As reported on Wired.
- BY JOE HANSON
Juvenile great white shark on exhibit at the Monterey Bay Aquarium. Credit: Monterey Bay Aquarium/Randy Wilder
Before hitting the road for a long drive, it’s a good idea to fill up the gas tank. Before great white sharks set out on their great migrations, they do the same. Only instead of unleaded, they fuel up on fat, young seals.
Using satellite data from tagged great white sharks, scientists determined that these elite predators depend solely on built-up stores of high-energy fat to power them during their long journeys to the open ocean, some of which can stretch nearly 4,000 kilometers. By analyzing how fast the sharks were able to dive along their travels, the scientists were able to estimate how the sharks lost fat, and therefore buoyancy, over time. It’s the first such study of how migrating sharks store energy and burn it along their journey.
Barbara Block, a marine biologist from Stanford and lead investigator on the new work, likens a Pacific great white’s pre-migration meal to visiting an Outback Steakhouse stocked with elephant seals. However, no one knew if they ate again along the way.
This white shark, photographed at the Farallon Islands off Northern California, has been tagged with an acoustic tag (front) and a pop-up satellite tag (rear) as part of the TOPP research program.Credit: TOPP
Gen Del Raye, who undertook this research while an undergraduate at Stanford, set out to answer that question by taking a new look at data already collected from tagged sharks. “I’m hoping that these kinds of creative adaptations of the data we have will be able to tell us more about the physiological adaptations that allow sharks to undergo these migrations, and what role it plays in their life history,” Del Raye explains.
Del Raye hypothesized that a well-fed shark would drift down more slowly than a lean one. A great white’s fatty liver, which can make up more than a quarter of an adult’s body weight when fully stocked with buoyant fats, has the same effect as a life preserver, making it hard to dive quickly. (Unlike bony fish, great whites don’t have gas bladders, so they depend on this oily organ to keep from sinking.)
Of course, Del Raye and Block can’t check the liver of a migrating shark to see how much fat it has stored up.
Instead, Del Raye began by observing a captive great white at the Monterey Bay Aquarium, measuring its downward drift as it coasted through its tank. Because this shark has a carefully regulated diet, Del Raye could calculate how its buoyancy changed with time after feeding.
He then applied these dynamics to wild great whites in the open ocean. As he’d predicted, the longer the sharks were away from their seal snack bar, the faster they drifted downwards as they coasted along. That’s consistent with the idea that they become less buoyant as they burn through their liver fat. Because their buoyancy appeared to decline steadily throughout their migration, the sharks probably don’t refuel along the way, the researchers conclude in a paper published this week in Proceedings of the Royal Society B. In addition, the tracking data showed no evidence of swimming patterns indicative of hunting.
This study highlights the importance of protecting not only a single species, great white sharks, but also their prey. Salvador Jorgensen, a scientist from the Monterey Bay Aquarium who contributed to the study, recorded one great white returning to the same seal rookery 26 years after it was first photographed there. “It really underscores how important these hunting sites are for white sharks,” he says.
“With these data we can look at shark migration in a slightly different light, not dwelling on ‘Where and when are they feeding during the trip?’ and more on the need for food at the beginning and end of the trip,” adds Robert Hueter, a shark researcher at Mote Marine Laboratory who was not involved in the research.
Of course, why sharks migrate away from such a well-stocked seal buffet remains a mystery. Block and Jorgensen plan to examine that question as part of the Wired Ocean initiative, which includes a mobile app called SharkNet that lets users track shark migrations in real time.