The spread of sea lice (Lepeophtheirus salmonis) from salmon farms probably contributes to declines of some native Pacific salmon populations. Migration normally protects juvenile wild Pacific salmon from the marine ectoparasite in coastal waters by separating juvenile salmon from infected wild adults that are located offshore. Farmed salmon populations dwarf natural coastal host populations, particularly in winter, leading to biomagnification of louse populations. By spring, there may be large numbers of lice on farmed salmon, and this is associated with recurrent parasite infestations of wild juvenile salmon and depressed wild salmon stocks. Abiotic (eg temperature and salinity), biotic (eg predator abundance and food availability), and management (eg periodically emptying farms and applying chemical parasiticides) factors are thought to mediate the louse threat, but none have been well studied. Policy is needed that protects undeveloped juvenile salmon habitats and that supports long-term study of salmon ecosystems, to evaluate the sustainability of wild and farmed salmon.