With the speed of a tuna and the homing instinct of a salmon, the great white shark--once considered a homebody--is proving a transoceanic traveler. And not to be outdone, a less fearsome cousin called the salmon shark has now shown up in Hawaii, far away from its supposed home in Alaska. These findings complicate conservation efforts, experts say, as multiple countries may now have to take part in protecting the great white sharks, and perhaps one day, the salmon shark.
Over the past decade, satellite technologies have enabled marine biologists to follow the oceanic travels of animals tagged with transmitters. A team led by Ramón Bonfil of the Wildlife Conservation Society in New York, New York, used such technologies and visual markings on fins to observe 32 great white sharks for 15 months. A homing device attached to the dorsal fin sent data about geographical coordinates whenever a tagged shark surfaced, enabling Bonfil and his colleagues to plot a shark's journey. In addition, the researchers equipped the sharks with acoustic transmitters and tracked the animals' movements using microphones scattered in bays along South Africa's coast.
Great whites appear to have a serious case of wanderlust, Bonfil's team reports 7 October in Science. One female sped across the Indian Ocean and back at 4.7 kilometers per hour, covering 20,000 kilometers in less than 9 months. Other tagged great whites, thought to be looking for prey, regularly took 2000-kilometer trips up and down South Africa and into the waters off Mozambique.
In a separate study, also appearing in Science this week, Kevin Weng and Barbara Block of Stanford University Hopkins Marine Station in Pacific Grove, California, used remote-sensing satellites to track 48 tagged salmon sharks based in Prince William Sound, Alaska. The salmon sharks also migrated long distances. After wintering off Alaska, some headed south, sometimes going as far as Hawaii or Baja California before returning to Alaska. One covered 18,220 kilometers in just 640 days. As with the great whites, the salmon sharks took multiple trips but always seemed to return to familiar territory. "[Both] sharks use entire ocean basins as home ranges and show remarkable fidelity to areas," Block says.
"These two papers represent great leaps in our understanding of how top predators utilize the world's oceans," says Andrew Martin, an evolutionary biologist at the University of Colorado, Boulder. But the findings have a downside: The more spread out a species is, the harder it is to protect. "They are so much more exposed to being caught because they cover a much wider area," says Andre Boustany of the Hopkins Marine Station. Thus, "conservation management of this species and other highly migratory species must occur on an international level."