THE RATIONAL DESIGN OF POND FISH CULTURE WITH REFERENCE TO THE PRINCIPLES OF YIELD INCREASE Ⅰ. Rational structure and energy conversion efficiency of the community of major cultured fishes in ponds

Pond culture experiments (1987—1989) and indoor supple-mental experiments showed that the commonly used stocking ratio of 1 grass carp to 3 filter-feeding fish in polyculture systems fails to fully utilize the interspeeifie benefits among the grass carp, silver carp and bighead carp, resulting in a low energy conversion efficiency in the pond system. Because of the low food availability caused by high densities of silver carp and bighead carp, over-fertilization is frequently practised, resulting in deterioration of water quality and diseases and poor condition in the grass carp. Based on the food-supply potential of the feces produced by grass carp, and the energy flow pathyway acting on the filter-feeding fish, the weight ratio of stocked grass carp to filter-feeding fish was adjusted to (1.5—2.5) : 1 Ⅰ. Green plants were used as the major food, supplemented with pellet feeds. The growth rates of major cultured fishes and energy conversion efficiency of the pond were apparently higher than those in ponds using routine culture methods. With no exogenous fertilization and suitable green and pellet feed inputs in accordance with the biological characters of the grass carp, each unit weight of grass carp produced could yield 40—80% of its weight of filterfeeding fishes.