Damming creates winners and losers: life history traits fail to predict vulnerability in freshwater fishes

Increasing global use of dams is a major threat to freshwater ecosystems as they dramatically disrupt river flow regimes resulting in a cascade of negative impacts on fish and other aquatic biota. Quantitative reviews of damming impacts have focused on community level responses, but there is no synthetic understanding of species-specific effects. Such work is needed because freshwater fishes are among the most diverse clades in the animal kingdom, possessing a wide variety of life history traits that may facilitate resiliency in the face of increasing damming worldwide. To help fill this knowledge gap, we conducted a meta-analysis to evaluate whether key life history traits mediate fish population response to damming. We developed linear mixed models to evaluate if migration status, body size, reproductive strategy, trophic level, and taxonomy influence population response to damming. Largely, we observed both negative and positive population responses to damming suggesting that there is no universal response to this type of disturbance. Further, there was no single or combination of life history trait(s) that significantly explained variation in our models. There was weak evidence that maximum body length and parental care through nest guarding influenced the response of fish to damming. Taxonomic analysis revealed a single order, Eupercaria, that exhibited some evidence of positive response to damming. Overall, our findings suggest that dams create both positive and negative effects on fish populations without a clear signal from the life history traits we evaluated. Resilience to the effects of damming may depend on complex interactions in traits not measured in our analysis or other site-specific factors that result in population increases or declines. This suggests that case-by-case system assessments are likely still needed to understand and predict the consequences of dams. Such information is critical, given the increased prevalence of damming in major river basins that harbor a large fraction of global fish biodiversity.