Upper littoral zones of 13 lakes with fish and 10 lakes without fish were sampled to compare macroinvertebrate communities. Lakes with fish had lower macroinvertebrate species richness, diversity, and smaller-sized individuals than lakes without fish. Larger invertebrates, including most large invertebrate predators, such as dytiscids, corixids, odonates, Gammarus, and Chaoborus, were generally not found in lakes with fish. To determine the feeding behavior of a conspicuous invertebrate predator found in fishless lakes, the predaceous diving beetle ( Coleoptera: Dytiscidae), we used microcosm feeding experiments and a molecular technique for gut content analysis. Dytiscids were sit-and-wait predators in the laboratory, feeding on large, mobile species and avoiding smaller, benthic species. Gut content analyses of field-caught dytiscids, as assessed by native polyacrylamide gel electrophoresis, were not entirely consistent with laboratory selectivity studies, but the field results did confirm that dytiscids fed on large mobile species. However, we cannot rule out the possibility that small prey might also have been ingested. We conclude that fish impact macroinvertebrate diversity in the upper littoral zones of these arctic lakes by excluding large invertebrates and decreasing species richness, diversity, and size-distribution of individuals. Dytiscids in fishless lakes feed on large species as well, but clearly do not eliminate them from lakes as do fish.
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