Changes in fish assemblage structure upstream of impoundments within the Upper Wabash River Basin, Indiana

The effects of dams and impoundments on downstream fish assemblages have been well documented, but changes in fish assemblages in upstream tributaries have received little attention. We compared changes in abundance and composition in fish assemblages in streams fragmented by impoundments with those found in nearby unfragmented streams by sampling fish, in-stream habitat, physicochemical factors, and drainage features in 22 agriculturally dominated streams during the summers of 2002 and 2003. Eleven sampling sites were tributaries upstream of impoundments, while 11 were tributaries of free-flowing rivers. We tested the hypothesis that fish assemblages upstream of impoundments would differ from those found in streams without impoundments. Using multiple regression and canonical correspondence analysis, we partitioned the variation in species distributions into that explained by in-stream habitat, reach-level factors, drainage features, and temporal variation. Spatial patterns of species distributions indicated significant upstream effects of impoundment, fragmentation being the single largest predictor of species distributions. Mean fluvial specialist richness was significantly greater in unfragmented (6 species/reach) than in fragmented streams (3 species/reach), whereas mean fluvial generalist richness was significantly greater in fragmented (12 species/reach) than unfragmented streams (8 species/reach). A shift in piscivore abundance and composition was also observed, with smallmouth bass Micropterus dolomieu and redfin pickerel Esox americanus replaced by largemouth bass M. salmoides and white bass Morone chrysops in the streams fragmented by impoundments. Additionally, greater total richness and piscivore abundance was observed in tributaries upstream of impoundments. Unfragmented streams showed a significantly lower total abundance of piscivores (7 fish/reach) than upstream tributaries of impoundments (14 fish/reach). The downstream presence of an impoundment led to significant homogenization of fish assemblages through a significant increase in generalist richness and abundance in fragmented streams and a shift in the abundance and type of piscivores.