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Biting the shark Add to ...

From the dodo and the passenger pigeon to the shark and the whale, there seems no species that humankind can't annihilate or endanger if it sets its mind to it.

Consider the basking shark, a filter-feeding creature that has reached lengths of 12 metres and dines on plankton and small fish. Having been hunted in the past for its meat and oil, it is now among the sharks being killed for their fins to serve the Asian market for shark-fin soup; a basking-shark fin can command $50,000 or more. According to a study in the journal Animal Conservation, British and U.S. researchers used DNA analysis to detect the presence of the basking shark's fins in the Hong Kong and Japanese fin markets. The demand, said co-author Mahmood Shivji, "is continuing to drive the exploitation, surreptitious and otherwise, of this highly threatened species."

Canada did its part to make it harder for future generations to see the basking shark. According to Jeffrey Hutchings, chair of the Committee on the Status of Endangered Wildlife in Canada, until 1970 this country worked to eradicate the creature as a matter of public policy because it was getting caught in commercial salmon nets and destroying them. The population fell, and in the past decades there have been only six sightings of the shark off the West Coast. "It's Canada's longest fish, has the longest gestation period of any vertebrate [and]has an extremely slow rate of growth," he said, "and it's in big trouble around the world."

Join the club. A March 30 report in the journal Science by researchers from Dalhousie University and two U.S. universities cited studies off North Carolina that registered declines since 1972 as great as 99 per cent in the number of tiger, blacktip, bull, smooth hammerhead and other sharks migrating through the area. As those great sharks vanished, the populations of species they preyed on (small sharks, skates, rays) increased, and those creatures accordingly devoured a much greater proportion of even smaller prey, such as bay scallops, soft-shell clams, hard clams and oysters, in the process devastating the century-old bay scallop fishery in North Carolina. There might be other explanations for this ripple effect, but, as the report put it, it makes more sense to attribute it to the elimination of the great sharks as predators than to assume "numerous coincidental increases" in the population of the rays and other prey. This was familiar territory for one of the authors, late Dalhousie biologist Ransom Myers, who had catalogued a vertiginous decline in the populations of such species as bluefish tuna and giant blue marlins.

The authors mention a similar reported ripple effect from the hunting of killer whales, with sea otters thriving and urchins and kelp bearing the brunt, and a boom in the population of longheaded eagle rays in Japan at the expense of shellfish. Memo to overfishers: The law of unintended consequences has not been repealed.

What do we learn from all this? Beyond the obvious lesson that great animals may not recover if you kill them in great numbers, there is the corollary that overhunting has unpredictable effects. "We know better now why sharks matter," said Dalhousie's Julia Baum, a co-author of the Science article. "Keeping top predators is critical for sustaining the health of the oceans." The casual way in which sharks are killed for their fins, or swept up as bycatch in nets set for other creatures, can shake the ocean's ecology to its core.

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