The levels of toxins that accumulate in a fish depend on its environment. At our restaurant, we sometimes get hamachi from Japan, Hawaii, Australia, etc. I don't trust fish that comes from any area near an urban concentration. The only real way to know where your fish comes from is by buying it from a reputable fishmonger. I wouldn't recomend buying fish at a grocery market. To know the exact concentration of mercury I think you'd have to analyse thi fish as an individual.

My two cents.