Fish stock manipulation techniques in aquachulture for higher yield
- Part 2 -

Mrinal Kanti Datta *

MAJOR OPTIONS:

o Harvest the larger fish prematurely
o Apply supplemental rearing
o 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.

PERFORMANCE:

o Individual fish growth performance: higher growth
o Overall fish biomass at harvest: Positive impact
o 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. The process is most popular in Assam and Tripura.

The stocking density can reach as high as 30,000 fingerlings/ha. Periodical harvesting of large sized fishes is followed by restocking of half to the same number of seeds in the same pond. The system needs special care for water quality and oxygen management.

STOCK MANIPULATION IN TILAPIA CULTURE:

This is a scheduled monthly activity of grading the fish stock into different size groups to reduce the adverse effect of uneven growth and association of “size of hierarchies” within the fish population. Smaller fish are harvested later when they reach the desired size and weight.

EXAMPLE OF STOCK MANIPULATION IN ASIA:

An attempt was made to modify the stock manipulation technique as practiced in Taiwan to suit Philippine conditions. Three 1500 m2 ponds were each stocked with three size groups of milkfish, viz., half grown, 1500/ha; post-fingerling, 15000/ha; and fry, 3000/ha. One month after stocking, another batch of fingerlings was added to the ponds at the rate of 15000/ha.

This was followed by another batch of fingerlings at a stocking rate of 15000/ha. Two months after the initial stocking, 50% of the marketable size (stocked as half grown) were harvested. The other 50% was harvested one month later. In the fourth month after the initial stocking, 50% of the marketable size fish (stocked as the first batch of post-fingerlings) was harvested.

One month later, the remaining 50% was harvested. A monthly harvest was thereafter programmed until the last batch of fingerlings was harvested. The results showed a significant difference in milkfish production between stock manipulation ponds and non-stock manipulation ponds.

Production in the former ranged from 734 to 961 kg/ha, while in the latter, it was from 613 to 702 kg/ha, indicating that stock manipulation can be a positive technique for yield enhancement.

CONCLUSION:

The technique of stock manipulation is a boon for the small and marginal farmers who have small water bodies, may be seasonal in nature and cannot invest huge amounts in fish culture for a longer period. Without much increase in input cost and considering the local demand, this technique can enhance the fish yield and profit.


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