Image: COURTESY OF BURNEY J. LE BOEUF/UCSC
White sharks' speed and tendency to spend a majority of time below the surface make tracking the globally distributed fish a challenge. But by utilizing new electronic tagging technology researchers have assembled the most detailed record yet of the beast's ecological niche. Intriguingly, according to a study published today in the journal Nature, it seems that the creatures dive deeper and wander farther from shore than previously thought.
Barbara Block of Stanford University and the Monterey Bay Aquarium and colleagues attached so-called pop-up satellite tags to the backs of sharks while they were feeding on seals off the California coast. The tags record temperature, pressure and light-level data every two minutes before detaching from the fish at a predetermined date. Once they reach the surface, the tags transmit their stored data via satellite.
The researchers tracked six white sharks for up to six months and found that the fish experienced wide depth and temperature ranges as well as broad migratory movements. Initially, all of the sharks spent time near shore. Then, according to Block, "the surprise came in winter when four sharks tracked for longer durations all headed offshore into the central and eastern Pacific." One tenacious traveler made a 3,800-kilometer journey, covering 71 kilometers a day, to the waters off the coast of Hawaii. According to the report, the results indicate that sharks spend a surprising amount of time offshore and undertake extensive oceanic travel. "What they were doing out there is a mystery," notes study co-author Burney Le Boeuf of the University of California, Santa Cruz. "Since they were hunting for seals when tagged, such a long migration suggests a possible rendezvous for mating."
The tags also tracked the fish as they dove. When close to shore, all the sharks tended to stay within 30 meters of the surface. Those that made significant ocean journeys, however, showed a different diving pattern: they spent 90 percent of their time either within five meters of the surface or between 300 and 500 meters down. The authors caution that it is unclear whether these offshore movements represent feeding or breeding migrations, but they note that "the offshore phase lasted for at least five months, suggesting that it is an important period in the life history of white sharks in the North Pacific."