Marine pathogens can cause large population declines and heavy losses for fisheries and aquaculture globally. In disease ecology, sublethal effects of pathogens on host traits are largely ignored despite their potential importance to host survival. Two traits that may be susceptible to sublethal effects of pathogen infection are competitive foraging ability and body growth, both of which are common determinants of survival, especially for marine fish. We assessed whether wild juvenile sockeye salmon experience reduced competitive foraging ability and growth when they are highly infected with sea lice – an ectoparasite whose abundance on wild salmon is elevated in part from open-net salmon farms. To address these questions, we performed a competitive field experiment and examined daily growth rings in the otoliths of wild juvenile sockeye captured during their ocean migration from the Fraser River.
Juvenile sockeye salmon that were highly infected by sea lice had lower competitive foraging abilities relative to lightly infected or uninfected individuals. We also found that heavily parasitized sockeye grew more slowly than uninfected fish in the ten days preceding capture. In addition, larger fish grew faster than smaller fish, and there was evidence of an interaction between body size and infection status, indicating the potential for parasite-mediated growth divergence. Our results highlight the potential survival consequences for salmon from the sublethal effects of pathogens like sea lice. As the aquaculture industry expands, understanding the impacts of pathogens on wild fish – including those which occur indirectly – may be critical for conserving vulnerable populations.