Multiple-stressor effects on freshwater fish: Importance of taxonomy and life stage

Interactions among multiple anthropogenic stressors threaten freshwater fish and pose challenges for fisheries management and conservation. Previous studies of multiple‐stressor effects on freshwater fish suggest a prevalence of antagonistic interactions. However, taxonomy, life stage and/or environmental context likely modify the magnitude and direction of fish responses to multiple stressors. Stressor intensity, impact mechanism, exposure time and ecosystem size may further affect interaction outcomes. Large‐scale studies quantifying how these variables moderate stressor interactions are lacking. To address this knowledge gap, we performed a meta‐analysis of 29 factorial multiple‐stressor experiments to examine the influence of seven potential moderator variables on the magnitude and direction of stressor interactions. Using weighted random‐effects meta‐analytic models, we demonstrate the importance of taxonomic identity and life stage for interaction outcomes. In particular, Cypriniformes showed stronger antagonisms than Salmoniformes, as did larval fish compared to juveniles. Interaction outcomes also varied among the measured fish responses with survival yielding stronger antagonisms than biomass. Increasing experimental duration and volume of the experimental units both drove interactions towards synergisms, supporting findings from previous studies that synergisms take time and space to develop. In an era when the number of stressors affecting freshwater systems is increasing rapidly, our study provides a vital step towards identifying generalities in multiple‐stressor outcomes and thus improved predictions of multiple‐stressor impacts. Furthermore, our meta‐analysis complements studies in real streams, rivers and lakes by providing an experimentally derived context for the growing number of multiple‐stressor assessments in research, management and conservation of freshwater fish.