Far-traveling shark enlightens scientists
Fish migrated from Monterey to Japan
Saturday, August 3, 2002
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A blue shark that was tagged off Monterey has been caught near Japan, setting a distance record for the species -- and leading some researchers to call for greater international protection for sharks on the high seas.
The shark, a 4 1/2-foot juvenile, was tagged off Monterey in 2000 by staffers of the Pelagic Shark Research Foundation, a marine fish advocacy group based in Santa Cruz. It was caught recently by Japanese fishermen 560 nautical miles east of Japan, traveling farther than any other known blue shark.
"We've had five tags returned from (the mid-Pacific) since tagging started in the late 1980s," said Valerie Taylor, a California Department of Fish and Game biologist who monitors shark tagging for the agency. "I don't know of any tagged fish that was caught farther west than this one."
Blue sharks are pelagic or open ocean sharks, reaching lengths of 13 feet and weights in excess of 500 pounds. They are handsome fish: sleek, streamlined, colored a striking cobalt blue on their dorsal surfaces and white on their ventral surfaces.
Blue sharks are relatively prolific reproducers. But they are slow to reach sexual maturity and are vulnerable to international long-line and drift-net fisheries. Anecdotal observations from West Coast fishermen indicate they are not as common off the continental U.S. as they once were.
"They were once ubiquitous," said Sean Van Sommeran, the executive director of the shark research foundation. "Wherever you went rockfishing, you had to move after 20 minutes because the sharks were biting all the fish off the lines. Now they're difficult to find."
The recovery of the tag indicates blue sharks are a pan-Pacific migratory species similar to tuna, and thus particularly vulnerable to open ocean fishing methods, he said. It "suggests that fishing anywhere in the Pacific could affect their populations."
Steve Crooke, a senior biologist for state Fish and Game and a member of a multiagency team that is devising a management plan for migratory marine fishes, said he doubted blue sharks are truly migratory, the recent tag recovery notwithstanding.
"I don't think anyone has proved that, though it's true they wander like mad," Crooke said. "They're not schooling fish, like tuna are." Crooke also disputed the contention that Pacific populations of blue sharks were depleted.
"The National Marine Fisheries Service has been observing blue shark by- catch on Pacific swordfish, mako shark and thresher shark boats since 1990," said Crooke. "We've noticed no downward trend on blues -- the amount of by- catch has remained stable throughout that period."
Crooke said the maximum catch for blue sharks set by the Pacific Fishery Management Council for the North Pacific "is about 160,000 tons a year, and we aren't anywhere near that yet."
But Robert Hueter, the director of the Center for Shark Research at Mote Marine Laboratory at Sarasota, Fla., vigorously objected to Crooke's contentions. Hueter recently published a peer-reviewed study on North Atlantic blue sharks.
"We showed that they migrate in a very purposeful manner, utilizing different stations for different parts of their life history," Hueter said. "For example, they mate in the western Atlantic, then give birth in the eastern Atlantic off Spain and Portugal."
The idea that blue sharks are nonmigratory is "archaic, ignorant," said Hueter. "It's a stereotypical and outdated notion that sharks simply roam about aimlessly," he said.
Hueter said he hadn't seen recent population assessments for Pacific blue sharks, but gives credence to anecdotal accounts of a decline.
"We showed that some segments of the Northwest Atlantic population of blues declined 80 percent from the mid-1980s to mid-1990s," Hueter said. "These fish are relatively prolific as sharks go, but they are not immune to overfishing.''
E-mail Glen Martin at glenmartin@sfchronicle.com.
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