A new study is shattering old beliefs about the great white shark--one of the largest, most awe-inspiring predators in the sea.
Scientists 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 January 3 issue of the journal Nature reveals that white sharks can range hundreds of miles across the open ocean. In fact, one male tagged along the Central California coast migrated thousands of miles to the warm waters of Hawaii and remained there for nearly four months.
The Nature study involved six marine scientists from three California institutions: Burney Le Boeuf and Scott Davis of the Institute of Marine Sciences at the University of California, Santa Cruz; Peter Pyle and Scot Anderson of the Point Reyes Bird Observatory (PRBO) in Stinson Beach, Calif.; and Barbara Block and Andre Boustany of the Tuna Research and Conservation Center, a joint project of Stanford University and the Monterey Bay Aquarium.
Le Boeuf, a professor emeritus of ecology and evolutionary biology at UCSC, said he was shocked by the results.
"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," Le Boeuf said. "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 tags to the backs of six adult white sharks near seal rookeries in California between 1999 and 2000. Every two minutes, the tags recorded data on the sharks' location, the depth of their dives, and the temperature of the surrounding waters. 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 Stanford's Hopkins Marine Station.
"Until this study, white sharks had only been tracked for a few days around seal colonies," said Stanford's Barbara Block. "With the advent of new electronic tagging technology, we can now track their movement, depth, and temperature preferences over many weeks and months," she said.
A legendary hunter, the white shark 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--three near Southeast Farallon Island about 30 miles (48 kilometers) west of San Francisco, and three near Año Nuevo Island about 55 miles (88 kilometers) to the south. Sharks are attracted to these islands by the large number of elephant seals that congregate there.
Researchers used different techniques to tag the sharks. At the Farallon site, PRBO researchers Peter Pyle and Scot Anderson, with assistance from Stanford graduate student Andre Boustany and UCSC graduate student Scott 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 jump in the 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 cutout of a seal. Sharks approached the decoy, sometimes mouthing it, and when a shark surfaced near the boat Davis would apply a tag to its back.
Satellite tagging data confirmed that, in the fall, white sharks appear near these coastal seal rookeries just as young elephant seals arrive to rest prior to the annual mating ritual of the adult seals. Data showed that, while near shore, tagged sharks spent most of their time close to the surface, usually diving no more than 90 feet (30 meters) down and swimming in temperate waters ranging between 50 degrees F and 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. 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," Block said. "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," Le Boeuf said. "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. "
Another possibility, he added, is that the sharks were migrating on the same track to Hawaii as Tipfin, but the electronic data record ended before they reached their destination.
Pop-up satellite tags also revealed interesting diving patterns of the four sharks during their transits across the open sea. While they sometimes dove as deep 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.
White sharks are endothermic or warmblooded fish, like tunas, capable of maintaining large temperature differences between their bodies and the surrounding water. The new study shows that they can tolerate a broad range of water temperatures.
"As the sharks moved offshore to the southwest, they increased their diving activity and experienced a broader range of ambient temperatures," the authors wrote. The sharks encountered temperatures ranging 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 the surface.
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," the study's authors concluded.
"Increased surveillance using electronic tagging should provide more information about the movement patterns," they wrote.
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."
_____
Editor's note: Reporters may contact Le Boeuf at (831) 459-2845 or leboeuf@cats.ucsc.edu, and Block at (831) 655-6236 or bbl
