The last years they have hovered around 95% being very good or good. So statistically you are very wrong. But sure there are some localities that have bad conditions, and those should be closed and new localities should be opened for those farmers. Such as offshore localities when the technology is mature. Many smart people are trying to figure that out with strong scientific backing, but the report from havmerden is avaliable to read if you understand norwegian
In norway there are many environmental institutions following the state of the fjords, so I wouldn't be too concerned.
They are _still_ using hydrogen peroxide to combat lice for example, despite it's effect on shrimp and plankton.
I am all for farming fish, but on land in closed systems.
One of the concepts is closed-containment farming, where the fish are kept in a sealed enclosure. The fish can be kept below the sea lice level, and all waste can be collected. The downsides are cost, failure modes, and welfare (the fish have less area than open-pen near-shore farms, and will never see sunlight, for example).
We take extreme care not to flush our own sewers into the sea, yet happily permit salmon farms with the equivalent fertilizing effect of a town of 25000 having all their toilets flush everything out on the shore.