Understanding how Pacific salmon respond to multiple human activities is essential for predicting vulnerability and informing management in increasingly altered freshwater ecosystems. Stressor-response relationships quantify changes in biological responses as exposure to environmental stressors varies. Estimating these relationships has become increasingly important for predicting the cumulative effects of activities on species, including salmon. Salmon ecology literature is vast, yet existing evidence describing stressor-response relationships for Pacific salmon are fragmented across stressors, activities, and species.
We conducted a comprehensive synthesis and quantitative reanalysis of stressor-response relationships linking human activities to Pacific salmon responses in watersheds of the Pacific Northwest. We first developed a pathway-of-effects framework linking major human activities (e.g., forestry, agriculture, urban development, mining, hydroelectric infrastructure) to freshwater stressors (e.g., temperature, flow alteration, sediment) and salmon responses (e.g., survival, recruitment, fecundity). This framework informed search terms for a systematic literature review. Studies were screened using predefined inclusion criteria, including restriction to stressors in fresh water and studies conducted in the Pacific Northwest (e.g., Alaska, British Columbia, Washington, Oregon, and Idaho).
For studies reporting underlying data, we extracted or digitized observations and fit stressor-response curves across studies and species using generalized linear mixed models. Models included species-level random intercepts and slopes and study-level random intercepts to account for hierarchical structure and cross-study variability. We quantified where stressor-response curves changed most rapidly and compared the strength of responses at those locations across stressors using standardized metrics.
Across synthesized stressors, Pacific salmon responses frequently exhibited nonlinear dynamics, with pronounced changes in response occurring over relatively narrow ranges of stressor exposure. The magnitude and location of these changes varied among stressors and species, with many relationships exhibiting weak, linear responses. Many stressor-response relationships lacked sufficient data for robust estimation and studies testing for interactive effects among multiple stressors were scarce. This synthesis integrates pathway-based review and quantitative reanalysis to provide stressor-response curves that can be used in cumulative effects frameworks and help prioritize future research and management actions for Pacific salmon in freshwater ecosystems.