How does farmed salmon affect wild salmon?

Coho salmon

Wild salmon populations in many areas of the world have been declining for several decades.  Farmed salmon was thought to be the answer to this problem, but many studies have found harmful effects of aquaculture on the environment (for example, high concentrations of nitrites and ammonia polluting the waters surrounding fish farms).  Despite these problems, farmed fish was still thought to be more helpful than harmful to wild fish populations.  This is now known to be false.   

According to a new study, wild salmon populations living near salmon farms have lower rates of abundance and survival than wild populations unexposed to the farms.  Study authors Ford and Myers looked at five different salmonid species – pink, chum, coho, and Atlantic salmon, as well as sea trout.  They studied populations of these fish living in Ireland, Wales, and Scotland, as well as Canadian populations in Newfoundland, New Brunswick, and British Columbia.  They defined a wild salmon population as “exposed” if at least one fish farm discharged water into wild spawning grounds, or into a bay downstream from spawning grounds (where juvenile fish had a high likelihood of swimming by fish farms).  In almost all cases, having a fish farm upstream dramatically reduced the chances that wild fish would return to their natal spawning grounds.  In many cases the rate of return was less than 50%.   

Some of the causes of this might be pollution or spread of parasites from the fish farms, or problematic interbreeding of escaped farmed fish with wild fish.  Ford and Myers’ study shows that salmon farms have strong negative effects on nearby wild populations. Further research may show what factors cause these effects.

Sources:

-Ford, J., and Myers, R. (2008) A global assessment of salmon aquaculture impacts on wild salmonids. PLoS Biology 6(2): e33. doi:10.1371/journal.pbio.0060033

-Gross, L. (2008) Can Farmed and Wild Salmon Coexist? PLoS Biol 6(2): e46. doi:10.1371/journal.pbio.0060046

Posted by: Anne Accettullo