Fish Life History Strategies Determine Assemblage Responses to Altered Flow Regimes

The flow regime is the most influential driver of ecosystem processes in fresh waters, yet our understanding of how the magnitude, timing, and variability of flows shape freshwater fish diversity remains limited. Using pre-existing records of stream discharge, impoundment data, and fish occurrence data from surveys throughout the United States, we examined whether river regulation by dams impact life history composition of freshwater fish assemblages. To accomplish this we first identified ‘triplets’ of dams, gages, and fish surveys throughout the continental United States that met particular criteria.  For each triplet, flow regime and the degree of flow alteration were characterized using hydrologic metrics describing flow magnitude, duration, frequency and timing of flow events based on daily mean flow records for at least 15 years before and 15 years after impoundment.  Fish life histories were characterized along a continuum between three end-point strategies of opportunistic, periodic, and equilibrium as defined by Winemiller (1989) using surveys data collected from 1989 to 2000 in altered (post-impoundment) conditions.  Life history theory and recent empirical work suggest strong linkages between fish life histories and the flow regime; here we tested whether life history theory predicted relationships between the fish assemblages and the flow regime both in free-flowing and regulated rivers. Our results show that altered flow regimes affect life history trait composition of freshwater fish communities, with the direction of these changes being largely in agreement with predictions from life history theory. This study contributes to the understanding of how fish species and biodiversity respond to anthropogenic changes in the hydrologic regime and has implications for flow regulation and restoration via managed flow releases from dams.