Fish raised in a fish farm is reasonably sustainable, especially those raised in lakes.
It's not though -- thats the myth to be debunked. Also the parasites were disgusting -- farmed fish are slowly being eaten alive -- for every four salmon produced only 1 makes to the chopping board for human consumption. The others succumb to the parasites and are simply discarded creating their own pollution problems.
I didn't know this before the documentary. He covers advanced countries like Norway and Japan as well. So this isn't like some developing country who can't even get onboard with human rights or international law.
There are also serious welfare issues with fish farming - for example, horrible mechanical and chemical treatments of salmon to remove lice, high use of antibiotics, terrible life expectancy of feeder-fish (which are introduced to eat lice off of salmon), poor environmental conditions for the fish (largely because so many fish are packed into such a small area), high rates of disease (again largely because of high stocking density).
For example, aquaculture can be moved offshore to large farm, mobile farm vessels, so the pollution is spread about instead of being concentrated at a small section of coastline. These vessels are new though, and an accident or freak weather even could result in millions of farmed fish being released to the wild - farmed fish are not the same, genetically, as wild fish, so the effects on wild stocks from that many escapes could be significant.
Another new solution is closed-containment aquaculture, where the fish are placed inside a sealed sphere in the ocean. This approach allows all waste to be collected at the bottom of the vessel, and then turned into fertiliser. There are welfare concerns around increased stocking densities, and shutting hundreds of thousands of fish off from the outside world.
Another solution that is (inexplicably, IMO) seeing huge investment are land-based, RAS (Recirculating aquaculture system) farms. These too can collect all fish waste, and it's touted as a way to prevent disease. There are concerns here too though - using arable land for fish farming doesn't seem like a very good idea, RAS farmers use horribly high stocking densities (they must, to be profitable), and there have been several incidents where disease has been inadvertently introduced from the outside (in such close quarters, the effects are of course disastrous).
OK, so that still all sounds pretty gloomy - realistically the only way for things to get much better is through regulation. But better welfare increases farming costs, and like anything, there is always going to be a race to the bottom (e.g. if Norway increases costs too much for farmers, more move to Chile etc).
Moving farming to vegetarian species like tilapia would solve this but it's going to take a monumental effort to change the attitudes of generations.
I know the united states and Australia have some cleaner operations, but they’re much lower density and nowhere near as profitable. I’m personally fine with that, but I don’t think most people are. Hence the constant races to the bottom
E.g. for salmon
> On a dry-dry basis, 2–4 kg of wild-caught fish are needed to produce 1 kg of salmon. The ratio may be reduced if non-fish sources are added. Wild salmon require about 10 kg of forage fish to produce 1 kg of salmon, as part of the normal trophic level energy transfer. The difference between the two numbers is related to farmed salmon feed containing other ingredients beyond fish meal and because farmed fish do not expend energy hunting.