THE FEEDING HABIT OF SILVER CARP AND BIGHEAD AND THEIR DIGESTION OF ALGAE

The numerical ratios of phytoplankton to zooplankton in the intestinal contents of the silver carp (Hypophthalmichthys molitrix) and bighead (Aristiehthys nobilis) were 248:1 and 4.5:1 respectively. Volumetrically, the average size of a single individual of zooplankton was estimated to be about 10 times that of phytoplankton. It was concluded that the silver carp fed mainly on phytoplankton while the bighead on zooplankton. An investigation of the intestinal contents of 30 silver carp and 4 bighead caught in Qingling Lake between 1984—1985 reveals that the ratio of phytoplankton to zooplankton was 48.78:1 in silver carp and 7.95:1 in bighead which are equivalent to 1:47.6 and 1:292.15 in biomass respectively. Considering that the biomass of the zooplankton in the food of bighead was nearly 6 times bigger than that of silver carp, the former concept regarding the major food items of the two fishes is not unreasonable; however, the biomass of zooplankton taken by either fish was bigger than that of phytoplankton. As the plankton composition in the Lake agreed with the food composition in the gut of these fishes, it was suggested that there was no selectivity on food during feeding. The digestibilities of some algae by these fishes were also investigated and discussed.