Fish stock manipulation techniques in aquachulture for higher yield
- Part 1 -

Mrinal Kanti Datta *

Economic fish production through mono and polyculture system is dependent upon various factors, such as pond preparation, control of weeds, control of predatory animals, liming, proper stocking, monitoring of physio-chemical parameters, use of manure, supplementary feeding, netting, harvesting, etc.

Important cultivable species are carps i.e., Rohu, Catla, Mrigal, Grass Carp, Silver Carp, Common Carp. Rohu, Catla and Mrigal are called Major Carps. Silver Carp, Grass Carp and Common Carp are called exotic Carp. Other species in culture are in Kalabasu, Pangus, Silver barb, Kai, Magur, Singi, Moka etc. These fishes are of high economic importance and are largely used for culture.

Growth of fish also depends upon appropriate stocking and stock manipulation. Proper stocking means maintaining scientific proportion of different types of fishes in ponds. Ponds can support either a large number of very small fishes or small number of large fishes.

Though growth of fish in ponds with thin stocking may b very satisfactory, but it is not economical. Total fish population that a given pond can support depends on its carrying capacity in terms of food contained in the body of water or the supplementary food that can be supplied.

Hence manipulating stocking density, sizes etc. may be a tool for higher fish yield and the technique is called fish stock manipulation.

SALIENT FEATURES OF STOCK MANIPUATION:

Hence stocking rate of fish pond in aquaculture can be increased by different kinds of stocking systems such as:
o Multiple sizes stocking
o Same size stocking in a system of ponds
o Double cropping
o Sequential harvesting

MULTIPLE SIZES STOCKING:

This may be defined as stocking of the same species in different sizes to make more efficient use of water space. If suitable density of fry is stocked at the beginning, the rearing pond will be overcrowded when fry reach adult size.

Growth will be retarded with low survival. The carrying capacity of pond in different age group of fishes is greater from same age group. Multiple sizes stocking requires periodical harvesting of the largest fishes when they reach marketable sizes. After the stock is thinned out smaller fishes are added.

Continuous harvesting and stocking method not only gives the farmers a constant income and a higher average price, but also thins out the stock and thus improving the growing conditions for the younger stock remaining in the ponds. This methodology can enhance total yield significantly.

SAME SIZE STOCKING IN A POND SYSTEM:

Stocking of fish of one size in one pond. When more space is needed, transferring them to a larger adjacent pond. The smaller ponds are prepared for using then for rearing of the succeeding batches. The number of harvests increases from 1-2 times under this type of operation.

DOUBLE-CROPPING:

Culture of two species in the same pond in different seasons, taking the advantage of different thermal requirements of the two species. This method may increase the income more than 100 percent. Crops may be for monsoon the post monsoon with carp and during winter with cat fishes as well as air breathing fishes.

STOCK MANIPULATION METHD:

o mono-specific
o multi-size
o medium-water-depth method

POND PREPARATION:

This particular method calls for pond bottom drying and fertilizing with organic and chemical fertilizer then watering with 10 to 15 cm of water and slowly deepening this level to 75-100 cm at which time planktonic food would be well established. Stock of fish is then introduced in two to four sizes. Usually the bigger ones are the hold-overs from previous crop.

STOCKING:

o About 12,000 nos./ha of (6-8 inches) 15-20 cm or 8-12 fish to a kg
o About 14,000 nos./ha of (4-5 inches) 10-13 cm or 40-50 fish to a kg
o About 18,000 nos./ha of (2-3 inches) 5-8 cm or 150 fish to a kg

PERIODICAL HARVEST AND STOCKING:

Subsequently the pond water is gradually increased to 100 to 150 cm as the fish grows. The above numbers may vary depending on the carrying capacity of the pond as observed by the farmers. After 60 to 90 days, the big ones are harvested either by cast or drag net. Another batch of small size seeds are again introduced.

Every 30 to 50 days, the larger fish are harvested until it is decided to stop the culture and a final harvest is made. The remainder smaller fish are stocked in holding ponds for the next culture period. 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.

To be continued.....