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.
Some 90% of tuna caught in the northeast Atlantic Ocean and more than 60% of yellowfin samples caught in the Gulf of Mexico contained pollutant levels worthy of health advisories in pregnant and nursing women, not to mention young children. 
In January 2017, the FDA and the EPA released advice regarding eating fish. The guidelines state people can safely eat 2-3 servings of canned tuna per week, 1 serving of either fresh yellowfin tuna or canned, fresh, or frozen albacore tuna. Bigeye tuna should be avoided due to high levels of mercury.
Most of the tuna the team looked at would be considered safe under current guidelines.
But each of the 10 samples of fish contained a particularly toxic set of compounds known to interfere with proteins which regulate cell membranes and defend against toxins, according to Nicklisch. 
“These compounds might lead to accumulation of chemicals in these tuna, because the proteins usually block those compounds in fish, but also in us, in humans.”
The best way to protect yourself from toxins in tuna is to find out where it was caught, but that’s no easy feat.
Dick Jones, president of Ocean Outcomes, a non-governmental organization based in Portland, Oregon, that focuses on improving fisheries, said:
“At the retail level right now, it’s only country of origin that’s required. The only time we see a more detailed description of where the fish was caught is when [companies] want to take advantage of the marketing opportunity.” 
Seafood fraud also makes it difficult to know where your fish comes from. An Oceana report released in 2016 revealed that up to 1 in 5 seafood samples tested worldwide are mislabeled, and fraud can occur all the way up the seafood supply chain.
Human rights abuses, and illegal, unreported, and unregulated fishing also make seafood-traceability a challenge.
Nicklisch said he is hopeful the study will result in improved testing methods for chemicals in food, and improve public information and labeling of seafood.
“The most important part of the take-home message is that it’s important to know where your fish was caught.”