A key challenge in stream fish ecology and biomonitoring is to partition local and regional influences on assemblage structure. Numerous studies have identified local determinants of species composition (i.e., competition, predation, habitat availability), but regional influences remain poorly understood. Here, we test the hypotheses that (1) fish dispersal from adjacent streams influences local fish assemblage structure, and (2) the effects of interstream dispersal are mediated by local environmental conditions. We evaluated fish and physical habitat data from the U.S. Environmental Protection Agency's Environmental Monitoring and Assessment Program in western Virginia streams (n — 55). We found significant effects of adjacent stream size on localspecies richness, mean reproductive age, and riverine species richness. Large adjacent streams (greater than third-order) were associated with increased species richness in second-order sites. Fourth-order sites showed increased riverine species richness and decreased mean reproductive age in the presence of large adjacent streams. The nonrandom effects of adjacent stream size among sites of various stream orders suggests that local environmental conditions mediate the effects of dispersal from adjacent streams. Measures of channel shape (i.e., depth, width, and sinuosity) and microhabitat complexity (i.e., mean substrate size and woody debris) were associated with local assemblage structure in some cases, but did not account for significant variation in fish metrics explained by adjacent stream size. These results indicate that the ability of fish biomonitoring metrics to detect anthropogenic impacts may be improved by calibrating scoring criteria based on the size of adjacent streams.
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