Hi P - link with some further info may be of interest
http://www.nrdc.org/health/effects/mercury/gui de.asp suggesting contamination levels in seafish.
Also as tuna is becoming an endangered species round the world - Mediterranean as well as Pacific - and that it takes 20 tons of white fish to feed 1 ton of tuna it is gaining in momentum to:
avoid if there is a more sustainable and so preferred alternative to choose; to consider the ethical and ecological impact - dolphin friendly catch etc; to reduce the amount consumed anyway due to the tuna being at the top of the food chain so (as noted) will absorb the prey contents of contaminents.
Fish stocks are the subject of loads of debate and comment but as a percentage of farmed species like salmon can also contain other harmful chemicals like PCBs from water run off etc, be fed antibiotics that stay in the fish, the pollution of the seas is continuing - especially in areas that have high deforestation - chemical run off into water courses leads to the rivers feeding the residues into the seas near fishing grounds etc etc, try to get some sort of clarification for the sellers as to the supply source when buying./
The mercury in the tuna by the way is not the one you find in barometers etc - it is still a harmful chemical and advice is avoid if pregnant or very young, and limit the quantity consumed in any given period.
In the UK the Seafish authority (or at least one of them) has a useful site including certificates of the trawlers that have registered to take up the Responsible Fishing scheme - desgined to ensure both commercialy viable rates for the catch but also be environmentally friendly to the stocks as well. Link at:
http://www.seafish.org if you want some info, plus recipes, etc etc.