Flow Regime, Juvenile Abundance, and the Assemblage Structure of Stream Fishes

Abstract

The assemblage of fishes in a second—order stream in east—central Illinois was compared through seine sampling for two years with distinctly different flow regimes. In both years adult (age >0) fish were most abundant in late spring and early summer while juvenile (age 0) abundance peaked in late summer or autumn. Total density of adult fish differed little between years but large changes in juvenile abundance occurred between years in association with differences in hydrologic regime. High stream discharge had little influence on abundance of juvenile suckers and darters, but some minnow species and all sunfish species exhibited large increases in juvenile abundance during stable to low flow conditions. The largest increase in juvenile abundance occurred among species with prolonged breeding seasons: bluntnose minnow (Pimephales notatus) and striped shiner (Notropis chrysocephalus). Increased juvenile abundance during stable flow conditions resulted in increased species richness in small habitat patches and annual changes in species composition of the community. Models of fish assemblages in small streams need to incorporate ontogenetic changes in the role of stochastic environmental variables in regulating the population size of component species.