Optimization of hatchery Chinook salmon releases in the Salish Sea through ecosystem-based management: adapting hatchery practices to pink and chum salmon abundance

Producing more hatchery Chinook salmon juveniles has recently been proposed in Washington State with hopes of enhanced ecosystem and fisheries benefits. However, it has recently been shown that even the vast North Pacific Ocean cannot support an infinite number of salmon (Ruggerone and Irvine 2018), and it appears that mortality of salmon in the Salish Sea can also be density dependent (Ruggerone and Goetz 2004). Density-dependent effects between pink salmon in particular (the most numerous species, but also chum salmon) and Chinook salmon have been documented by an increasing number of studies (e.g., Ruggerone and Nielsen 2004; Ruggerone et al. 2003), suggesting that salmon compete for food in the ocean, which can lead to reduced growth, delayed age at maturation, and lower survival rates (Ruggerone and Irvine 2018). Preliminary analyses of the relationships between Chinook salmon hatchery production and numbers surviving to adulthood suggest reduced Chinook survival in years when large numbers of juvenile pink salmon also out-migrate. In some regions, this density-dependent mortality may be so strong that large increases in hatchery releases may limit the number of returning adults.
We propose to evaluate the following questions: what Chinook salmon juvenile release abundance values are associated with greatest marine survival rates? How do survival rates vary with different numbers of juvenile pink salmon out-migrating in the Salish Sea? How do these relationships differ among Chinook salmon stocks from various regions of the Salish Sea?