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% [+ or -] 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. Les parasites peuvent moduler le succes d'approvisionnement, mais ce phenomene n'est pas bien compris en ce qui concerne les poissons marins, dont les groupements et la repartition parcellaire des proies creent des conditions favorables a une concurrence intraspecifique intense. Nous avons verifie si les infections aux poux du poisson sont associees a la plenitude stomacale des saumons rouges (Oncorhynchus nerka) juveniles sauvages dans le detroit de Johnstone (Colombie-Britannique) durant leur migration vers la mer a partir du fleuve Fraser. Caligus clemensi represente 98,6% des poux du poisson preadultes et adultes et 86,5% des copepodites (individus recemment fixes), alors que Lepeophtheirus salmonis constitue le reste. Nous avons constate que l'etat d'infection est un important predicteur de la plenitude stomacale relative pour les saumons rouges juveniles (masse humide du contenu stomacal divisee par la masse corporelle), comme l'indique la selection de modeles a effets mixtes, et que la plenitude stomacale relative de poissons fortement infectes est de 17%[+ or -]8% inferieure a celle des poissons peu infectes. Cette reduction de la plenitude stomacale relative associee aux poux se produit au moment ou les jeunes saumons rouges en migration passent par un milieu ou la nourriture est restreinte et ou la concurrence est probablement forte. Comme la croissance precoce en mer des saumons juveniles est souvent une variable predictive de la survie, nos resultats soulignent l'importance d'une bonne comprehension des effets subletaux des parasites sur les salmonides et possiblement d'autres especes de poissons. [Traduit par la Redaction]
Introduction Foraging success is tightly linked to growth and survival of individuals, both of which are predictors of fitness and population dynamics (Crombie 1947; Sutherland 1996). Fish frequently experience patchy [...]