McClatchy Newspapers: Near the end of a 12-day cruise in the Gulf of Mexico to study the habitat of just-hatched Atlantic bluefin tuna, scientist Jim Franks came upon fields of oil sheen as far as he could see. Mixed with the oil were large amounts of sargassum, the golden brown alga that drifts at the whim of winds and tides and shelters the quarter-inch-long bluefin tuna larvae. How the young will fare and what will happen to the population of bluefin tuna will affect a wide range of people, …