Marine biophysical conditions influence the vertical and horizontal distribution of sub-adult Chinook salmon in nearshore marine waters

ABSTRACT: The present study quantified the vertical and horizontal distribution of sub-adult Chinook salmon Oncorhynchus tshawytscha in nearshore marine environments. Depth-specific hook and line sampling was conducted along the Pacific Ocean coast of Washington, USA. Our analysis, based on 187 Chinook salmon from 1299 sampling deployments and 6616 hooks, revealed a wide distribution of salmon in nearshore marine waters, with distinct patterns associated with fish size and age. Chinook salmon that spent one winter in marine waters were more likely to be caught at greater depths than those in their first year at sea, and larger fish were found at greater depths than smaller fish. The probability of Chinook salmon capture varied with depth, showing a higher likelihood of capture at midwater (>15 m from the surface and >5 m from the bottom) and near (<5 m) the bottom compared to near (<15 m) the surface. Additionally, environmental variables such as sea surface temperature, sea surface chlorophyll a, minutes to low tide, and boat speed unimodally influenced capture probability. Our study contributes valuable insights into the spatiotemporal ecology of Chinook salmon, offering a more mechanistic perspective for their management and conservation. The identified relationships between environmental covariates and Chinook salmon distribution can be used to inform life cycle models used to manage and protect this at-risk species and the ecosystem processes that depend on them, particularly in the context of changing oceanic conditions and their role as both predator and prey in marine ecosystems.