Higher Early Marine Mortality of Steelhead Associated with Releases of Hatchery Coho Salmon but Not Chinook Salmon

Understanding the drivers of mortality during critical life history periods is an important part of increasing our capacity to rebuild depressed salmonid populations. For threatened steelhead Oncorhynchus mykiss in Puget Sound, Washington, early marine predation has been implicated as a key source of mortality. Yet, the agents that mediate predation pressure are poorly understood. In this study, we characterize abundances of juvenile Coho Salmon O. kisutch and Chinook Salmon O. tshawytscha in Puget Sound and relate these abundance patterns to weekly steelhead survival to better understand whether pulses of hatchery-released salmonids mediate steelhead survival. We found that weekly abundances of hatchery Coho Salmon and Chinook Salmon smolts vary by several orders of magnitude across weeks, indicating that large resource pulses are available to salmonid predators. We further found that weekly steelhead survival was significantly negatively related to abundances of hatchery-released Coho Salmon but not Chinook Salmon, which had considerably smaller body sizes than both Coho Salmon and steelhead smolts. Together, our results suggest that releases of Coho Salmon into Puget Sound mediate mortality of steelhead smolts, possibly via increased predation pressure by shared predators.