For the first time, researchers have found evidence that sharks at the top of the food chain are being killed by predators, according to a study published Tuesday in the journal Frontiers in Marine Science.
The team of scientists who wrote the study have been researching pregnant porbeagle sharks and tracking their movements as they are endangered in many parts of the world. The scientists come from Oregon State University, Arizona State University and the Rhode Island-based Atlantic Shark Institute.
One of the sharks studied measured 8 feet and had been tracked for five months, said James Sulikowski, director of the Oregon Coastal Marine Experiment Station at Oregon State University.
What researchers have discovered is that large, warm-blooded sharks have fallen prey to other warm-blooded predators – likely other sharks.
Porbeagle sharks are eaten by warm-blooded predators, the team said
Sulikowski said the team put two different tags on the subject shark.
The first tag, called a finmount tag, sits on the shark’s fin and gives scientists “a very accurate geolocation of when the fin is out of the water,” he told USA TODAY on Tuesday.
The second tag, called the pop-off satellite archive tag, records the temperature, as well as the depth of the shark in the ocean.
“That’s how we know if the shark was actually eaten or attacked,” Sulikowski said, pointing to archive tags.
The second shark, also a porbeagle, died about a year after the first shark and sank to the bottom of the ocean before the tags emerged, scientists said.
Sharks have been tracked for hundreds of miles
The dead shark had been tracked for hundreds of miles as it made its way from New England to Bermuda, scientists said.
The shark has spent time at depths between 1,640 feet and 3,280 feet. Because the shark has been swimming far away from the sun, the temperature is cooler. Suddenly, while still deep in the ocean, one of the shark tag readings went from 15 degrees Celsius to 25 degrees Celsius.
“We knew something was going on,” Sulikowski said. “We know that the tag is inside a warm-blooded creature… And we know that it’s not a whale or a mammal, because mammals are warmer than that.”
The predators that eat porbeagle sharks are most likely other lamnid sharks, Sulikowski said, adding that the “three iconic” lamnid sharks are porbeagles themselves, great whites and mako sharks.
Body temperature is usually between 25 and 27 degrees, he said.
“I think it’s probably a mako or a white shark because they’re bigger than a porceagle,” he said.
The second shark fell to the bottom of the ocean
There are also other sharks that the researchers collected data from that they found interesting. A porbeagle shark was swimming about 1,968 feet deep when it suddenly sank to the bottom of the ocean, Sulikowski said.
The team thinks someone killed the shark without eating it or tagging it. After the shark sinks, the sign − will come out when the shark is stagnant for a longer period of time − reappears after three days.
“Both sharks are attacked at the same depth, in the same location, half of the year,” Sulikowski told USA TODAY.
What does this mean for porbeagle sharks and science?
Sulikowski said, with sharks as big and fast as porbeagles, the only animals they prey on are other sharks that are bigger than them.
Researchers expected smaller sharks to prey on larger sharks, but the 8-foot one was unexpected, he said.
The fact that this happened is proof of how little is known about the ocean, he said.
“It made us want to learn more and learn more about how other big sharks get eaten and who is the top dog out there.”
Saleen Martin is a reporter on the USA TODAY NOW team. He is from Norfolk, Virginia – at 757. Follow him on Twitter at @SaleenMartin or email her at sdmartin@usatoday.com.
This article first appeared in USA TODAY: Do big sharks eat each other? Researchers say there is a possibility.
