Effects of flow regime on fish assemblages in a regulated California stream

The fishes in Lower Putah Creek, a regulated stream in the Central Valley of California,were sampled over a 5-yr period, 1994-1998. Distinct fish assemblages were observed in thelower 37 km of stream using two-way indicator species analysis (TWIN-SPAN) and canonicalcorrespondence analysis (CCA). The assemblages segregated in an upstream-downstreammanner. Distinct differences were found between assemblages of native and nonnatives fishesand their association with environmental variables and habitat use. Native fishes tended to clusterin areas with colder temperatures, lower conductivity, less pool habitat, faster streamflow, andmore shaded stream surface. Numbers of nonnative fish were negatively correlated withincreased streamflow, and numbers of native fish were positively correlated with increased flow.Hydrologic variability between years and seasons indicated that flow regime had a large effect onthe fish assemblages. This study provides a clear demonstration of how native fishes in streamsof the western United States exhibit different habitat requirements and respond to temporalvariation in flow in a different manner than nonnative fishes. It supports the concept thatrestoration of natural flow regimes, in company with other restoration measures, is necessary ofcontinued downward decline of native fish populations in the western United States is to be recovered. http://dx.doi.org/10.1890/1051-0761(2001)011[0530:EOFROF]2.0.CO;