Survival Patterns of Wild and Hatchery Coho Salmon in the Salish Sea

Abstract

Returns of coho salmon to the Salish Sea have declined over the past three decades. Studies in the Strait of Georgia have suggested that early marine environment is a critical survival bottleneck for multiple stocks whereas forecasting efforts in Puget Sound have highlighted the variable survival among stocks within a single year. A study which investigates spatial and temporal patterns in coho marine survival is needed to explain these observations. The objectives of the current study are to determine the spatial and temporal coherence in smolt-to-adult survival for coho salmon stocks, to compare hatchery and wild smolt-to-adult survival, to determine and compare early marine survival estimates among basins, and to evaluate the contributions of marine versus freshwater survival to overall productivity of wild coho salmon populations. Marine survival estimates were derived using coded-wire tag data and adult versus smolt count data from coho salmon stocks representative of regions both inside and outside the Salish Sea. A cluster analysis was used to identify the appropriate spatial scale to consider environmental factors driving marine survival. In addition, two approaches were used to determine the contribution of survival in the marine environment to overall returns of coho salmon. The first approach examines whether early marine survival, measured from Strait of Georgia and Puget Sound trawl surveys, explains variation in total marine survival of hatchery and wild coho salmon. The second approach examines whether variation in freshwater versus marine survival best explains the cohort sizes of wild coho salmon. The results of this work identify the appropriate spatial scale to develop ecosystem indicators of marine survival and describe the importance of the marine environment in driving coho salmon returns to the Salish Sea over the last three decades