for National Geographic News
North Atlantic sharks are in serious trouble. Scientists searching through fifteen years of fishing logbooks have spotted a precipitous drop in shark populations.
Of the 17 shark species studied, all but two have seen their numbers slashed in half in less than two decades.
"These species are declining, and they're declining really rapidly," said Julia Baum, a biologist at Dalhousie University in Nova Scotia and lead author of the study, which appears January 17 in the journal Science.
Hammerhead sharks showed the most serious decline with an 89 percent decrease in population since 1986.
To Catch a Shark
The U.S. National Marine Fisheries Service (NMFS) has been following the decline of sharks spotted in North Atlantic fishing areas. But the state of several individual species, such as the hammerhead, has remained unclear. And sharks that range across the open ocean, known as oceanic or pelagic sharks, have been an even bigger mystery.
The International Union for Conservation of Nature and Natural Resources (IUCN) puts out a "red list" of threatened species. Of the 21 shark species it assessed in the North Atlantic, 20 received question marks when it comes to population trends, Baum said.
To piece together the status of sharks, the team pored through fishing logbooks from the National Marine Fisheries Service. The boats of U.S. pelagic longline fleets, or boats that head to the open ocean, record the number and type of caught fish. These longlines, which dangle hundreds of hooks into the deep, are set to catch swordfish and tuna.
But sharks can rise to the bait as well. Fishermen record this accidental hooking, called bycatch, in their logbooks along with their regular hauls.
The researchers also looked at shark counts from trained shark observers that sail out on some fishing boats. After poring over 15 years' records, the team ran their shark counts through the wringer, considering under-reported shark catches in log books and checking seasonal changes and fishing conditions that might skew the statistics.
Now, these fishing records have started filling in the blanks. "It's the first time anything has been recorded for thresher sharks and oceanic white tips," Baum said. The numbers of sharks in these two groups, both oceanic, dropped by more than two-thirds.
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