White Sharks pack their lunch in Liver Fat on long migrations

Each year, great white sharks that forage in waters off the central California coast migrate as much as 2,500 miles (4,000 km) over the open ocean, to other feeding grounds in the Pacific Ocean. A team of scientists investigating the tracks of four great white sharks, using data from satellite tags, have found evidence that these non-stop journeys are fueled by lipids, or fat, stored in the sharks’ large livers. This new research shows that great white sharks power their non-stop journeys of more than 2,500 miles with energy stored as fat and oil in their massive livers. The findings provide novel insights into the biology of these ocean predators.

Great white sharks are not exactly known as picky eaters, so it might seem obvious that these voracious predators would dine often and well on their migrations across the Pacific Ocean. But not so, according to new research by scientists at Stanford University and the Monterey Bay Aquarium.

Blubber is rich in fat and a valuable source of energy for many migratory animals during their long migratory journey Just as bears put on fat to keep them going through long months of hibernation, ocean-going mammals such as whales and sea lions build up blubber to burn on their long migrations. Until now, little was known about how sharks, which carry fat in their massive livers rather than external blubber, make similar voyages. However, the exact food supply for these sharks during their migrations is not known.

Gen Del Raye of Hopkins Marine Station and the University of Hawaii and his colleagues collected satellite tagged data from four great white sharks near the central California coast.

The researchers hypothesised that migrating great white sharks were tapping into a supply of lipids stored in their liver. Fat is a major component of lipids, a collection of molecules required for cell health and function, especially energy storage. The liver of an adult great white shark accounts for more than a quarter of its body weight. When it’s well-stocked in energy reserves, as much as 90% of the liver is taken up by lipids.

Great white sharks move in alternating phases of swimming and drift diving. Drift diving is an energy-saving strategy to travel more distance. When a shark stops swimming, its momentum continues to carry it forward as it drifts downward. The rate of descent during a drift dive depends on the buoyancy of the shark; the higher its buoyancy, the slower its rate of descent.

As it turns out, buoyancy is also an indication of a number of lipid reserves left in the liver. Since lipids are less dense than water, more lipids in the shark’s body make it more buoyant. If a great white shark were using lipids to fuel its swimming, as hypothesised by the scientists, the satellite tagging data should show the shark’s rate of descent gradually increasing over its migratory journey. This would indicate that the animal is losing buoyancy, which also means that its supply of liver lipids is decreasing as it gets converted to energy for swimming.

The researchers' findings, published in Proceedings of the Royal Society B, reveal previously unknown details of how great white sharks power themselves and stay buoyant on these non-stop trips of more than 2,500 miles. The discoveries have potentially broad implications for conservation and management of coastal waters

In a press release, Barbara Block, professor of marine sciences at the Stanford Woods Institute for the Environment and a member of Del Raye’s team, said:

“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. It helps us understand how important their near-shore habitats are as fueling stations for their entire life history.”