There are a couple of bright spots in the bleak picture for wild tuna, whose overfished and mercury-laden stocks are collapsing worldwide. It's U.S. Pacific albacore, caught by troll or pole, which is endorsed as a "best choice" by Monterey Bay Aquariaum (MBAYAQ) Seafood Watch. This albacore, from the only tuna fishery certified as sustainable by the Marine Stewardship Council (MSC), is in season now, from July through September. A younger, smaller tuna, caught within 200 miles from the coast, it has high levels of heart-healthy omega-3 fatty acids, and moderate mercury levels rather than the high levels found in its overfished cousins, including bigeye and the noble bluefin. British Columbia albacore, and skipjack, if troll or pole caught, are also on MBAYAQ's green-go-fish list.
This eco-albacore, which has pinkish, almost white flesh when cooked, is to be distinguished from canned albacore, which has higher mercury levels than canned light (chunk) tuna, generally a moderate mercury fish. For more info and where to buy, see PacificAlbacore.com. We can safely eat six ounces of moderate mercury fish once a week, per the FDA's advice, while more cautious organizations such as the Environmental Working Group recommend less frequent tuna noshing calibrated to your weight. To be safest, aside from eating no tuna, children and women of childbearing age should probably only eat six ounce tuna steaks about once a month. For more info on seafood safety, see Environmental Defense's great overviews on fish contamination and its Seafood Selector card, and the FDA's site.
A correction to Monday's daily bit: Only Pacific sardines are MBAYAQ best choices; Herring, or Atlantic sardines, which are trawled and can thus damage the ocean floor, have been demoted to good alternatives (still not bad).