which salmon organs would contain most of say the heavy metals? they could be selectively cut out, and any below-regulation levels sold (domestically and abroad) can be seen as a free heavy metal remediation effort in those bays and fjords.

Making a closed loop system possibly including IMTA either on land (why not right at the salmon processing facilities saving transport costs and assets like boats, its cheaper to have no vehicle to maintain) or in water like the "Eggs" described in the article sounds like a no-brainer for the industry, who cares about 10x capex when the bottleneck is not capex but bureaucracy to get expansion permits? Once you realize capex is not the bottleneck (you can reuse the new system), it allows a lot of other costs to be eliminated, like disease and parasites: commingling fish with expensive health monitoring and management at density ρ can be replaced with smaller cells, each cell their intake and exhaust tubing, and the reservoir contact rate falls precipitously for the same density ρ: is the switchover investment more expensive than the current system? yes, but growth isn't limited by capex. This switchover would be desirable (true damage like the harmful fish feces concentration is prevented; false damage like bioaccumulating harmful heavy metals inserted by other human activities becomes none of the fish farmer's problems at all). The taxation excuses will dry up.