Freshwater growth can provide a survival advantage to Interior Columbia River spring Chinook salmon after ocean entry

Abstract

ABSTRACT: A prerequisite to effectively managing fish populations is to understand what factors and processes, including predation and changing environments, affect the survival of individuals. In anadromous fishes, the transition from freshwater to marine habitats is considered a critical period regulating population abundance due to high and variable mortality rates. During this period, conditions experienced in freshwater may influence size- and growth-selective mortality in the ocean. To determine if size- or growth-selective mortality occurred in juvenile Interior Columbia River spring Chinook salmon Oncorhynchus tshawytscha as they migrated through the Lower Columbia River and Estuary (LCRE) and during early marine residence, we examined 2 cohorts in years with differing survival (2016 and 2017). We reconstructed the size and growth of individual Chinook salmon from otoliths and compared these attributes in fish caught at 4 sites in the LCRE to those caught in the ocean off Oregon and Washington. We observed evidence of growth-selective mortality in 2017 but not 2016. Specifically, in 2017, when overall survival was lower, individuals caught in the ocean grew significantly faster in freshwater than individuals caught in the estuary. Given that the fish had resided in the ocean for an average of 30 d, these results indicate growth-selective mortality in 2017 occurred soon after ocean entry. The finding that growth in freshwater may impact marine survival adds to the growing body of evidence that processes occurring both prior to and after ocean entry impact the marine survival of this species.