Fattened livers prep white sharks for extreme migrations

A white shark’s big fat liver, which can plump up to more than a quarter of an animal’s body weight, turns out to be the fuel tank for extreme migrations.

White sharks (Carcharodon carcharias) in the eastern Pacific take a springtime swim from California to Hawaii and return in late summer. A one-way, 4,000-kilometer trip takes about a month.

By combining data from two kinds of tracking tags attached to the animals, an unusual analysis shows that sharks fatten up for the demands of migration much the way birds do, says Gen Del Raye of the University of Hawaii in Manoa.

As the sharks use up calories stored as oily lipids, their bodies lose buoyancy. The tracking data revealed signs that during the trip to Hawaii sharks sink progressively faster during glides, Del Raye and his colleagues report July 17 in the Proceedings of the Royal Society B. Tagged sharks on the California coast didn’t change in glide trajectories.

One of the tag types the researchers analyzed logs location and dive depth. It eventually pops loose from the animal, bobs to the surface and uploads its data to a satellite. Another kind of tag records the animal’s acceleration. Del Raye used these records to select episodes when sharks stopped swimming and just drifted downward at a steady, shallow angle.

The data give researchers their first evidence that sharks bulk up for the ordeal of migration, says Aaron MacNeil of the Australian Institute of Marine Science in Townsville.

A white shark’s liver is its largest visceral organ, and, at its maximum, 90 percent of its volume comes from high-energy lipids. Waters off California and Mexico offer a great opportunity to bulk up on elephant seals and other marine mammals. But prey become scattered and scarcer on the trip to Hawaii. Making a long journey into food-poor waters is a risky endeavor, so biologists expect sharks get some kind of big payoff, perhaps in mating.

Marine ecologist Nigel Hussey would like to know whether white sharks out in the central Pacific reload their livers to some extent to fuel the journey back to California. Hussey, of the University of Windsor in Canada, notes that recent calculations argue that white sharks may need to feed more often than biologists thought.