Heavy sea louse infection is associated with decreased stomach fullness in wild juvenile sockeye salmon

Foraging success can be mediated by parasites, but this is poorly understood for marine fish whose aggregations and patchy prey fields create conditions for intense intraspecific competition. We evaluated whether sea louse infection is associated with decreased stomach fullness of wild juvenile sockeye salmon (Oncorhynchus nerka) in Johnstone Strait, British Columbia, during their marine migration from the Fraser River. Caligus clemensi comprised 98.6% of the pre-adult and adult lice and 86.5% of the copepodites (freshly attached juvenile lice); the rest were Lepeophtheirus salmonis. We found that infection status was an important predictor of relative stomach fullness for juvenile sockeye (wet stomach content mass divided by body mass), as indicated by mixed-effects model selection, and that highly infected fish had 17% ± 8% lower relative stomach fullness than did lightly infected fish. This louse-associated reduction in relative stomach fullness occurs as the juvenile sockeye migrate through a food-limited environment and, presumably, elevated competition. Given that early marine growth for juvenile salmon is often a predictor of survival, our results highlight the importance of understanding sublethal effects of parasites on salmonids and possibly other fish species.