Great White Sharks Eat Far More Than Thought
The sharks eat three to four times more food than previously thought.
Great white sharks, the world's largest predatory fish, eat three to four times more food than previously thought, an Australian study shows.
U.S. research from the 1980s estimated a 30-kilogram, or 66-pound meal of mammal blubber could sustain a one-ton shark for more than six weeks.
That perpetuated assumptions that large sharks could survive long periods without eating.
However, a University of Tasmania-led study published this week in Scientific Reports on the nature.com website found that 30 kilos was only enough for 12-15 days.
Researchers tagged a dozen great white sharks at Neptune Islands off South Australia and calculated their metabolic rate derived from swimming speeds.
They worked out how much energy the sharks burned and how much food they required.
Senior research scientist Jayson Semmens, lead author on the study, said the amount of energy required by great white sharks was equivalent to eating a seal pup every three days.
"They picked a shark that probably wasn't working very hard at the time when they did it," Semmens told ABC television.
"At the time it was a really novel study. They did some metabolic work similar to us but on one shark.
"The white sharks at a seal colony where we worked, they're working pretty hard, ... they're coming up to some pretty high speeds to catch the seals.
"Their metabolic rate or the engine that runs them is much faster than what we had assumed.
"These animals are probably going to be feeding you know every few days, rather than multiple weeks."
The research also sought to improve understanding of how sharks fit into the ocean ecosystem.
"We don't have a good handle on the population sizes of white sharks. We know that sharks in general are under pressure around the world from overfishing," he said.
"They're quite vulnerable because of their life history, they're long-lived, they reproduce late in their life and they produce a small number of offspring."
Semmens said the knock-on effects of removing white sharks from the ocean were thus far more important than realized.
"They're keeping under control a lot more animals than we thought," he said.
The apex marine predators, which grow up to five meters (yards) long and weigh an average 1,300 kilos, are protected and found around the globe.
Aug. 17, 2012 --
Conservation biologist Joshua Drew just wanted a fun project to take his mind off his job search, so he turned to the massive anthropological collections in Chicago’s Field Museum where he was a postdoc. Curious about the relationship between cultural and biological diversity, he focused on sharks and the Pacific Islanders who relied on them. He studied shark-teeth weaponry from the 1800s that was crafted and used by people of the Gilbert Islands, an island chain in the country Kiribati that straddles the Equator. “Sharks are an incredibly important ecosystem engineer, but they’re also incredibly important to the culture of the Gilbertese people,” Drew said. His research turned into more than a mere relaxing distraction. After landing a lecturer position at Columbia University, Drew recently presented his findings at the Ecological Society of America’s annual meeting in Portland, Oregon. It turns out the ferocious-looking weapons were made from sharks that scientists had never seen in the Islands.
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Drew got help for his project from the Field Museum’s anthropology holdings collections manager to get access. The shark-tooth weapons, originally made for combat between villages and initiation rites, ranged from smaller swords to extremely long spears. A few were frayed because they'd been used. Most items were stored in vaults and some were in a separate oversized collection. Six-foot-tall Drew lay down next to one lance and found it was more than double his height. Anthropologists who went to the Gilbert Islands with early missionaries observed how the weapons were originally made, he said. First, holes were drilled into the teeth using a snail shell. Then each tooth was lashed to a wooden core with coconut fiber, and wood slats buttressed it on either side. Cords added grip to handles. The Gilbertese had customs about who could fish sharks, what they were allowed to do, and what they weren’t, Drew said. Shark fishing was clearly an important cultural component so the weapons were probably crafted from the sharks they caught, he added. In all, the Field Museum has 124 of these weapons.
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The numerous shark-tooth weapons mainly came from a wealthy British collector who amassed them through auction and left them to the museum in his will. All the weapons date from the early 1840s through the 1880s, and they were assumed to have been constructed relatively soon before they were purchased. These collections are remarkable for several reasons. For one, the collection dates predated the first thorough scientific fish surveys of the Gilbert Islands by several decades, Drew said. Ichthyology didn’t start to take off in the area until an Australian museum tour in the 1920s. And, with such a large number of weapons to study, similarities among them were easier to spot. "When we found something atypical, it really stood out."
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Different species can be identified by carefully examining the type and locations of serrations on the tooth, but no official key currently exists. Drew persevered using multiple tools and resources. He downloaded free shark guides with illustrations from the Food and Agricultural Organization (FAO) online and also tapped into the Field Museum’s holdings to look at shark jaws up close. “A couple of the species are fairly tough to discriminate unless you have a really, really good picture of them so it’s great to have that reference material right there,” he said of the jaws. Poring over the ridges on the shark teeth from the weapons and checking scientific records, Drew found three species -- Carcharhinus sorrah, Carcharhinus obscurus, and Carcharhinus altimus -- that weren’t listed in the earliest surveys and haven’t been recorded since. Although locally extinct, sorrah or spot-tail sharks are found today in the Solomon Islands. Obscurus, the dusky shark, is a continental shark mostly found across Australia and Asia. Altimus, the bignose shark, has been recorded in Hawaii but tends to inhabit extremely deep waters. The Gilbertese have a cultural record of deep fishing, Drew said. “It doesn’t surprise me that they were picking that up.”
One question that Drew often fields about the missing species is whether the Gilbertese simply traded for those shark teeth. “We're making the implicit assumption that the teeth on the weapons came from the reefs where the people were fishing,” he said. But the island is also extremely remote. In addition, Drew said there’s no archaeological, ethnographic or linguistic evidence that the Gilbertese people traded with anyone in areas where those shark species are found today. Commercial shark fishing in the Gilbert Islands was known as far back as 1910. Shark fins were easy to trade because they kept without refrigeration, didn’t weigh much, and the value per ounce was high, Drew said. The capital had a largely Chinese immigrant population that, as it grew, saw opportunities to ship shark fins to China. By the 1950s, the Gilbert Islands were exporting more than 6,500 pounds of shark fins annually. “They’re large fish, they grow slowly, they don’t give birth until they’re fairly old, and when they do give birth they don’t give birth to a lot,” Drew said. “It’s a perfect combination if you were trying to come up with a species to drive to extinction, which is really unfortunate for sharks.”
Drew isn’t absolutely sure why those three shark species whose teeth were woven into weapons are no longer there. He can’t say whether factors like environmental declines, disease, or introduced species played a role, but he does suggest that human activity is a strong candidate. “The world that the Gilbertese Islanders now experience, the reef that they now experience, is fundamentally duller than the ones that their forefathers experienced,” Drew said. In a way, the Field Museum collection is a biodiversity time capsule that reveals shadows of how vibrant the Gilbert Island reefs once were before scientists ever visited them. Drew calls this idea “shadow diversity.” However, Drew doesn't want the public to get so stuck on the loss that it becomes discouraged about shark conservation. “These reefs used to have large, vibrant populations of predatory fishes,” he said. "There's no reason why they can't today."
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