Footage of great white sharks often shows them ravenously feeding on marine mammals, and now a new study reveals that such feasts often occur after the sharks go on long journeys.
The study, published in the latest issue of Proceedings of the Royal Society B, reveals how important protection of established shark feeding grounds are, since so much concentrated feasting takes place at these sites.
“We know from researchers observing white sharks feeding on whale carcasses that one shark can eat more than 30 kg (66 pounds) of blubber in a single feeding,” lead author Gen Del Raye told Discovery News. “This has been estimated to be sufficient energy to allow a shark to survive for 1.5 months.”
“We also know that the sharks feed fairly frequently on juvenile northern elephant seals at certain seasons,” added Del Raye, who is a researcher at Stanford University’s Hopkins Marine Station and the University of Hawaii. “One of my co-authors, Salvador Jorgensen, for example, has identified the same shark feeding on three juvenile elephant seals in the course of one week.”
Del Raye, Jorgensen and their colleagues assessed great white shark fat stores over long periods by examining depth records from pop-up satellite tags affixed to sharks in the Eastern Pacific Ocean. Changes in shark buoyancy served as a proxy for how much weight the toothy predators were packing.
As a shark’s single largest organ, the liver, can account for 28 percent of an adult’s body weight. Other fat is stored in the shark’s muscles. While sharks fill up on fatty food after long journeys, no shark has ever been classified as obese. Their lifestyle is probably inherently too active for them to keep the pounds on.
Great white sharks go on long-distance migrations covering more than about 2,485 miles per trip. It's not certain why they undertake such lengthy journeys, it's likely foraging, mating or both.
“Our study is another piece of mounting evidence that the major feeding point in the shark’s yearly cycle occurs along the coast,” Del Raye said. He added that they may go offshore to mate or move away from the California Current when their main prey, juvenile elephant seals, are not present. And they "may be avoiding the seasons with the coldest water temperatures near shore.”
A spot in the eastern North Pacific has been nicknamed “the great white café” since so many of the sharks gather there at times. It’s unclear if the sharks are there to dine, to woo and mate, or all three.
What is clear is that the sharks use efficient “swim-and-glide” and “drift diving” to get from one place to another. Body weight affects these moves and others, so it’s essential for the sharks to have enough fuel, but not so much that it weighs them down.
“White sharks need to swim constantly to breathe and most fish that do this tend to be slightly negatively buoyant,” Del Raye explained, adding that some other species “are very nearly neutrally buoyant,” allowing them to hover over the ocean bottom for long periods of time. “Increasing the buoyancy too much in either kind of shark would disrupt their preferred movement patterns.”
Kevin Weng of the University of Hawaii at Manoa also studies sharks. He told Discovery News that great white sharks seem to feast and practically fast, putting them into what he called “boom-and-bust” food periods.
Great white sharks never forget a good meal either.
“One shark, for example, has been observed to return to the same northern elephant seal rookery every year for 26 years,” Del Raye said. “So it is clear that the sharks are relying on a dependable food source at the end of their journey.”