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Fisheries > Culture Fisheries > Seafishes >Seabass

Seed Production

The required important facilities for a seabass hatchery complex are (a) broodstock holding tanks, (b) maturation tanks, (c) spawning tanks, (d) egg incubation tanks, (e) larval rearing tanks, (f) Artemia hatching tanks, (g) live feed (algae and rotifer) culture tanks and (h) nursery rearing tanks. The saline water can either be drawn from a borewell in the intertidal area or from open sea. Water drawn should be stored in reservoirs and filtered through biological filters, rapid sand filters and ultra-violet, filter to maintain the required water quality.

Broodstock development and maintenance

The source of the broodstock can be either from the wild or from rearing ponds/cages. Brood fishes range in size from 2.0 to 10.0 kg, smaller size for males and larger ones for females, which can be induced to mature and spawn within 6 months. Transportation can be done using open or closed containers lined with soft materials like rubber foam mattress. Before transferring the fishes into the broodstock holding tanks (12m x 6m x 2m or 7m x 4m x 2m; rectangular in shape: with PVC inlet and outlet), they should be acclimatized in the holding tanks for. 2-3 days.

Stocking

Stocking of 1 kg fish biomass/m3 is recommended for a 100 tonne of water, i.e. 10 females each of average weight 6.0 kg and 16 males each of 2.5 kg.

Water quality management

Water exchange to an extent of 70-80% of the total volume should be done daily. A flow through arrangement for water exchange is desirable. Important parameters of water quality to be maintained are as follows: water temperature: 28-32°C; salinity: 29-32 ppt; alkalinity (CO3): 80-120 ppm; pH: 6.8-8.0; dissolved oxygen: above 5 ppm; phosphate: less than 10 ppm; unionised ammonia: less than 5 ppm; ionised ammonia: less than 1.5 ppm;

Feed management

Trash fishes like Tilapia/sardines can be procured, cleaned and packed as 2-3 kg blocks and frozen in deep freezers. At the time of feeding this can be thawed and given as feed to the broodstock @ 5% of the total biomass. Conditioning the broodstock fishes to feed on frozen trash fish should be done gradually, first with live and frozen fish and later with frozen fish only. Excessive feeding may be avoided and left-over feed should be removed immediately, since it will pollute the rearing medium.

Health management

Parasites like Caligus sp., Lernanthropus sp. are common in seabass. The most problematic parasite in seabass broodstock maintenance is the monogenic trematode Diplectanum latesi. Infected fishes do not feed, become lethargic and swim in isolation. Bath treatments with 100 ppm formalin for crustacean parasites and 1 ppm organophosphorus pesticide Dichlorovos, for 1 hr, is effective in controlling the parasites.

Maturation

Seabass matures spontaneously in captivity during June to October. The maturation process can be accelerated by hormonal pellet implantation. The maturity stages of the broodstock should be monitored every fortnight. For assessing the maturity stage of female, ovarian biopsy is done by catheterisation. A polythene cannula of 1.5 mm diameter is inserted into the oviduct through the genital opening and the eggs are collected. When the diameter of eggs is more than 0.450 mm in size, the fish can be induced to spawn In matured males, white creamy milt will ooze out when the abdomen is gently pressed.

Induced spawning

Matured seabass can be induced to spawn by hormonal manipulation. The hormones like LHRH-a (Luteinizing Hormone Releasing Hormone analogue), HCG (Human Chorionic Gonadotropin), ovaprim, ovatide and carp pituitary extract can be used for induced spawning, of which, the LHRH-a hormone is known to give consistent results. Female and male fish in the ratio of 1:2 are selected for hormone treatment. Hormone is administered normally in the early hours of the day to facilitate the spawning in the evening hours of subsequent day. Full moon or new moon days are preferred for spawning. LHRH-a hormone is administered intramuscularly in a single dose @ 60-70 mg/kg body weight for females and 30-35 mg/kg body weight for males and the fishes are released into spawning tanks of 15-20 tonne capacity. The abdominal swelling and the courtship behaviour of the fish externally indicate the ovulation response. Good water quality and aeration should be provided in the spawning tank. After 35-36 hrs of hormone injection, the fish spontaneously spawns. Seabass is a protracted spawner (i.e. ova are spawned in batches by the same fish). The same female can be induced to spawn 3 times in a season with an interval of 15 days. Fertilization is external and the fertilized eggs, which are transparent, measure 0.78-0.80 mm in diameter and float on the surface. The unfertilized opaque eggs sink to the bottom. The rate of fertilization may be 60-90%. From a single spawning, 0.75 to 1.5 million eggs can be obtained.

Incubation and hatching

The floating fertilized eggs can be collected from the spawning tanks by a scoop net made of bolting cloth and stocked @ 80-100 nos./ litre in cylindro-conical shaped incubation tanks of 500 litre capacity. Hatching takes place 17-18 hrs after fertilization. Newly hatched larvae measure 1.4-1.6 mm in length and the healthy ones are transferred to larval rearing tanks.

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