Effects of host migration and aquaculture on disease threats to wild fish populations

Ocean ecosystems and fisheries are in decline. At the same time global fish consumption continues to rise and this has driven the rapid growth of aquaculture. But does aquaculture benefit wild fish? I will discuss recent research on parasite interactions between wild and farm salmon in Pacific Canada. Salmon exemplify a new consequence of host migration for disease ecology, which I term migratory allopatry – a migration driven allopatric barrier to parasite transmission among host age classes. Migratory allopatry protects wild juvenile salmon from parasitic sea lice (Lepeophtheirus salmonis) for the first three months of salmon marine life. Aquaculture can undermine this disease refuge because farm fish are sympatric with wild juvenile salmon. I will describe field surveys and experiments that are synthesized by mathematical models. The models account for salmon migration, parasite dispersion, infection, and development leading to quantitative estimates of the magnitude and spatial extent of sea lice transmission from farm to wild juvenile salmon. The model yields estimates of mortality in wild juvenile salmon populations caused by farm origin parasites that range 9-95% for pink (Oncorhynchus gorbuscha) and chum (O. keta) salmon depending on the number of salmon farms and exposure time along any of several studied migration routes. I then adopt a simple mathematical model that is empirically parameterized to predict salmon population persistence or collapse in response to parasite transmission from farm fish. This disease threat may challenge the sustainability of coastal ecosystems and economies.