Parasitism and food web dynamics of juvenile Pacific salmon

There is an increasing realization of the diverse mechanisms by which parasites and pathogens influence the dynamics of host populations and communities. In multi-host systems, parasites may mediate food web dynamics with unexpected outcomes for host populations. Models have been used to explore the potential consequences of interactions between hosts, parasites and predators, but connections between theory and data are rare. Here, we consider sea louse parasites (Lepeophtheirus salmonis), which directly increase mortality of juvenile salmon hosts (Oncorhynchus spp.). We use mathematical models and field-based experiments to investigate how the indirect effects of parasitism via predation influence mortality of sympatric juvenile chum salmon (O. keta) and pink salmon (O. gorbuscha). Our experiments show that coho salmon predators (O. kisutch) selectively prey on pink salmon and on parasitized prey. Preference for pink salmon increased slightly when prey were parasitized by sea lice, although there was considerable uncertainty regarding this result. Despite this uncertainty, we show that even the small increase in preference that we observed may be biologically significant. We calculate a critical threshold of pink salmon abundance above which chum salmon may experience a parasite-mediated release from predation as predation shifts towards preferred prey species. This work highlights the importance of considering community interactions, such as predation, when assessing the risk that emerging parasites and pathogens pose to wildlife populations.