Before a shark makes a long trip, it has a snack – a seal snack.
Scientists have found that the great white shark makes its 2,500-mile-long journey through the western Pacific only after feasting on seal fat and packing the oil in its gargantuan liver. That oil is the shark's sole sustenance during the food-scarce haul from California to Hawaii.
“Scientists have known that the size of sharks' livers fluctuates widely, and that this variation reflects food availability,” says Salvador Jorgensen, a post-doctoral research associate at Stanford University and Monterey Bay Aquarium and a co-author on the study, published in Proceedings of the Royal Society B.
“We have been able to finally demonstrate that a well-fed white shark in our aquarium becomes more buoyant with increasing energy stores, and in the wild, the sharks are using those stores to fuel vast migrations,” he said.
Many animals pillage from stored fat during migrations – some birds double their body weight in packed fat before departing for a long trip south. Great white sharks are one of nature’s greatest migrators, making month-long journeys of more than 2,500 miles from coastal areas abundant with seals to offshore waters, where food is scarce. Still, it had been unclear if sharks packed fat for the sea jaunt or hunted their way through the trip.
To analyze if great white sharks drew on fat stores, researchers studied changes in the shark’s buoyancy throughout its migration. Those changes revealed the gradual depletion of the shark’s packaged liver fat as it chugged through the journey.
Shark livers are formidable fat-packs: the liver is the predator’s largest organ, composing almost a third of its body weight and putting it at about 1,000 lbs. About 90 percent of the liver’s weight is liquid fat – equivalent to about 550 gallons of oil. One large meal of blubber will pack enough fat into that spacious container to feed the shark for about six weeks.
When a shark’s liver is filled to capacity, its weight provides a buoyant lift that neutralizes its weight. Over time, as those fat stores are gradually depleted, the shark becomes less buoyant, in an effect similar to air being released from swim floaties. That will affect the shark’s dive time: a less buoyant, running-on-empty shark will drift down faster than a buoyant, full one.
“Sharks face an interesting dilemma. 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,” said Dr. Jorgensen. “So, if they draw on those reserves, they become heavier and heavier.”
Researchers first took data from a captive juvenile shark at the Monterey Bay Aquarium in California. That salmon-fed shark drifted downwards more quickly over time as it burned through its fat supplies and as its buoyancy decreased.
The captive shark’s data was then compared to migration pattern data received from satellite-tagged sharks off the Californian coast. Those sharks behaved similarly to the captive shark, diving faster as they swam way from their seal feeding venues near California. Since the sharks’ buoyancy consistently decreased, the sharks were not hunting and refilling their liver during the migration. Except for some brief snacking around Hawaii, the sharks were relying on their seal oil storage from the Californian coast
It is still unknown why great white sharks make that extended trip away from seal-meals and out to a food-barren sea, said Jorgensen.
“Long distance migration is a costly and risky endeavor for animals, so there has to be big pay-off, generally in the form of feeding or mating opportunities,” he said, noting that current, preliminary evidence suggests that mating opportunities are driving the migration.
The shark’s extreme dependence on coastal seals emphasizes the importance of protecting those animals in addition to sharks, which as a vulnerable species are already among the world’s most protected animals, the authors said. More research is also needed on shark hunting patterns around Hawaii, to better understand how those food sources factor into the shark’s ecosystem.
“A lot of policy is aimed at counting and conserving single species of concern, in this case great white sharks,” said Jorgensen.
“This study shows how important a concentrated prey base can be,” he said. “In turn, the seals that white sharks prey on also need a reliable prey base. So we really need to consider the entire food chain.”