PS 14-184 - Microbial infectious agents are associated with sea louse infection in juvenile Pacific salmon (Oncorhynchus spp.)

Abstract: At the intersection of food security and infectious disease lies the issue of animal agriculture; such operations, on land and in the ocean, can harbour pathogens that spill over into at-risk wildlife populations. There is concern that open net-pen salmon farms, which overlap with the migration routes of wild salmon, elevate the risk of pathogen transmission from farmed to wild fish. Although ectoparasitic sea lice (primarily Lepeophtheirus salmonis and Caligus spp.) are the most well-documented pathogen associated with salmon farms, transmission of microbial pathogens from farmed to wild fish is an area of growing concern. While there is reason to believe sea louse parasitism may indirectly or directly predispose fish to secondary infection, there has never been an evaluation of how sea lice co-occur or interact with other pathogens in wild Pacific salmon (Oncorhynchus spp.).

To determine the co-infection patterns of sea lice and viral, bacterial, and fungal infectious agents on wild juvenile Pacific salmon, we coupled molecular-genetics infectious-agent data with assessments of sea lice on outmigrating Pacific salmon. In 2015 and 2016, the Hakai Institute Juvenile Salmon Program (JSP) collected juvenile sockeye (O. nerka), pink (O. gorbuscha), and chum (O. keta) salmon by hand-pulled purse seine from 14 sites in coastal British Columbia located before, during, and after the wild salmon migrated past salmon farms. For each fish, the abundance of 2 species of sea louse (Caligus clemensii and Lepeophtheirus salmonis) was visually determined and gill tissue was collected and assayed for 47 infectious agents of concern (i.e., viruses, bacteria, and fungi). We evaluated the presence and infection intensity of sea lice and infectious agents, then established how microparasite infection is related to that of sea lice across salmon species using generalized linear mixed-effects models.

Our preliminary results have identified several key infectious agents that are associated with sea louse infestation in wild juvenile salmon. These findings may suggest that sea lice facilitate infection by other pathogens­­––potentially as a vector­­––or simply that individual hosts have underlying conditions causing them to be secondarily more susceptible to infection by multiple agents. Regardless of the mechanism, this work emphasizes the importance of aquaculture management strategies that reduce the risk of pathogen transmission between wild and domesticated salmon.