Farmed salmon decimate wild populations by exposing them to parasites

The next time you buy salmon from your local supermarket, think about the hidden costs in each succulent fillet. Compared to wild fish, farmed salmon is far less likely to burden your wallet. But by buying it, you may be placing a much larger burden on the environment. Fish stocks around the world are declining due to over-fishing and 'aquaculture' - the farming of fish - was originally thought to help. But farming brings with it a host of ecological problems.

If the farmed fish are meat-eaters, as salmon are, they must be fed on the proteins and oils of wild fish, which does nothing to alleviate the stress on wild populations. Domesticated farmed fish are also genetically different to their free counterparts, and escapees risk spreading their genes and replacing local genetic diversity.

But the biggest and most immediate problem may be to do with the spread of parasites. Large, crowded and trapped animal populations are an easy target for parasites, and salmon farms are no exception. Farmed salmon are often infested with sea lice, parasitic relatives of prawns and shrimp, that cause direct damage, starve their host and increase vulnerability to disease. But the real problem comes when infected farmed salmon pass their parasites onto wild fish.

Martin Krkošek and colleagues from the University of Alberta believe that salmon farming may be disrupting behaviour that evolved in salmon to protect their young from parasites.

Salmon are known for their massive and demanding migrations in order to mate and lay eggs. Because of these treks, the young salmon enter the ocean several months before the adults and their parasite passengers return. In this way, the youngsters gain precious months' respite from infections during which they can develop unhindered. It's the same strategy that human parents use when they move to the country to raise their children in safer surroundings.

But this safe period is shattered if the ocean is crammed with lice-ridden farmed populations. Across the North Atlantic from Canada to Norway, wild juveniles are being infested with sea lice. The farms are providing the lice with new routes for infecting even more hosts, and the young salmon are not ready for them.

To adults, sea lice are irritating but tolerable, but to the much smaller young, carrying more than two lice is always lethal. Not only do they take up valuable nutrients the juveniles need to grow but they make them more vulnerable to predators and weaken their immune systems.

Krkošek analysed data on salmon and lice populations off the western coast of Canada and revealed disturbing trends. The lice were decimating many local populations of salmon, killing up to 95% of juveniles in some regions.

As aquaculture continues to spread, this study provides us with a harsh reality check and consumers around the world have the power to reverse the trend by protesting with their money. Choosing wild fish over farmed varieties sends a message to the fishing industry that the benefits of buying cheaper fish are outweighed by the costs to wild populations.

Reference: doi: 10.1126/science.1148744