Cooperative Fish and Wildlife Research Units Program: Idaho
Education, Research and Technical Assistance for Managing Our Natural Resources

Reservoirs are often managed similar to natural lakes because they are assumed to be functionally comparable. However, direct comparisons of fish assemblage-environment relationships between these two ecosystems are rare. We investigated associations of fish assemblage structure from 45 natural lakes and reservoirs in Iowa, USA. Fish sampling was conducted with benthic trawls in summer in conjunction with modified-fyke nets and night electrofishing in fall to characterize lentic fish assemblages and reduce gear selectivity. High species diversity in reservoirs was most strongly related to morphometric characteristics (i.e., larger surface area, increased depth); whereas, fewer species were observed in natural lakes with low water clarity and high suspended solids. Fish assemblage structure between natural lakes and reservoirs was consistently dissimilar for all sampling methods. Structuring of species composition in reservoirs was correlated with a variety of limnological and physical characteristics, but was inconsistent among sampling methods. In contrast, trophic structure of fishes in reservoirs was weakly associated with the environmental factors and was similar to taxonomic structure of fish in natural lakes. Fish trophic composition of natural lakes was most strongly related to waterbody size. Overall, distinct differences in fish assemblage structure were observed between natural and artificial lentic ecosystems. Differences were likely the result of dissimilar management activities (e.g., rotenone, stocking) between waterbody types that had overriding effects on assemblage-habitat relationships. Our results emphasize the need to consider waterbody origin and management legacies on fish assemblage characterization and subsequent inferences made from environmental correlations.