VERY good question, MJYP, and one that has taxed my brain considerably. As you say, there is a consumer demand, and it will need to be filled somehow.
I found myself in the odd position of defending salmon nets in the recent debate on the 'Atlantic Salmon' board - although nothing would please me more than to see them all gone! In my view, a moderate harvest from identified, prolific runs of fish is the 'least worst' option, at least until a better way of salmon farming can be found. There are undoubtedly rivers that can sustain some netting effort without threatening the survival of the species, although it obviously isn't good for rod fishermen.
The problem in the UK is that salmon have moved in the last 10-15 years from a luxury product to one of the cheapest forms of animal protein you can buy. As a result, the demand is dramatically higher than before, and only a really dramatic health scares, like the recent one, will put that genie back in its bottle. But trying to educate the consumer - to reduce or cut off the market for farmed salmon - is one way to help, and is an integral part of Bruce Sandison's Salmon Farm Monitor campaign over here.
Incidentally, I've just seen a post by Bruce on a UK board, noting that a planning application has just been lodged here for a land-based farm. The only figures I've seen (which were here some time ago) indicated that this would likely make the product so expensive that it would not be able to compete in the market. But if it can be made to work, even if the consumer has to pay a bit more, I think this is probably the best solution. The ecological problems associated with marine aquaculture (notably effluent, lice/diseases and escapes) can all be more or less controlled in an inland farm.