HALIFAX, CANADA—In the fall of 1990, a few humpback whales showed up off the coast of western South Africa where they had rarely been seen before. Over the next couple years, a few more showed up, then a few more. Today, nearly 200 of the giant ocean mammals mill around a piece of ocean smaller than a U.S. football field for several months out of the year. Now, scientists think they know what’s luring what may be the largest global gathering of these cetaceans: masses of free food. Hungry humpbacks travel thousands of kilometers to feast on a rich buffet of tiny crustaceans called krill, researchers reported here this week at the biennial meeting of the Society for Marine Mammalogy.