I’m old enough to remember when salmon was a luxury food in Britain but now it’s commonplace. So far as I know, that’s largely due to the salmon farming industry of Scotland. It’s great being able to eat salmon so often and so cheaply but as Hugh Fearnley-Whittingstall and others have pointed out, farmed salmon are fed on wild fish. Infectious diseases affecting wild fish are a concern too, and so is pollution from the fish farms.
James Simpson at the Marine Stewardship Council sent me some references about how come salmon gets the MSC logo. Meanwhile, Canada´s Atlantic Salmon Federation wants more salmon farming to be done onshore.
Science on the Land 
“James Simpson at the Marine Stewardship Council sent me some references about how come salmon gets the MSC logo, and I’m sorry to say that I haven’t yet read those, but I will read them.”
It’s an issue I’ve been wondering about myself, so I’d love to know what he says.
It’s in a comment to my post here about supermarket fish.
The link is http://www.msc.org/business-support/environmental-improvements
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