Effects of Stocked Fish on Nutrient Cycling and Algal Dynamics

Daniel Schindler Department of Zoology, University of Washington, Seattle

Peter Leavitt University of Regina, Regina, Saskatchewan

Stocking of sport fishes into fishless lakes has been a widespread activity throughout western North America for most of this century. Many of the effects of stocking on invertebrate and amphibian communities are now well described. However, the ecosystem level consequences of stocking have been more subtle and as a result are not as well understood. Stocked fish alter ecosystem structure and dynamics through a combination of perturbations to predator-prey interactions and re-structuring of nutrient cycles. Fish predation usually results in elimination of large-bodied zooplankton, which in turn decreases zooplankton grazing of phytoplankton. Fish also feed on benthic prey. In fact, benthic fauna dominate the diets of most fish stocked into fishless lakes. Benthivory results in both changes in benthic food web interactions, and a net flux of limiting nutrients that are excreted by fishes and transported from benthic habitats to pelagic habitats. As a result of these complex interactions, one of the main consequences of fish stocking for amphibians is a decrease in the productivity of benthic habitats relative to the productivity of pelagic habitats in lakes.

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