Aquaculture-Induced Changes to Dynamics of a Migratory Host and Specialist Parasite: A Case Study of Pink Salmon and Sea Lice

Exchange of diseases between domesticated and wild animals is a concern for conservation. In the ocean, many species display life histories that separate juveniles from adults. For Oncorhynchus gorbuscha and parasitic Lepeophtheirus salmonis, infection of juveniles in early marine life occurs near salmon sea-cage aquaculture sites and is associated with declining abundance of wild salmon. Here, we develop a theoretical model and use it to explore the effects of aquaculture hosts, acting as reservoirs, on dynamics. Because pink salmon have a two-year lifespan, even- and odd-year lineages can have consistently different relative abundances. This phenomenon, termed "line dominance", serves as a useful probe for the dynamical effects of introducing aquaculture hosts into this host-parasite system. We demonstrate how parasite spill-over and spill-back with aquaculture hosts can either increase or decrease the line dominance in an affected wild population. The direction of the effect depends on the response of farms to wild-origin infection. If aquaculture parasites are managed to a constant abundance, independent of the intensity of infections from wild to farm, then line dominance increases. On the other hand, if wild-origin parasites on aquaculture hosts are controlled proportional to their abundance then line dominance decreases.