Host-pathogen-environment interactions determine survival outcomes of adult sockeye salmon (Oncorhynchus nerka) released from fisheries

Using weekly nonlethal gill biopsies and high-throughput qPCR, we measured up to 46 infectious agents with host stress and immune gene expression. Marine-captured fish had less severe infections than river-captured fish: a short migration (100 km, 5-7 d) that produced profound infection differences. At 14°C, river-captured fish survived 1-2 weeks less than marine captured fish.

All fish held at 18°C died within 4 weeks unless they were not handled, and few river-captured fish survived. Gene expression correlated with infections in river-captured fish, while marine-captured fish were more stressor-responsive. Our results implicate river-derived infections as causal factors of stressor-mediated early mortality in migrating adult sockeye salmon, whereas cumulative stressors were detrimental regardless of initial infections due to extreme physiological disturbance.

We propose that river entry decreases stressor resilience of adult Pacific salmon by shifting infections and redirecting host responses toward disease resistance.