Most larger, longer living, cultivated fish are contaminated, not tainted. While Yellow fin, and dolphin are highly guarded as a delicacy, they are high in Mercury deposits. Due to their growth time, and harvested weights.

Pacific Salmon, wild caught, and farm raised are used quite a bit in Sushi, along with eel, and tuna. The older the fish, the higher the content.

There are many factors in the farm raising, and cultivation of Salmon that can introduce these fish to Mercury. The feeding, and area in which they are being raised can lead to Mercury contamination, and along with the processing plant, can also introduce these fish to being tainted.

The overall best practice is to use younger harvested fish, and not wild caught. Being a chef for over 20 years, I have purchased accidentally, higher mercury contaminated tuna. Albacore is considered the best, and most expensive, but after some research I discovered White Albacore to be the highest in Mercury content, and stopped buying it.

While in Sushi production, I switched to a salt water eel variety, rather than a fresh water species. I also started using processed seaweed, rather than the fresh, again to lower toxins in my productions.

I'd rather know I'm serving higher quality, with a lower quality fish, than serve a considered high quality fish, with a toxin contamination. Being practical is beneficial.