A new study is shattering old beliefs about the great white shark - one of the largest, most awe-inspiring predators in the sea.
Scientists studying white sharks along the California coast have long believed that these powerful carnivores spend most of their lives relatively close to shore, pursuing seals and sea lions.
But a study in the Jan. 3 issue of the journal Nature reveals that white sharks can range across vast stretches of the open ocean. In fact, one male tagged along the Central California coast migrated thousands of miles to the warm waters off Hawaii - and remained there for nearly four months.
The Nature study was co-authored by six marine scientists from three California institutions:* Barbara A. Block and Andre M. Boustany of the Tuna Research and Conservation Center (TRCC) - a joint project of Stanford University and the Monterey Bay Aquarium;* Peter Pyle and Scot D. Anderson of the Point Reyes Bird Observatory (PBRO) in Stinson Beach, Calif.;* Burney J. Le Boeuf and Scott Davis of the Institute of Marine Sciences at the University of California-Santa Cruz (UCSC).
"The migrations and environmental preferences of white sharks have remained elusive," observed Block, the Charles and Elizabeth Prothro Professor of Biological Sciences at Stanford's Hopkins Marine Station in Pacific Grove, Calif.
"Until this study, white sharks had only been tracked for a few days around seal colonies. With the advent of new electronic tagging technology, we can now track their movement, depth and temperature preferences over many weeks and months" she added.
"I was shocked by the results," noted UCSC biologist Le Boeuf. "Going into this, what we expected was that white sharks were just coastal animals that breed in Southern California, then migrate a few hundred miles north to feed on seals. But it turns out they've got a life at sea, and when they're in the open ocean, they're diving very deep at times."
To monitor long-distance migrations, researchers attached "pop-up" satellite archival tags to the backs of six adult white sharks near seal rookeries in California between 1999 and 2000.
The electronic tags recorded data every two minutes on water depth, temperature and light.
"Light-level data allow you to calculate when sunrise and sunset occurs," said PRBO biologist Pyle, who along with Anderson, has been has been studying white sharks for 15 years. "From the light data, we can calculate the longitude and latitude of the fish on Earth."
Each tag was programmed to detach from the animal on a specific date, then pop up to the surface, where the data were transmitted via the Argos satellite system to computers at Hopkins Marine Station.
A legendary hunter immortalized in the book and motion picture Jaws, the white shark (Carcharodon carcharias) is the world's largest predatory fish, reaching 21 feet (7 meters) in length and weighing up to 4,800 pounds (2,100 kilograms).
The six animals tagged during the Nature study - four males and two females - ranged in size from 11 to 15 feet (3.7 to 5 meters). All six were tagged in the fall - four near Southeast Farallon Island, a national wildlife refuge about 30 miles (48 kilometers) west of San Francisco, and two near Año Nuevo Island about 55 miles (88 kilometers) to the south. The sharks are relatively easy to tag because they are attracted to the large number of elephant seals that congregate on these islands.
Researchers used different techniques to tag the sharks. At the Farallon site, PRBO researchers Pyle and Anderson - with assistance from Stanford graduate student Boustany and UCSC graduate student Davis - stood watch on the island until they spotted a shark preying on a seal.
"These feeding events can last a half-hour or more," said Pyle. "When we see one, we launch our research boat and carefully approach the area where the sharks are feeding. We tag them as they swim around the boat."
At the Año Nuevo site, Davis placed a decoy in the water: a plywood cut-out of a seal. A shark will approach the decoy, sometimes mouthing it, and when the shark surfaces, Davis applies a tag to its back.
Satellite tagging data confirmed that, in the fall, white sharks appear near coastal seal rookeries at Año Nuevo and the Farallones just as young elephant seals arrive to rest prior to the annual mating ritual of adult seals. Data showed that, while near shore, tagged sharks rarely dove more than 90 feet (30 meters) below the surface, swimming in temperate waters that ranged from 50 F to 57.2 F (10 C and 14 C).
"The surprise came in winter when four sharks tracked for longer durations all headed offshore into the central and eastern Pacific," noted Block.
One male - named Tipfin by PRBO researchers - migrated from the Farallones to the Hawaiian island of Kahoolawe some 2,280 miles (3,800 kilometers) to the west, traveling at a minimum velocity of 43 miles (71 kilometers) per day. The animal stayed in Hawaiian waters the entire winter and spring.
"White sharks have only rarely been reported in recent times in Hawaii," said Block, "but ancient cultures had tools that were made from white shark teeth, so it's clear they have been around the islands for some time."
Three other tagged sharks - two females and a male - migrated to a subtropical region of the eastern Pacific hundreds of miles west of Baja California. The three sharks remained in the open ocean for several months, never venturing near any coastline.
"What they were doing out there is a mystery," noted Le Boeuf. "Since they were hunting for seals when tagged, such a long migration suggests a possible rendezvous for mating, or a move to feed on different prey."
Pop-up satellite tags also revealed interesting diving patterns among the four sharks during their transit across the open sea. While they sometimes dove as far as 2,040 feet (680 meters) below sea level, the animals seemed to prefer swimming at two discrete depths - one within 15 feet (5 meters) of the surface, the other 900 to 1,500 feet (300 to 500 meters) down.
All four sharks spent up to 90 percent of the day in these two diving zones and little time at intermediate depths, according to the Nature study.
"As the sharks moved offshore to the southwest, they increased their diving activity and experienced a broader range of ambient temperatures," wrote the authors - from 79 F (26 C) at the surface to 41 F (4.8 C) at their maximum diving depth some 2,000 feet (667 meters) below sea level.
"White sharks are endothermic fish, like tunas," explained Block. "They power their migrations with muscles warmed from their own metabolic heat. Clearly they can tolerate a broad temperature range, which provides access to prey over a wide ecological niche."
Satellite data indicate that sharks spend at least five months in the open ocean, "suggesting that it could be an important period in the life history of white sharks in the North Pacific," conclude the authors of the Nature study.
"Increased tracking using electronic tagging should provide more information about the movement patterns," they write.
The research team is currently spearheading new shark studies at Año Nuevo and the Farallones islands. In November, Tipfin returned to the Farallones, giving Pyle, Anderson and Boustany the opportunity to attach another pop-up tag on his back. The tag was programmed to record data for nine months and could answer some of the questions about the shark's round-trip migration.
"We see the same sharks return to the Farallones again and again," said Pyle. "Males come back yearly, but females return every other year, which means they may be going farther afield than males as part of a two-year breeding cycle. So long-range data on females will be of particular interest."
Tagging of Pacific Pelagics
In November 2000, Block and her colleagues launched TOPP - the Tagging of Pacific Pelagics, a major research initiative whose objective is to complete the first comprehensive analysis of marine life in the Pacific using electronic tagging technology. As part of the global Census of Marine Life, TOPP's international research team plans to electronically monitor up to 4,000 fish, birds, mammals and large squid. TOPP scientists are working with engineers to develop even more sophisticated electronic tags that increase the amount of information sent back to researchers. The scientists will launch a pilot program in the next two years to tag salmon, blue and white sharks, albatross, bluefin tuna, elephant seals, blue whales and squid.
"As the Nature study showed, electronic tagging and remote sensing technologies herald a new era for biological oceanography," Block explained. "Acquiring data on the behavior of large marine predators is difficult. Knowing where they go and what they do has been challenging."
New electronic tags are making it easier to work in extreme ocean environments far from land, she added.
"This is coming at an important time, as white sharks - along with many open-ocean animals - require increased protection from international fisheries," concluded Block. "The first step in protecting their future on Earth is knowing where they go."
Relevant Web URLs:
The above story is based on materials provided by Stanford University. Note: Materials may be edited for content and length.
Cite This Page: