HE TARPON’S FINS scissored back and forth as it searched for baitfish, and as I watched from the deck of a panga, sixty feet away. It was working the outside edge of a miles-long smear of brown-gold “macro-algae”—a form of seaweed called “sargassum.” I’d first noticed the field of free-floating weed from the air, as we flew to Belize’s Ambergris Caye. From far above, it looked like blobs in a lava lamp. Up close, it was more like an oil-slick made
