Katrin Lundstedt-Enkel: As this is my field of research... Farmed salmon have much lower content of contaminants than wild, especially if the wild come from the Baltic Sea. The problem with wild is more ecological. That escaped farmed salmon change the gene-pool in the wild population, that they use medicine in the farms that leach into nature, that the farms cause local eutrophication (nutrient enrichment) below the cages etc. These problems are actively worked with as I see it. The Norwegian fish industry do NOT want to be known as any nature villains.
Mercury... I would not hesitate to eat fish from the sea. Very low mercury levels over all. Some local contamination in some parts of the world, mostly in the tropical part. I would hesitate to eat fish from the Amazonas... (Or anywhere where they use Mercury in the gold mining). Some lakes in Sweden have high levels of Mercury. These were "black listed" until a few years ago. Now it is up to the local council to keep these lakes under surveillance and fish cannot be sold if they originate from one of these (more than 100 000!) lakes.
Dioxins... well another of my field of research. I do not hesitate to eat fish from the sea at all. The dioxin levels are well below the 4 pikogram WHO TEQ/g fresh weight that is the "safety level". And this level is well within the margin for "safe eating" by adding safety factors etc when calculated. The problem is that fatty fish from the Baltic Sea is well above the 4 pg WHO TEQ /g ... My herring and salmon from the Baltic Sea are around 12-30 and 8-12 pg WHO TEQ/g, respectively. For the Norwegian farmed salmon the level is well below 4, difficult to say as most dioxins and furans are below the detection limit. If one calculate the levels as if they were ON the detection limits (if I remember correctly at this late hour) (and I think I do) the level is below 1 pg WHO TEQ fresh weight in the farmed salmon.
In fish from the open water, well away from any human activity the contaminant concentrations can also be high. Biomagnification (my speciality!) causes high levels of some contaminants in the Arctic Sea food web. Human activity is messing with the whole planet, the "pristine nature" is a thing in the past...
