Sea lice amplified by salmon farm proximity devastate wild salmon populations, causing welfare harm to millions of individual wild fish as they migrate past farm sites during their most vulnerable life stage.
The welfare of wild salmon migrating past farm sites is a significant but underappreciated issue. Juvenile salmon undergoing smoltification are already physiologically stressed by the transition from fresh to saltwater. Acquiring dozens of external parasites during this vulnerable period causes direct physical harm through tissue destruction and secondary infection, and impairs the osmoregulatory process essential for saltwater survival. Fish that survive initial lice acquisition may die later from the secondary consequences. The scale of this welfare harm across millions of wild salmon in major farming regions makes it one of the larger-scale welfare problems in marine environments, driven by a land-use planning failure that allows farm placement near wild salmon migration routes.