by Mike Barrett , Natural Society:
Scientists from Scripps Oceanography say in a new report that tuna caught in industrialized areas of the Pacific and Atlantic oceans contains 36 times more pollutants than tuna caught in remote parts of the West Pacific. 
For the study, published in June 2017 in the journal Environmental Health Perspectives, researchers looked at toxin levels in tuna caught around the world and found that the source of the fish can affect how safe it is to eat, as much if not more so than its species.
Lead author Sascha Nicklisch, a postdoctoral researcher at Scripps Institution of Oceanography at UC San Diego, said:
“The pollutant levels in seafood — and tuna in our case — can be heavily determined by the location where it was caught. It is important to know the origin of catch of the fish, to know the amount of pollutants in your fish.”
The researchers tested the tuna for the presence of pesticides, polychlorinated biphenyls (PCBs), and flame retardants. The 3 contaminants are part of a class of chemicals known as persistent organic pollutants, or POPs, which accumulate in body tissue and progress up the food chain.  
Greater concentrations of toxic pollutants tend to exist in large fish and predators, so tuna is a decent way to track them. And since yellowfin is a fairly big fish and has a shorter range than other tuna species, researchers were able to use them to study regional pollutants. 
“They stay in the location where they are born and hunt, so we tried to use these tuna to create a snapshot of local contamination.”
For the study, 10 types of fish were gathered from Tonga, Panama, Louisiana, Hawaii, Guam, and Vietnam. Researchers screened the fish for 247 toxic compounds and calculated concentrations of pollutants from each of the 8 regions.
On average, toxin levels in tuna sampled from the most heavily polluted areas were 36 times higher than those found in the least polluted areas. Among the individual fish, toxic levels in the most and least contaminated tuna samples varied by a factor of 180, the researchers found.