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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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