparation procedures should be followed.

Nursery rearing and stocking

Before releasing into grow-out pond, the hatchery-produced seeds of uniform size are stocked at a stocking density of 500 no./m² in cage or hapa of 2 x 2 x 1.5 m dimension having 10 mm mesh size mounted on PVC floating structure installed in the pond until it attains 50 g size and thereafter it is released into the pond. This helps to reduce cannibalism. Ideal stocking density for grow-out farm is 1 no/m².

Feeding

Feeding in pond culture is same as that explained for the cage farming.

Care and monitoring

Periodic liming at 250 kg/ha is done to manage the pH. Water is added intermittently to maintain a water depth of 1-2 m. Plankton bloom is essential for early stages of culture (until 100 g). If the pond water appears clear, a mixture of cow dung at 100-150 kg/ha and inorganic fertilizers at 10-15 kg/ha can be applied at regular intervals to develop natural food organisms. Sufficient water level is maintained in the pond to reduce the growth of benthic algae. The water depth in the shallow part of the pond should be at least 100 cm. Water quality can be maintained by exchanging 10% of the water once in a week for the first 3 months. After 3 months, this is increased to 20% per week and to 30% per week after 6 months. Paddle wheel aerators are placed in the pond to maintain the DO level above 5 ppm, which is vital for proper growth and survival. Growth is monitored by periodic sampling.

Harvesting

Harvesting of seabass is carried out using the drag net. To maintain the freshness and quality of harvested fish, washing in clean water and chill killing can be adopted.

Source : Department of Fisheries, Government of Kerala