July 17, 2013
Great White Sharks Use Stored Liver Oil As Fuel For Long Trips
Brett Smith for redOrbit.com - Your Universe Online
While not quite as exciting as last week's Sharknado, a report from Stanford University just published in Proceedings of the Royal Society B has revealed new details on how great white sharks can make non-stop voyages that extend 2,500 miles or more.
According to the report, the sharks store excess energy after feeding in the form of liver oil, which they can tap into on long trips.
"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," said study co-author Barbara Block, a professor of marine sciences at Stanford. "It helps us understand how important their near-shore habitats are as fueling stations for their entire life history."
Ocean mammals, such as whales and sea lions, utilize a similar method - accumulating blubber for use during long migrations. The new study illustrates how sharks make those same types of voyages.
To reach their conclusion, researchers began by examining a well-fed great white at the Monterey Bay Aquarium. They recorded an increase in buoyancy as the shark's body mass grew, which they attributed to the increase of stored oils in its liver.
Next, the team looked at records from 97 electronically tagged white sharks swimming around the eastern Pacific Ocean. Using location, depth and water temperature data, the scientists looked for periods of "drift diving," which is when marine animals passively descend and drift forward like an aquatic hang glider.
As a quicker drift dive means less buoyant liver oil in the shark and a slower descent means more oil, the research team was able to estimate the amount of oil each fish was carrying.
"Sharks face an interesting dilemma," reflected co-author 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."
According to the report, each shark's buoyancy reliably decreased over the course of time, indicative of a slow but steady reduction of liver oil. The researchers said this meant the sharks were using mostly stored energy before they left on their journeys.
"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 co-author Gen Del Raye, an undergraduate researcher at Stanford.
The researchers noted that their findings have implications for the management of the sharks' coastal feeding grounds. These feeding grounds are also an important source of calories for whales and other marine mammals that make long migrations.
In a press release, the Stanford researchers speculated that other ocean animals such as sea turtles could have a similar energy storage method. They called for additional research to be performed in the form of either completely new studies or new analyses being applied to on-going studies.