Roberta De Carolis For the first time, predation by a white shark on a porbeagle, a pregnant female, has been documented. The study was led by Arizona State University (USA) and demonstrated how the large shark actually fed on the female porbeagle
@Canva
In what scientists described as a “first recorded case of predation” using modern tracking techniques, a pregnant porbeagle shark was attacked and eaten by a great white shark. The study is the result of work directed by Arizona State University of the USA.
Porbeagles are native to the Atlantic Ocean and the southern Pacific but can also be found in the Mediterranean. These active robust sharks reach up to 12 feet and weigh up to 507 pounds.
They are also long-lived, with life spans of up to 30 and as much as 65 years. Females are not reproductive until at least age 13 and typically give birth to an average of four pups every one to two years, after an eight to nine months gestation period.
Yet, regardless of their features, which barely qualify them as “weak” animals, they are also a potential prey in natural ecosystems. Predator incidents on great whites had never been documented until now.
“This is the first documented predation event of a porbeagle shark worldwide,” said Brooke Anderson, lead author of the research. “In one incident, the population lost not only a reproductive female who could have contributed to population growth but also all of her developing pups.”
This is likely not to be the first incident, and it’s probably not the last one either. Unfortunately, this is not good news, nor is it simply a scientific curiosity.
“If this predation were general and more widespread than previously thought, there could be serious repercussions on the porbeagle shark population, already in a dire state due to overfishing,” added Anderson.
Because of their slow reproductive cycle, the populations of porbeagle hardly recover from persecution, recreational fishing, bycatch, and loss and degradation of habitat to which they are presently exposed.
Not surprisingly, porbeagles in the Northwest Atlantic are considered one of the threatened species according to the International Union for the Conservation of Nature, while the populations in the Northeast Atlantic, as well as in the Mediterranean are critically endangered.
How did they arrive at this conclusion?
In the recent inquiry into shark migration, scientists captured several porbeagles briefly off Cape Cod in Massachusetts in 2020 and 2022 and tagged them with a couple of satellite tags: a fin-mounted satellite transmitter coupled with a pop-off satellite archival tag – PSAT.
When returned to the ocean, the porbeagles were tracked, and the fin-mounted tags told the satellites the location of the sharks every time their fin broke to the surface.
Meanwhile, the PSAT continuously measured depth and temperature, storing this information until the tag detached after a set period. It would then float to the surface and transmit the stored data to satellites.
Among the tagged porbeagles was a pregnant female, 7.2 feet long. Scientists were hoping to get data from this female that would help identify important habitats for porbeagle mothers and their newborns.
It was the female with PSAT which suddenly, 158 days post-release off the coast of Bermuda, began transmitting as expected-the indication being that the PSAT had detached and was floating at the surface.
The information being transmitted at the moment told them that the shark spent five months swimming in waters at night between 328-656 feet and between 1,968-2,625 feet during the day, with temperatures fluctuating between 43.5°F to 74.3°F. Meanwhile, during the period when the data were transmitted, only one transmission had been received from the fin-mounted tag, thus confirming the animal remained submerged practically all of the time.
Then again, from March 24, 2021 onwards, in fact, the temperature recorded by the PSAT flatlined at approximately 72°F for four consecutive days at a depth range from 492 to 1,968 feet. The poor porbeagle must have become prey for an even larger predator, with the PSAT released on or about four days later and then transmitting.
“Two potential predators were large enough to prey on mature porbeagles and were in the vicinity and during the time of the predation event,” the authors wrote. “The great white shark, Carcharodon carcharias, and the shortfin mako shark, Isurus oxyrinchus.”
The latter, therefore, will feed on cephalopds, bony fish, small sharks, porpoises, sea turtles, and seabirds, while Great White Sharks also feed on whales, dolphins, seals, and rays.
Of the two possibilities, great white shark was the more likely aggressor, given that shortfin mako tend to dive in an oscillating pattern between sea surface and deeper while in open water during the day, a movement pattern not captured by the PSAT.
“The predation of one of our pregnant porbeagles was an unexpected find,” Anderson said. “We often think of large sharks as apex predators. But with the technological advances, we’ve started to find out these interactions with large predators are even more complex than we might have imagined.”
Research will continue with monitoring efforts in determining what cascading impacts these interactions could have on the ecosystem.
The research was published in Frontiers in Marine Science.
Sources: EurekAlert / Frontiers in Marine Science
