Phenological mismatch, carryover effects, and marine survival in a wild steelhead trout Oncorhynchus mykiss population

Abstract

• Annual phenological mismatches decrease survival of steelhead trout smolts. • Larger steelhead smolts at outmigration had higher ocean survival. • Optimal outmigration date varied annually. • Freshwater and marine growing conditions act together to influence ocean survival. • Match/mismatch hypothesis may be a useful tool for predicting salmonid recruitment. Climate-driven changes in the oceans, such as shifts in prey timing and abundance, could influence variability in population productivity of marine fishes. For example, according to the match/mismatch hypothesis, the temporal matching of the young salmon outmigration from freshwater to the ocean relative to the timing of availability of their prey could influence their marine survival. Indeed, understanding patterns and processes of marine survival is particularly pressing in many salmon and steelhead trout populations due to recent declines. To determine whether phenological mismatches between juvenile salmonids and their prey could contribute to low ocean survival, we analyzed the migration timing and ocean survival of 22,116 tagged juvenile steelhead trout Oncorhynchus mykiss over 12 years from the Wind River, Washington State, USA. We used a Bayesian multilevel modelling approach with variable selection to assess how survival was associated with body size, river exit date, the biological spring transition date (the day when northern zooplankton first appeared in the coastal region near the Columbia River estuary), a…