Fishing vessels in the North Atlantic have long caught tuna and swordfish using 100-kilometer-long lines that can hold as many as 1200 baited hooks. But as the numbers of these fish have decreased, the ships have been catching more sharks, which they sell for their meat and their highly lucrative fins. The trouble is that sharks, as slow-growing top predators, are vulnerable to overfishing. Just how vulnerable? To find out, an international group of researchers tracked where migratory species of ocean-going sharks spent most of their time. They put electronic tags on six species in the North Atlantic, tracking 99 individuals for an average of 80 days. They found—somewhat surprisingly—that sharks cluster predictably in areas with abundant food, such as the Gulf Stream and the Azores islands. To measure how often they might cross paths with commercial fishing ships, the team also analyzed location data for 186 vessels that fish for sharks with home ports in Spain and Portugal. Comparing the locations, the researchers found an 80% overlap in the ranges of fishing vessels and the most common species, the blue shark (see photo) and the mako shark, they report this week in the Proceedings of the National Academy of Sciences. The researchers determined that blue sharks spent an average of 2 days per month within 50 km of long-line hooks, but that one spent as many as 20 days a month in close range to the fishing vessels before being caught. The fleets’ “intense focus” on sharks puts the sustainability of the populations at risk, the researchers conclude. They add that the most important step to prevent the decline of shark species would be to place international limits on how many can be caught. If that were to occur, they say, fishermen could prevent shark captures by outfitting their fishing lines so that sharks can bite their way free.