ON THE DIGESTION AND UTILIZATION OF MICROCYSTIS BY FINGERLINGS OF SILVER CARP AND BIGHEAD

The digestion and utiization of Microcystis by fingerlings of silver carp and bighead were studied in laboratory during the summer of 1980—1985. The digestibility of Microcystis by silver carp (154 individuals, body length 9.88±0.38cm, body weight 16.78±1.86g) and bighead (194 individuals, body length 10.09±0.58cm, body weight 19.4±2.58%) were determined in July-August, 1980—1982 at a water temperature of 30℃, a dissolved oxygtn of 8mg/L and a light intensity of 2800 lx. The digestibilities of dry matter, protein, fat and carbohydrate of Microcystis by the two fishes were 29.45±4.37% and 26.07±2.15%, 39.15±3.44% and 33.7±2.93%, 34.03±5.38% and 30.29+2.16%, 36.61±4.15% and 30.39±2.81% respectively. The content of amino acids in the feces of silper carp was 26.50% lower, and that in the feces of bighead is 23.45% lower, than the amino acid content in Microcystis. The effects of Microcystis on growth of silver carp and bighead were studied in July-August, 1983—1985. In the experiments, water temperature was held at 30℃, dissolved oxygen was over 8mg/L, pH was between 7.0/8.0 and light intensity was 2800 lx. Microcystis was supplied to silver carp and bighead. The results showed that the growth of fish was related to the concentration of Microcystis in the water. The body weight of fish increased in the experimental group where the algal densities in the water were between 0.4—0.8g/L, but decreased in the group where the algal densities in the water were 1—1.3g/L. Thus, 0.8g/L is the maximum concentration for the effective filtration by silver carp and bighead.