Originally Posted by
YVR Cockroach
Do note that a lot of farmed fish, especially atlantic salmon requires a lot of feed derived from the by-catch of (over)fishing other oceans.
Farmed atlantic salmon also contains more heavy metals and mercury as well as PCBs for some reason.
Overpopulation leading to overfishing is the major factor. Many fisheries are far offshore to be affected by pollution as you cite but by overfishing.
For example, it is reported the orange roughy and patagonia toothfish (a.k.a. "Chilean sea bass") take a long time to mature (decades?) but fishing pressure is so heavy that the stock is starting to collapse.
Many deep sea fish are bred in estuaries on the coast.
And far off shore - how far? Pollution travels. In the case of Stellwagen Bank is only 25 miles east of Boston - which makes it 15 miles away from the effluent pipe that dumps fresh water and chlorine, where there is a growing dead zone.
Overpopulation by demanding more food or by pollution - same end result.