Differential responses of body shape to local and reach scale stream flow in two freshwater fish species

Stream flow is recognised as an important influence on the physical traits, distribution and diversity of stream fishes. However, measures of stream flow' are not consistent among studies and can refer to variables characterising microhabitat (e.g. flow velocity) or reach-level (e.g. discharge) parameters. Because of the ambiguity associated with the term stream flow', the differential effect of these two aspects of local stream hydrology on freshwater fishes is unclear. Using two frequently sympatric freshwater fish species, Etheostoma nigrum and Pimephales notatus, we tested for correlations between population-level body shape and two measures of stream flow, microhabitat flow velocity (m/s) and reach-level discharge (m(3)s(-1)). Etheostoma nigrum body shape was correlated with flow velocity, while P.notatus body shape was correlated with mean annual discharge. The respective correlations are potentially manifestations of behavioural differences among the two species. Etheostoma nigrum is a benthic specialist rarely venturing far from its associated microhabitats. Pimephales notatus is a habitat generalist and uses many different microhabitats but is unable to escape the overall discharge pressures of a particular stream. These results demonstrate the importance of considering the spatial scale of variables when predicting phenotype-environment associations.