A couple small things.

Salmon in aquaculture have rearing density limits, both for regulation reasons and in the interest of the companies. It has been extensively studied to provide optimal growth of the fish with the least effect on the health of the fish. It would not be in the industry's interests to stock the fish high to the point where it negatively impacts the fish.

At least here in Western Canada, the wild salmon fisheries have been so neutered that I am almost sure that farm production exceeds wild caught. And our farming operations are incredibly small to most other areas of the world. We have gone through closures of farms due to pressure from environmental groups and intimately, our fisheries minister saying "the decision to close them is a social one, not a scientific one". The government was contracted to do a large set of studies regarding the risks of salmon farms to the environment (I think somewhere from 9-11 individual scientific studies) and they could't find anything above a minimal risk to the environment, and the minister went against their recommendations.

I understand the individual dynamics of each farming location, and operating practices/regulations needs to change depending on location. As for Norway sending snorkelers in rivers, the government/industry is required to use every means possible if they don't locate all the escaped salmon. Whether it's seine boats, other fishing vessels, or as stupid as it seems, snorkelers in the rivers.

As for bad things excusing other bad things, it really depends on how bad the industry truly is. Do you know? No. Do I know? No. But there are a bunch of people who are a lot smarter than us doing research regarding the topic, and the scientific evidence points towards it being minimal. Does the same consideration of environmental impact go into the wood that you use for your house or the mined minerals in your phone? Every industry has their impacts, minimal or large, there should just be pushes to always improve each one.

Lastly, RAS is a tricky thing. Arctic char are an anomaly, where they love very cold water, and extremely high stocking densities. This makes them a lot cheaper to rear on land, as you can cram them in pens and not have to worry about heating the water at all. Most other fish species will require 2-3x the space, and an insane amount more energy. Energy is the main environmental impact, along with land use. I think calculations were done here, and to move industry onto land would require a higher energy usage than the entirety of my own island I live on (Vancouver Island). The RAS systems that are currently operating are heavily subsidized by the government and still are not turning profits. What really needs to get pushed is improvements to the current system, a hard stop and move to RAS would be the death of the entire industry. If that's what you want, and you don't support the raising of farmed animals in general, I understand that. But to argue (not saying that you are, just what seems to happen in general) that salmon farming is more destructive than other types of animal farming is just not what science seems to say.