Dynamic coastal pelagic habitat drives rapid changes in growth and condition of juvenile sockeye salmon (Oncorhynchus nerka) during early marine migration

Migrating marine taxa encounter diverse habitats that differ environmentally and in foraging conditions over a range of spatial scales. We examined body (RNA/DNA, length-weight residuals) and nutritional condition (fatty acid composition) of juvenile sockeye salmon (Oncorhynchus nerka) in British Columbia, as they migrated through coastal waters that varied oceanographically over tens of kilometers. Fish were sampled in the stratified, productive northern Strait of Georgia (NSoG); the highly mixed, unproductive Johnstone Strait (JS); and the transitional zone of Queen Charlotte Strait (QCS). Body and nutritional condition responded rapidly to changes in prey availability and were lowest in JS with low prey availability, supporting the Tropic Gauntlet Hypothesis, additionally we saw signs of compensatory growth in QCS. Juvenile salmon leaving the SoG in 2016 had significantly lower condition than in 2015, despite higher zooplankton biomass in 2016. We propose that this was due to the higher abundance of low food quality southern zooplankton species in 2016. This study highlights the importance of including food quality as a parameter to understand changes in fish condition and survival between years. Furthermore, small scale variation in oceanographic dynamics impact foraging conditions and need to be considered when assessing early marine survival of juvenile salmon. ### Competing Interest Statement The authors have declared no competing interest.