High and minimally variable adult marine survival among multiple stocks of Chinook salmon

This is the authors' abstract. We don't add key points for this paper.

Estimates of individual survival across space and time provide critical information on processes that regulate population abundance. Here, we use state‐space models fit to data from more than 500 acoustically tagged Chinook salmon to better understand the factors that shape survival during the final summer and fall of marine residence—a life stage where mortality may be driven by both harvest and predation. Median cumulative survival rates from the continental shelf of Vancouver Island to terminal migration areas were relatively high, ranging from 0.65 to 0.86 across stocks. In most cases, survival rates were also stable along each stock's migration corridor. Survival rates to terminal areas were strongly and positively related to lipid content, but weakly related to body size, which suggests complex trade‐offs between foraging risk and growth. Tagging date had a direct positive effect on survival rate, likely because tags deployed earlier in the year provided more opportunities to observe mortality. Individuals from cohorts with higher exploitation rate indices had lower survival rates, and accounting for exploitation rate effects strongly reduced among‐stock variability in survival rate. High, spatially stable survival rates suggest that adult Chinook salmon marine migrations, unlike those of many juvenile Pacific salmon, are not associated with severe survival bottlenecks. After accounting for variability in exploitation rates, our results are consistent with natural mortality rate assumptions used in current stock assessment models. These findings provide empirical benchmarks that can inform stock assessment model development and reinforce the importance of incorporating harvest dynamics into ecosystem‐based management of Chinook salmon.