Recovery of zooplankton assemblages in mountain lakes from the effects of introduced sport fish

Impacts of introduced fish on zooplankton assemblages of lakes may persist for decades following fish removal. We tested this hypothesis by comparing zooplankton assemblages from four categories of lakes located in western Ca nadian mountain parks including lakes without and with fish that differed i n their fish community complexity and fish-stocking history. Zooplankton sp ecies richness was greatest in lakes with a complex community of fish and l east in pristine fishless lakes. Canonical correspondence analysis showed t hat taxonomic shifts in zooplankton assemblages could be attributed to diff erences in fish-stocking history between the study lakes. In fishless lakes , larger copepods (Eucyclops agilis, Diaptomus leptopus), cladocerans (Diap hanosoma, large Daphnia), and chaoborids were abundant, whereas in the pres ence of fish, small crustaceans were more common and chaoborids were relati vely rare. Once introduced trout were absent from lakes, recovery trajector ies for zooplankton showed a general taxonomic shift towards assemblages ch aracteristic of fishless lakes that had never been fish stocked. Based on s eparation between previously stocked fishless lakes and naturally fishless mountain lakes in ordination space (chi-squared distance), taxonomic recove ry by zooplankton assemblages from the influence of introduced salmonids ma y require an average of 19 years.