Shark species are in big trouble. A new study--the broadest to date--reveals shocking declines. Even the least affected populations, those of mako sharks, have shrunk by 40% in the past decade or so, scientists estimate; hammerhead numbers have plummeted nearly 90%. The study makes the case for more active conservation of shark species by limiting fishing and creating no-fishing areas.
Observers have suspected that intense fishing of open-ocean fish, such as tuna and swordfish, is bad news for shark species. Many are accidentally caught in longline fishing, which involves kilometer-long lines with hundreds of baited hooks, and their slow reproductive cycle makes it difficult for them to rebound. Fishing of sharks themselves complicates the problem. Studies have shown local drops in certain shark species, but scientists have had trouble assessing the overall drop because many sharks roam widely, making them tough to tally.
To bring the big picture into better focus, Julia Baum, a graduate student in the Department of Biology at Dalhousie University in Halifax, Nova Scotia, looked at data gathered from U.S. longline fishing boats in the northwest Atlantic from Newfoundland to northern Brazil. For eight types of shark, Baum and colleagues examined records of catches between 1986 or 1992 and 2000. After running a battery of statistical tests to rule out confounding factors, they concluded that all species other than makos declined by more than 50%. The team reports its results in the 17 January issue of Science. Fishing is "definitely, without a doubt," what caused the decline, says fisheries biologist Ransom Myers, Baum's adviser.
Although scientists had suspected a sharp decline in shark populations, this study offers the first large-scale proof, says shark biologist Robert Hueter, director of the Center for Shark Research at Mote Marine Laboratory in Sarasota, Florida. He cautions that the percentages of decline are likely more accurate for pelagics, such as the white and blue sharks, than for more coastal species, such as the hammerheads. The only way to stem the decline, say Hueter and Myers, is to have international regulations--such as limits on longlining and no-fishing zones--in areas that are important in the sharks' life cycles.