Reasons Sought for Decline of Basking Shark
Basking sharks, the three-ton gentle giants of the ocean, seem to have disappeared from many coastal areas around the world, and researchers want to know why.
Plankton-eating animals, named for their tendency to float on the surface, they grow to 30 feet long or longer. They are the second-largest fish, after the whale shark.
Little is known about the basking shark's biology, habits and population. But what is known is worrisome, said Sean Van Sommeran, executive director of the Pelagic Shark Research Foundation in Santa Cruz.
"We have aerial photos of the California coast from the 1940's and 50's showing thousands of basking sharks, with hundreds in Monterey Bay alone," he said. "But in our last survey of the bay, we found only one."
In 1990-91, the foundation spotted 300 sharks in Monterey Bay and tagged 81. Similar decreases have been reported in Ireland, Norway, Korea and Japan, he said.
The foundation, part of the Earth Island Institute, has just received $5,000 from the World Wildlife Fund to set up a network of shark-watchers, often charter and commercial fishing boat captains, to track the population and find out why it is dwindling.
One likely culprit is humanity. Until the 1940's, the basking shark was hunted around the world for oils and liver. In British Columbia, specially equipped boats once deliberately rammed the sharks and killed them because fishermen considered them a danger to navigation.
West Coast commercial fishing has halted, but people still take a toll. Last year a pair of harpooned basking sharks with their fins cut off was found off Santa Barbara. Shark fin soup, a popular delicacy, has been blamed for similar attacks on other shark species.
Slow-moving, they make easy targets. "It is technically legal to go out and just harass them or commercially fish them," Mr. Van Sommeran said. "We've found basking sharks with arrows shot into them."
While Mr. Van Sommeran would like the basking sharks protected, as white sharks were recently, he said scientists must produce more authoritative data.
Direct attacks may explain some decreases, but there are even more disturbing possibilities, including pollution and climate changes, said Dr. John McCosker, a leading shark researcher with the California Academy of Sciences.
"A creature as large as it is is like a canary in an oceanic coal mine," he said. "As their numbers decrease, it's a sign that if we're not causing direct reduction, we're probably causing indirect reduction because of water quality."
Not all researchers are sure the population is in decline. "I frankly don't think there is enough observation on basking sharks to tell us anything," said Greg Calliet of the Moss Landing Marine Laboratories. "There's no way you can target basking sharks; they move around too much."
Mr. Van Sommeran contends that the decline is too steep to be explained by natural cycles. He notes that sharks give birth to only a few young. "There are basking sharks that reach up to 40 feet, and we believe it takes them 100 years to get that big," he said, "so if you kill even one, it takes a long time to replace."