Predation intensifies parasite exposure in a salmonid food chain.
Parasites can influence ecosystem structure, function and dynamics by mediating predator-prey interactions. Recurrent infestations of the salmon louse Lepeophtheirus salmonis associated with salmon aquaculture may mediate interactions between juvenile salmonids. Louse infection increases pink salmon Oncorhynchus gorbuscha susceptibility to predation, resulting in the trophic transmission of lice (with an adult male bias) to coho salmon O. kisutch predators. While experimental evidence is accumulating, the extent to which trophic transmission structures the distribution of lice among juvenile salmon in the wild is unknown. We used a hierarchical modelling approach to examine the abundance and sex ratio of salmon lice on juvenile pink and coho salmon, collected from a region of salmon aquaculture during sea louse infestations, to test the hypothesis that trophic transmission of salmon lice increases infection on coho that feed upon infected pink salmon prey. As predicted, coho had higher adult and pre-adult louse abundance than their pink salmon prey, and louse abundance was more adult male biased on predators than sympatric prey. We estimate that trophic transmission accounts for 53-67% of pre-adult and adult louse infection on coho. Synthesis and applications. These results suggest that, by evading predation, salmon lice can accumulate up juvenile salmon food webs. Predators, such as coho, can experience a two- to threefold increase in parasite exposure through predation on infected prey than would otherwise occur through passive exposure to infective larvae. Thus, predation may intensify parasite exposure and undermine the protection to ectoparasites conferred by the larger body size of predators. For larger predatory wild juvenile salmon, the risk of louse transmission from farmed salmon may therefore be greater than previously appreciated. These findings argue for an ecosystem perspective in monitoring and managing the marine environment in areas of intensive salmon aquaculture that includes the productivity and ecological interactions of all salmonid species.