Assessing the Influence of Stream Network Topology on Stream Fish Occurrence Rates

Stream network topology has been predicted to influence stream fish communities, with mainstems predicted to serve as refugia leading to increases in stream fish species occurrence rates closer to mainstems.  Alternatively, close proximity to mainstems may result in the displacement of stream species by riverine species reducing stream species occurrence rates near mainstem confluences.  Our objectives were to determine which species showed positive and negative responses to proximity to mainstems and to link species traits with those responses.  We used logistic regression to determine if the  presence of fish species at 384 sites in Missouri wadeable streams was linked to the distance to confluence with mainstem (5^th strahler order), with stream size (Shreve link magnitude) and habitat integrity (Percent of watershed classified as Forest/Grassland/Wetland) as covariates.  Of the 69 species evaluated 11 (16%) decreased and 17 (25%) increased in occurrence rates near mainstems (p <0.10).  Cottids (2/3), Percids (3/8), and equilibrium (6/18) and opportunistic (10/30) life history strategists increased occurrence near mainstems, whereas Catostomids (2/6), Pimephales spp. (3/3), and omnivores (8/19) decreased occurrence near mainstems.  Our results indicate that proximity to mainstem stream reaches may impact occurrence rates of many stream fish species.