Large sharks might be eating each other, scientists discovered when something strange happened to a huge porbeagle shark they had tagged.
Researchers were puzzled when they received strange data back from a 2.2-metre pregnant porbeagle shark (Lamna nasus) they had tagged. The clues led them to believe that the enormous shark was eaten by something even bigger – this is the first time scientists have recorded a porbeagle being eaten by an even larger predator. Their findings are published in Frontiers in Marine Science.
“This is the first documented predation event of a porbeagle shark anywhere in the world,” says lead author Brooke Anderson, a former graduate student at Arizona State University.
Porbeagles, which can grow up to 3.7 metres long, live in the Atlantic and South Pacific Ocean and the Mediterranean.
One mother-to-be was tagged as part of a research project looking into the movements of the species in the waters off Cape Cod in Massachusetts. By telling them where she went while pregnant, the researchers hoped the tags would help them learn which habitats are important for pregnant females and newborn pups.
The sharks were given two types of satellite tags – one would transmit their location to satellites every time the shark’s fin was above the water. The other would only send back the data after a set period when the tag was programmed to fall off.
So, the scientists were surprised when the second tag started transmitting early, meaning it had released and was floating at the surface. The data showed that the shark had spent five months swimming at depths of between 600–800 metres during the daytime and 100–200 metres at night in waters ranging from 6.4 to 23.5 °C.
Then the stats changed suddenly: for four days, the shark seemed to be between 150 and 600 metres deep but the temperature remained around 22 °C. The experts knew the only explanation was that a larger predator had eaten the shark and swallowed the tracking tag along with it. Four days later, it excreted the tag, and the researchers retrieved the data.
“The predation of one of our pregnant porbeagles was an unexpected discovery,” says Anderson. “We often think of large sharks as being apex predators. But with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought.”
The team had two suspects who could have killed this large shark: the white shark (Carcharodon carcharias), which usually eats whales, dolphins, seals and rays, and the shortfin mako (Isurus oxyrhinchus), which tends to eat porpoises, turtles, birds, cephalopods, small sharks and fish. Because the tag didn’t see movement between the depths and the shallows, which is typical of a shortfin mako, they believe the hunter was more likely to be a great white.
The team had two suspects who could have killed this large shark.
This finding has important implications for conservation. Porbeagles are already overfished and reproduce slowly. They don't reach maturity until around 13 years old, are pregnant for eight to nine months and only give birth to around four pups every year or two. This makes it more difficult for them to recover from threats so losing a pregnant female and her babies is particularly concerning. The Northwest Atlantic porbeagle population is currently listed as endangered by the IUCN Red List of Threatened Species.
When the expectant mother was eaten, “the population not only lost a reproductive female that could contribute to population growth, but it also lost all her developing babies,” says Anderson. “If predation is more widespread than previously thought, there could be major impacts for the porbeagle shark population that is already suffering due to historic overfishing.”
Credit: Porbeagle shark, frontiers marine science. Credit. James Sulikowski
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