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en prehistoric man arrived in new parts of the world, something strange happened to the large animals. Mammoths, mastodons, massive ground sloths, woolly rhinoceros, cave bears and large flightless birds suddenly became extinct. Smaller species survived. The large, slow-growing animals were easy game, and were quickly hunted to extinction. Now something similar could be happening in the oceans. That the seas are being overfished has been known for years. What Ransom Myers and Boris Worm of Dalhousie University in Halifax, Nova Scotia, have shown is just how fast things are changing. According to their paper in the latest issue of "Nature," the biomass of large predators (generally the most valuable species) in a new fishery is reduced on average by 80% within 15 years of the start of exploitation. In some long-fished areas, it has halved again since then. The researchers' data came from two sources. Those for fisheries on the continental shelves were derived from standardized research surveys of large bottom-dwelling fish such as cod, flatfish, skates and rays. Those for fish in the open ocean, such as tuna, billfish and swordfish, were estimated from figures collected by Japan's longline fishing fleet (composed of vessels that trail fishing lines with baited hooks at intervals along their lengths). The Inter-American Tropical Tuna Commission, an inter-governmental body that manages tuna fisheries in the eastern Pacific, has a different view of the status of tuna stocks. Robin Allen, its director, says longlines only catch older tuna, and the data the authors are using therefore comprise only part of the actual stock.