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Fisheries > Culture Fisheries > Edible Oysters

Hatchery Production of Spat

The technology for the mass production of both attached and free seed of C. madrasensis has been developed by CMFRI at its Shellfish Hatchery, Tuticorin. The criteria to be followed for site selection and the inputs required for the establishment of the edible oyster hatchery are the same as those of other bivalve hatcheries.

Hatchery operation

A) Brood stock: In the selection of brood stock, the area of occurrence of the oysters, their condition and age are considered. It is desirable to have a mixed stock, coming from several areas. The salinity and temperature at the collection site are recorded, and if required the oysters are slowly acclimatized to the hatchery conditions. Oysters in the length range 60-90 mm; with 30% of them (males) in 60-75 mm length range ensures the desired representation of the two sexes in the brood stock.

The oysters are cleaned thoroughly with a wire brush to remove the plants and animals adhering to the shell. A batch of 25 oysters is placed on a synthetic twine-knit PVC frame in 100 l FRP tank, and raw (unfiltered) pre-cooled seawater (at 20-22°C) is filled in the tank and aerated. About 2-3 litres of mixed algal culture, (1.5 to 2 million cells/ml) pre-cooled at 20-22°C is given as feed to the oysters twice a day. Thus the oysters are conditioned by holding them in the conditioning room at about 10°C below the ambient temperature.

B) Spawning: The conditioned oysters are subjected to thermal stimulation by suddenly transferring them to seawater with a temperature of 34 to 35°C. This sudden change in the temperature induces spawning. The spawning oysters are immediately separated and placed in spawning trays having filtered seawater at the ambient temperature. After the completion of spawning the oysters are removed.

C) Fertilization and developmental stages: The gametes are mixed and gentle aeration provided. Fertilization takes place immediately and the first cleavage within 45 min. At the end of 4 hrs morula stage is reached and the swimming morula are siphoned and reared in filtered seawater. At the end of 20 hrs the straight-hinge or D-shaped larval stage is reached.  

The various larval stages, their size and duration

Stage

Size mm

Hours/days

Straight hinge

60-70

20 hrs

Early umbo

100

3rd day

Mid umbo

150

7th day

Advanced umbo

260-270

12th  to 15th day

Eyed larva

280-290

13th and 17th day

Pediveliger

330-350

14th to 18th day

D) Larval rearing: As the larvae grow, their density in the rearing tanks is reduced and food ration increased. The schedule for the rearing density and food requirements of larvae is given below.

Larval stages

Density of larvae/ml

Algal cell concentration in ml/ larval/ day

Straight hinge

5

3000-4000

Umbo

3

4000-5000

Advanced umbo to eyed stage

2

5000-8000

Pediveliger

2

10000-12000

E) Spat settlement and rearing: Non-toxic, hard, chemically stable and clean spat collectors are used to produce attached spat. The shell is brushed well, washed in chlorinated water followed by soaking and repeated washing in seawater, so that the pH of the water in the rearing tanks is maintained.

The shell collectors are spread uniformly on the bottom of one tonne FRP tanks containing filtered seawater. The eyed oyster larvae are released in the tanks at a concentration of 1 larva/ml; the seawater is well aerated. In the following few days the larvae are set as spat on the oyster shells. The concave side of the shell usually has more settlement.

For the production of cultchless spat (also called free spat or single spat), pre-treated polythene sheet is spread as a lining on the bottom and sides of a FRP tank. Oyster shell grit of 0.5 mm size are washed thoroughly, sterilized in 10-ppm chlorine, further washed in running filtered seawater and dried. These shell grits are spread uniformly at the bottom of the tank and the larvae are released at the setting stage.

The spat are reared for about 3 weeks after setting in the hatchery before they are transferred to the field for nursery rearing.

clutch

Spat collecting tyre clutch

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