Makes sense about pole caught tuna. The issue is that mercury gets more concentrated as you move up the ocean food chain. Even little fish like sardines have some, but it's in such low concentrations as to be insignificant. The somewhat bigger fish that eat lots of the little fish have higher concentrations of mercury, and the bigger fish that eat them have still more. It continues that way up the food chain to the biggest predator fish like tuna, shark, and swordfish. It also follows that the smaller, immature predator fish (i.e., pole caught) would have lower concentrations than the ones who have been around longer to feast on other fish.
That said, I wouldn't worry too much about eating say, 4 cans of tuna a week, especially if it's the chunk light kind. (Solid white albacore is higher in mercury, apparently.)
Eating 4 cans a day seems a bit risky, however. Why not eat fish from lower on the food chain? Sardines in olive oil, for example, or herring? Tasty and very high in Omega-3s.