Fish stock manipulation techniques in aquaculture for higher yield
Mrinal Kanti Datta
Contd from previous issue
The pond then is drained, dried and the whole cycle is repeated. Sometimes a batch of smaller fish is introduced after the big ones are harvested if the food situation in the pond so warrants.
· Harvest the larger fish prematurely
· Apply supplemental rearing
· Reduce stock by transferring to other areas
ROLE OF AERATION IN STOCK MANIPULATION:
Along with stock manipulation, running water and aeration can also increase the dissolved oxygen of the pond water and hence productivity and yield.
· Individual fish growth performance: higher growth
· Overall fish biomass at harvest: Positive impact
· Monetary returns: higher than conventional systems
STOCKING IN NURSERY POND:
While nursery ponds are heavily stocked (3-6 millions/ha), the rearing ponds are lower stocked (3-5 lakhs/ha). Advance fry measuring 20-30 mm are usually indistinguishable. Therefore, rearing ponds are stocked without any discrimination of species. However, as they grow and attend 40-45 mm size, it is better to segregate major carps and discard the undesirable stock. For this purpose, stok manipulation i.e., discrimination of species must be maintained in nursery pond management. The shooter (larger one) should be thinned out to facilitate uniform growth of the spawn/fry to fingerlings for higher survibility and growth.
STOCK MANIPULATION IN COMPOSITE CULTURE IN INDIA:
Manipulation of the species ratio is for minimizing the interspecific and intraspecific competition for food available in various tropic levels and zones in ponds
1. 6 species culture: Surface feeders may form 30-40% (Catla 10-15%, Silver carp 20-30%), column feeder Rohu forms 15-20% (in moderately deep water > 2 m av. Depth) and 10% in shallow pond (<2 m av. Depth), bottom feeder (Mrigal and Common carp 40-35%) Mrigal 15-20% and Common carp 20-35% and macro-vegetation feeder Gras carp forms 5-15% depending upon the availability of a dependable source of weed supply with a yield of 3000-5000 kg/ha/year.
STOCK MANIPULATION IN NE INDIA:
Due to high demand of locally produced live carps to the consumers and takes less duration in culture, farmers used to market yearlings of sizes of 100-400 gm each from three months of culture onwards for lower risk, quick return and higher yield. The process is multiple stocking and multiple harvesting with higher stocking density. (To be contd)