Quantifying the Interactions and Cumulative Effects of Multiple Stressors on Salmonids

The cumulative effects of human activities and natural pressures pose significant threats to ecosystem functioning and global biodiversity. Assessing the cumulative impact of multiple stressors—whether acting simultaneously or sequentially and directly or indirectly—is challenging due to their complex interactions. Consequently, these interactions may be unintentionally overlooked or disregarded in management decisions. While existing reviews have focused on coastal and freshwater ecosystems, analyses specifically targeting salmonids as a focal group are lacking. This research presents the first quantitative and qualitative assessment of stressor interactions affecting salmonid biology and physiology. A focused literature search identified 118 experimental trials with multiple stressors on salmonids. From these, 46 cases were considered suitable for the quantitative analysis. We calculated Hedges’ g effect sizes to classify the interactions between multiple stressors as additive, synergistic, or antagonistic. Our findings revealed that additive effects were found most frequently (50% of interactions), followed by synergistic (30.5%) and antagonistic (19.5%) interactions. Additionally, we performed a network analysis including cases focusing on the influences of multiple stressors interactions (n = 38). Our qualitative analysis identified temperature, metals, and pesticides as the most paired stressors across the three types of interactions. The findings of this research highlight the potential vulnerabilities of salmonids and their habitats by identifying key interactions between multiple stressors, and priorities for future research. Understanding these interactions and cumulative effects, particularly in the context of climate change, can inform targeted conservation and management strategies, contributing to the preservation of these important fish species and their ecosystems, which are vital to local human communities.