Farmed salmon decimate wild populations by exposing them to parasites


Blogging on Peer-Reviewed ResearchThe 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.

i-10ca5fc7b704aefe50e00bdd0f173bc1-Sealice.jpgBut 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

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Thank you for posting about this. I live in Vancouver, B.C..
A lady scientist has been stating this ( with evidence ) for years but the politicans refuse to listen.
This year's return of adult salmon is very low on the Fraser river. Even sport fishing has been ban.

By rick munn (not verified) on 02 Aug 2008 #permalink

I live in Seal Rock, Oregon, a common bumper sticker around here is "Friends don't let friends eat farmed fish". This is a serious problem in so many ways. Farmed fish is an idea whose time has passed.

When did you ever see a sea lice on a wild salmon with the egg strings pointing forward? Your picture- more than likely provided by Ms. Alexandra Morton- could only have been produced using some special glue that allows her to attach these mature sea lice to weeks-old juvenile salmon. Your post may not be the "proper article" that you think it is.

Can you imagine trying to glue ANYTHING to a fish. They are rather slimy. I wish my dad, a chemist with a serious knowledge of adhesives, were still around so I could run that idea by him.

I'm glad you brought up the concept of farmed salmon. Only when enough consumers demand wild salmon and refuse to buy farmed salmnon will they stop the farming. I always cover this with my environmental science students. I need to buy Ray Troll's great T-shirt about fish farming to accentuate my lecture.

Wow who'd have thunk it?
Thats something I wouldnt have thought of before. And if I had, I would have imagined that wild salmon would have more genetic diversity and their immune systems would be better equipped to fight the parasites than the farmed ones..

I may have to find a new sushi restaurant.

Hi Ed,

Thanks so much for your research into this issue. Salmon farming in open net cages is causing very serious problems here in BC -- it's devastating wild salmon stocks and polluting the ocean ecosystem.

And there's a real urgency for a solution -- Krkosek's research also shows that wild pink populations may be locally extinct in 4 years if something isn't done about the sea lice problem (research published in Science last November.)

Check out for more information about the impacts of open net cages, the mounting scientific evidence against them, and for information about closed system aquaculture, a production system that would reduce or eliminate most of the threats posed by open net cages.

Also, check out for a list of restaurants and retailers that don't serve/sell farmed salmon.

--Tiffany Hilman, Markets Campaigner for the Coastal Alliance for Aquaculture Reform