On the decline of Pacific salmon and speculative links to salmon farming in British Columbia

Pacific salmon abundance along the West Coast of Canada has been in sharp decline since the early 1990s. Declines have been most severe for coho and chinook salmon despite large additions of hatchery-reared fry and smolts. There is particular concern for populations of wild coho because, in addition to low abundance, up to 80% of the juvenile coho in the coastal waters has been identified as being produced by enhancement projects. The most likely reasons for the decline in Pacific salmon stocks include a combination of climate change, overfishing, and freshwater habitat destruction. There have also been suggestions that salmon farming in British Columbia has contributed to the decline of salmon stocks. The hypothesized effects of salmon farming include potential ecological interactions as well as disease concerns. In this paper, we consider the effects of climate change on the abundance of wild salmon stocks as well as potential genetic, ecological, and disease concerns. Shifts in climate in 1977 and 1989 resulted in significant changes in production for a number of marine fish species including Pacific salmon. These climate-related changes, combined with local overfishing and the loss of freshwater habitat, have left some salmon stocks at very low levels. Large-scale salmon enhancement projects have also resulted in significant ecological and genetic interactions with wild salmon, particularly for coho and chinook stocks. These interactions have tended to reduce genetic diversity and result in the replacement of wild salmon by hatchery fish. Hatcheries also represent a potential source of disease pathogens although the magnitude of the problem is difficult to quantify because the disease agents of concern are widespread in both wild and hatchery Pacific salmon as well as a number of non-salmonid hosts. In addition, the same antibiotics used in the salmon farming industry are also used in salmon enhancement projects, making it difficult to identify the source of some pathogens. Although farmed salmon are also a potential source for these disease pathogens, surveys of pathogens in wild and hatchery fish show no patterns that could be attributed to salmon farming. Recent improvements in fish husbandry, including the development and widespread use of vaccines, have also reduced the risk of disease transfer from farmed fish to wild or hatchery fish. The combined evidence indicates that salmon farming, as currently practiced in British Columbia, poses a low risk to wild salmon stocks particularly when compared to other potential factors.