meco,
Are you perhaps referring to PCBs (poly chlorinated biphenyls.) If so, then the largest worry is for fish that spend their early lives in contaminated rivers (like sea bass in the Hudson). The Hudson's PCBs come primarily from GE's dumping of transformer oils over past decades and it lodges in the bottom mud almost forever.
The bigger worry for larger ocean fish like tuna is methyl mercury.
There's a push beginning to get amounts of these contaminants listed on the cans but it might be a long time coming.
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PCPs in canned tuna 
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