Great white sharks rely on fat/oil stores in their huge livers for energy during their non-stop long-distance journeys — some over distances of more than 2,500 miles, new research from Stanford University and the Monterey Bay Aquarium has found.
The fact that great white sharks don’t actually feed during long-distance migrations is a relatively recent finding — but until this new research it wasn’t exactly clear how they subsisted during these long journeys.
“We have a glimpse now of how white sharks come in from nutrient-poor areas offshore, feed where elephant seal populations are expanding — much like going to an Outback Steakhouse — and store the energy in their livers so they can move offshore again,” stated researcher Barbara Block, a professor of marine sciences and a senior fellow at the Stanford Woods Institute for the Environment. “It helps us understand how important their near-shore habitats are as fueling stations for their entire life history.”
In most mammals it’s relatively easy to observe animals preparing for food scarce times by putting on body fat — bears, sea lions, whales, etc, all simply get externally larger. But with sharks it wasn’t immediately as clear, as the energy was stored internally in the liver — carrying fat stores in their massive livers rather than external blubber.
Stanford University explains the research:
In a study initiated by a summer project of Stanford undergraduate student Gen Del Raye, researchers first looked at a well-fed juvenile great white shark at the Monterey Bay Aquarium. They documented over time a steady increase in buoyancy as the shark’s body mass increased, presumably due to the addition of stored oils in its liver.
The researchers then turned to detailed data records from electronically tagged white sharks free-swimming in the eastern Pacific Ocean. Using these data, which include location, depth and water temperature, the scientists identified periods of “drift diving,” a common behavior of marine animals in which they passively descend while momentum carries them forward like underwater hang gliders.
By measuring the rate at which sharks sink during drift dives, the researchers were able to estimate the amount of oil in the animals’ livers, which accounts for up to a quarter of their body weight. A quicker descent meant less oil was present to provide buoyancy. A slower descent equated with more oil.
Buoyancy consistently decreased over the course of each studied shark’s migration, indicating a gradual but steady depletion of oil in the liver. In other words, they were primarily running on energy stored up before they embarked on their journeys.
“Sharks face an interesting dilemma,” stated Sal Jorgensen, a research scientist at the Monterey Bay Aquarium. “They carry a huge store of energy in the form of oil in their massive livers, but they also depend on that volume of oil for buoyancy. So, if they draw on those reserves, they become heavier and heavier.”
“The most difficult thing about this research was finding a way to bring all of the different sources of data together into a coherent and robust story,” said Del Raye.
“Part of that story is the importance of calorie-stocked coastal feeding grounds, not just for mammals such as whales, but also for sharks readying for long-distance migrations. Could the same be true for other ocean animals such as sea turtles and a variety of fish? The study may help answer that question too through a novel technological approach that can be applied to ongoing studies of other large marine animals.”
The new research was just published July 17th in Proceedings of the Royal Society B.