STUDIES ON THE CARRYING CAPACITY OF CHINESE MITTEN CRAB IN MACROPHYTIC LAKES

The experiments on Chinese mitten crab (Eriocheir sinensis) stocked in cages and tanks with the transplanted submersed macrophytes were conducted to study effects of the crab sizes and densities on the biomass of different macrophytes, and to investigate the carrying capacity of the crab in macrophytic lakes. The small juvenile mitten crab (CW:7.00.6 mm) had little effect on biomass of the four submersed macrophyte species, at the stocking density of 50 to 200 indm-2. There were no floating, fragmented leaves or whole macrophytes found during the experiment for small crabs. It was observed that while feeding,these small crabs used their very small and weak claws to grip and tear foods, usually the surface parts of leaves or stems of submersed macrophytes or periphyton on them. The larger mitten crabs (18.022 mm CW) had noticeable effects on submersed macrophyte biomass, and the effects were significant at the higher crab stocking densities (40 indm-2). When the size of the crab became larger (35.03.6 mm CW), it had a much stronger effect on macrophyte biomass, in which, with stocking densities not less than 3 indm-2, whole V.spiralis with or without roots and its long leaves and the long stems of C. demersum, P. maackianus and M. spicatum were all found floating. Except for M. spicatum, plant biomass decreased sharply due to the largest crabs (60.05.7 mm CW) at stocking densities not less than 0.25 indm-2, and biomass of M. spicatum decreased significantly at stocking densities not less than 0.5 indm-2. While feeding, or searching for food, the largest crabs cut the leaves of V. spiralis from the lower end, and it resulted in floating long leaves or whole macrophyte. In the present paper, the carrying capacity is defined as the maximal standing biomass of the crab in the case that the macrophyte growth is not significantly affected by the crab. The different carrying capacities for the crabs of different sizes were estimated by analyzing the relationship between the crab density and the relative growth rate of the macrophyte. For the small juvenile crab (Carapace width=CW: 18.01.2 mm), the carrying capacity is 23 indm-2; for the larger crab (35.03.6 mm CW), the carrying capacity is 0.9 indm-2; for the largest crab (60.05.7 mm CW) in the experiments, the carrying capacity is about 0.083 indm-2. The carrying capacity of the matural crab in macrophytic lakes was calculated as 29.25 kghm-2 according to the relationship of carrying capacity-crab size.