Shellfish Breeding and Hatchery Management (2+1)

16.1.3 Hatchery seed production techniques and constraints

16.1.3 Hatching unit and constraints

Hatching unit

The berried females are dipped in 10 ppm malachite green / methylene blue / formalin for 5 minutes as a prophylactic measure and released into FRP tanks/ cement tanks of 500 litres capacity covered with black cloth to prevent the light. Providing darkness is found to enhance the reproductive activity of the crustaceans in general. Incubation period will last for 8-15 days.

Hatching usually takes place during early morning hours which lasts for 3-4 hours. Before the larvae hatch out, the abdominal flap of female makes jerking movements in quick succession, whereas the 2^nd & 4^th walking legs tightly tab the egg mass.

The zoea are positively phototropic and therefore soon after they are hatched out they will be attracted towards light. The freshly hatched zoea have to be stocked @ 200-400 nos per litre glass / cement rearing tanks of 2 ton capacity. Filtered seawater having salinity 30-35 ppt and temperature of 27-29⁰C can be used for rearing the larvae. Each zoea stage is of 3-4 days duration. After 5^th moult, the larva becomes Megalopa. Thus it takes 27-30 days for the first zoea to become first crab instar. Megalopa grows to juvenile crab within 8-11 days.

Constraints

During the transition period from zoea I to zoea V, heavy mortality (50-70%) may be observed. During the development of zoea V to Megalopa, less mortality (10-20%) is observed. During the growth of Megalopa to crab I stage, cannibalism is a serious threat. Cannibalism can be as high as 60% leading to a reduction of the desnity. The cannibalism will be severe if the stocking density is more (10 nos/ l). Cannibalism could be controlled by lowering the stocking density to 5 pieces per liter.