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White sharks fatten up for long migration

Fatty livers White sharks rely on fat stores in their livers to power their migration over thousands of kilometres across ocean basins, a new study has found.

PhD student Gen Del Raye, of the University of Hawaii, and colleagues, report their findings today in the Proceedings of the Royal Society B.

It's been known for a long time that migratory birds and whales burn body fat to fuel their migration, but to date it has been unclear as to whether sharks and rays also do this.

"The difference between sharks and a lot of other animals is that instead of having lipids stored in the muscle underneath the skin, in sharks the main lipid store is actually in the liver," says Del Raye.

"Shark livers can be gigantic. About a third of the body weight of a white shark can be its liver."

To study whether sharks are using fat to fuel migration you need some way of monitoring changing fat stores over the vast distances they migrate.

Del Raye and colleagues studied the white shark (Carcharodon carcharias), which migrates from the Californian coast to the Hawaiian islands - a trip of around 2000 kilometres that takes a bit over a month.

The researchers used a method of measuring fat stores in sharks that takes advantage of the shark's swimming pattern.

Buoyancy

The sharks glide down through the water column and then swim back up to their original level before gliding back down.

"From the rate at which they are sinking you can estimate what the buoyancy of the body is," says Del Raye.

The researchers used electronic tags linked to satellites to measure the rate at which the sharks sank through the water during the gliding phase.

As the amount of oil in the liver decreased, so too did the animal's buoyancy and the faster it sank into the water.

"The buoyancy of the shark is directly correlated to how much oil is in its liver," says Del Raye.

The researchers report that data collected from nine sharks revealed the animals were indeed using their fat stores to fuel migration.

"We showed that they are depleting their body fat in order to fuel their migration," says Del Raye.

Conservation

Del Raye says the findings have implications for conservation of white sharks.

"White sharks are heavily protected but are also very threatened," he says.

Del Raye says there is relatively little food out in the open ocean for sharks so they need to build up their fat reserves before they set out.

"It's like if you are going on a road trip, you have to go to the gas station and fill up the car. The Californian coast line is where the shark has to be able to do that," he says.

Del Raye says the findings show it's important that coastal habitats supply animals not only with enough food to sustain them, but also enough to build up their fat reserves to see them through their migration.

It is not known why sharks migrate, but it could be for mating, birthing or foraging, says Del Raye.