Assessing nutritional stress in juvenile Chinook salmon during the first ocean winter

Overwinter mortality driven by nutritional stress in juvenile Pacific salmon is one hypothesis linking foraging success to recruitment variability, but empirical data are limited. We investigated whether juvenile Chinook salmon (Oncorhynchus tshawytscha) experience nutritional stress during their first ocean winter in the Strait of Georgia, British Columbia. We sampled 1130 juvenile Chinook salmon originating from east coast of Vancouver Island rivers across three consecutive winters and assessed indicators of nutritional status. Contrary to expectations of severe winter food limitation, the proportion of empty stomachs remained stable throughout winter, and stomach fullness showed modest declines. While condition factor decreased significantly, values remained above those typically associated with starvation. Energy density peaked in December before declining to minimum values in March, suggesting seasonal shifts in energy allocation. Gastrointestinal somatic index, a measure of feeding intensity, declined initially before recovering in late winter, despite decreases in energy density. These results suggest that juvenile Chinook salmon in the Strait of Georgia may experience food limitation during winter, but patterns were not consistent with severe nutritional stress and increased mortality.