Great White Sharks Hit The Comeback Trail
(Inside Science) -- Perhaps it's time to come back out of the water -- or maybe to visit the beach and celebrate the revival of a much maligned marine creature.
Studies by two independent teams of marine scientists suggest that great white sharks are making a comeback on the East and West Coasts of the United States, following a period of decline.
"The animals are responding the same on both coasts," said George Burgess, director of the Florida Museum for Shark Research at the University of Florida in Gainesville. "Their population is on the rise."
"We showed that the white sharks' population has climbed back to about 70 percent of where they were historically, after declining by 63 to 73 percent in the 1970s and 1980s," said Tobey Curtis, a fishery policy analyst at the National Oceanic and Atmospheric Administration's Sustainable Fisheries Division in Gloucester, Massachusetts. He led the team that studied the East Coast population.
On the West Coast, Burgess added, "We showed the population was more than an order of magnitude higher than estimated."
"I'm very confident in their assessments of shark populations," commented James Sulikowski, professor of marine science at the University of New England in Biddeford, Maine, who did not participate in either study.
Researchers emphasize that beachgoers shouldn't worry about the increasing numbers. Despite the implicit message of the 1975 movie "Jaws," they say, great white sharks offer hardly any threat to people.
"Along the East Coast, we found only four fatalities attributed to white sharks over 200 years," Curtis said.
Burgess asserts that other sharks may have been responsible for reported attacks by great whites. "Most people who get bit aren't trained in shark identification," he said. "They're not going to take out calipers and measure the shark."
Sharks' popularity stems in part from recognition of the fishes' critical top predator role in the marine environment. "Having great whites and other apex predators is very important to the health of the ecosystems," Sulikowski said.
Those ecosystems span the globe. "In general, the species is found in every ocean," Curtis explained. "We think there's a Northwest Atlantic population, along with other populations off the West Coast, in the Mediterranean, and off South Africa and Australia."
Wherever they swim, great whites readily find food.
"In the Northwest Atlantic they spend summers off New Jersey, Long Island, and Cape Cod, and eat what's available. They'll stop off Cape Cod and see if they can grab a seal or feast on a whale carcass, one of their favorite foods," Curtis said. "They spend most of their winters off Florida, where they eat smaller sharks, sea turtles, and stingrays."
The sharks are difficult to track for several reasons.
Because they are at the top of the food chain, their numbers are relatively small compared with those of other sea creatures. They tend to swim alone, about 100 feet underwater, gathering only near food sources, and rarely coming to the ocean surface. And their size – adult males are more than 11 feet long and adult females at least 15 feet – makes it unlikely that fishermen will catch adult sharks in their nets, a traditional way of sampling fish numbers.
As a result, marine scientists have obtained only rough estimates of the sharks' numbers.
The two new studies, both of which appeared in June in the journal PLOS ONE, took different approaches to counting great whites.
The team headed by Curtis undertook an extensive study of data on great white encounters and sightings between 1800 and 2010. "We combed through all the available records -- newspaper clippings, reports of interactions with fishing gear, everything that recorded a white shark being captured," Curtis recalled. "When we brought all these things together, it gave us a relative trend."
The group led by Burgess expanded on a photographic study of two feeding sites in 2009. That project, headed by Taylor Chapple of Stanford University, suggested that the Pacific Ocean off central California contained just 219 adult great whites and sub-adults -- the latter are at least 8 feet long, but not full grown. The group interpolated that the entire Northeast Pacific, from Alaska to Mexico to Hawaii, held only double that quantity. "We threw out a ballpark figure," Chapple said.
"My colleagues and I gasped when we heard that number, because it was so low," Burgess remembered. "We remodeled that number and came up with 2,400 sharks for California alone, and a higher number for the entire area." That count covered great whites of all sizes, including sharks that aren't big enough to be considered sub-adults, but are feeding on marine mammals. It considered the entire California coast.
Taken together, Burgess said, "the two different methodologies on two coasts show that the animals are responding the same way. They're going up the hill from the valley they were in."
The great whites' numbers began to decline as a result of reductions in the numbers of their prey, such as seals and sea lions.
The "Jaws" movie exacerbated the effect. "It did a number on pretty much all shark species," Sulikowski said.
"Shark fishing became more popular and unregulated in the 1970s," Curtis added. "So the combination of commercial and recreational fishing depleted the population of great whites."
Regulation helped to turn the tide.
"The Marine Mammal Protection Act of the late 1970s protects their main food sources," Burgess said.
"Management of shark fisheries starting in 1993 helped," Curtis added. "And in 1997, whites and other sharks were prohibited for harvesting after they were caught."