Catastrophic response of lakes to benthivorous fish introduction

Introduction of the benthivorous common carp (Cyprinus carpio) has been ide ntified as one of the main causes of loss of biodiversity and water clarity in numerous shallow lakes and ponds worldwide. Recent observations in expe rimental fish ponds suggest that the effect of carp on the ecosystem is cat astrophic in the sense that a substantial impact occurs only when a critica l carp density is exceeded. In search for an explanation, we analyzed a sim ple model of the interaction between benthivorous fish and their invertebra te benthic prey, and of sediment resuspension resulting from fish feeding b ehavior. Our results suggest that benthic prey populations should be only m oderately depressed until predator fish abundance grows to a critical bioma ss at which benthos collapses due to overexploitation. This drop in prey de nsity is predicted to result in a sharp increase in water turbidity due to an increase in prey search activity of the fish. For less eutrophic and dee per lakes, where benthos productivity and hence benthivorous fish. carrying capacity are lower, water turbidity is predicted to be much less affected. The qualitative patterns are quite robust against assumptions on parameter values and correspond closely to the experimental results and data from la kes suggesting that the model may capture the essence of the mechanism caus ing a discontinuous effect of benthivorous fish on lake ecosystems.