Spatial and Temporal Patterns in Smolt Survival of Wild and Hatchery Coho Salmon (Oncorhynchus kisutch) in the Salish Sea

Understanding the declining smolt-to-adult survival (hereafter “smolt survival”) of salmon originating in the Salish Sea is a high priority for fish management agencies. We examined spatial and temporal coherence in smolt survival rates for Coho Salmon based on a decision tree framework organized by spatial hierarchy. Smolt survival of populations from the Salish Sea was compared with Pacific Coast reference populations at similar latitudes. Coherence in Coho Salmon smolt survival occurred at multiple spatial scales. The primary pattern within the Salish Sea was a declining smolt survival trend between ocean entry years 1977 and 2010. In comparison, smolt survival of Pacific Coast reference populations was low in the 1990s, but subsequently increased. Within the Salish Sea, smolt survival in the Strait of Georgia declined faster than Puget Sound. Smolt survival of Coho Salmon was synchronized at a more local scale than reported for other Pacific Salmon species, suggesting that early marine conditions are especially important for Coho Salmon. In order to address linkages between the marine ecosystem and smolt survival within the Salish Sea, researchers will need to carefully match spatial and temporal scales to their questions of interest.