Make them move their operations into concrete tanks, or some type of pond, on land, a properly setup operation would include the ability to use the fish poop for nutrients to also farm shrimp/prawns, the poop would be food for microorganisms/algae/sea weed, the shrimp/prawns would feed on that. The Israelis have setup this sort of system in a number of locations in the desert and have run them successfully for years, they use different fish than salmon of course, but the principles would be the same. This would also have the benefits of preventing escapements of the salmon, and keeping the seals from stealing from the pens, and also save seals from getting stuck in the nets of the pens and drowning (a fairly common occurrence I hear).