This one may seem like a bit of a surprise, as tuna is notorious for high mercury levels. But the Seafood Watch isn't just referring to any old albacore tuna. They're quite specific in their recommendation, as is often the case. Look for troll- or pole-caught albacore tuna (fresh, not canned) from the United States or British Columbia, whose populations are in fairly good shape, and fishing method results in little bycatch. In light of its high levels of omega-3s and reasonable sustainability levels, the Seafood Watch placed it on the list despite its moderate mercury levels.
Kids 6 years old and younger should eat no more than two servings per month, kids 6 to 12 years old no more than three, and adults should limit themselves to four servings per month.