Not sure if this is helpful, but I collected lots of fish from Lake Erie harbors in Ohio a few years ago also collected fish for the USEPA for 5 year snap shots of the lake.
Most of the mercury was attributed to coal power plants in the west (Illinois and Indiana) and the airborne mercury was blown to Ohio . The levels were going down due to better scrubbers on the coal plants.
During my testing PCB was rising. The belief was that we were doing so many cleanups that we stirred things up and made it available again. It was expected to be reduced with time.
There is a lot of variables in collection. Fillets, skin on fillets, ground whole fish.
Fatty fish (carp) were worse. Lean small fish that eat a good amount of bugs.... like perch were better.
Also think about how much a fish eats up the food chain. I don't know feed conversion for salmon.....maybe 10# of bait per # of salmon? That is 10x the contaminants levels of it bioaccumulates.