A pregnant porbeagle shark measuring over 2 metres has been murdered. But who is the culprit? In a scientific version of Cluedo, American researchers have now identified an even larger shark as the suspect. This mysterious case is being investigated thoroughly.
“This is the first documented predation – the phenomenon of some organisms killing and devouring others – of a porbeagle shark in the world,” said lead researcher Brooke Anderson of the Arizona State University. “In this case, the population not only lost a reproductive female, but also her unborn young. If this form of predation is more common than we previously thought, it could have major consequences for the already endangered porbeagle.”
Slow reproductive cycle
Porbeagle sharks, also known as porbeagle sharks, live in the Atlantic Ocean, the South Pacific Ocean, and the Mediterranean Sea. They are large and strong, can grow to 3.7 meters long, and weigh up to 230 kilograms. These powerful sea creatures do not become old and worn out until they are about 60 years old. They can live up to 65 years. Females do not become sexually mature until they are 13 years old and give birth to an average of four pups at a time once or twice a year, after a gestation period of eight to nine months.
Their slow reproductive cycle leaves porbeagle populations unable to defend themselves against threats such as overfishing, bycatch and habitat loss – the degradation of habitats through displacement, destruction, degradation and other causes that alter the ocean and marine landscape. The population in the northwest Atlantic is therefore on the Red List of the International Union for Conservation of Nature (IUCN) as endangered species, while populations in the northeastern Atlantic and Mediterranean are even critically endangered.
Migration research with smart transmitters
In their study of shark migration, Anderson and her colleagues captured porbeagle sharks off Cape Cod, Massachusetts, in 2020 and 2022. Each shark was fitted with two satellite transmitters: a fin transmitter, which transmits the shark’s location each time the fin surfaces, and a so-called pop-up satellite archival tag (PSAT), which continuously measures the depth and temperature of the environment. The PSAT is released after a few months, after which the transmitter floats to the surface and sends all the data to the satellite.
Bermudadriehoek
One of the sharks tagged was a pregnant female shark measuring 2.2 metres in length. The researchers were hoping to collect data that would help identify important habitats where porbeagle mothers and their young reside. But the cosmos had other plans. Unexpectedly, 158 days after her release, the female’s PSAT began transmitting signals from the Bermuda Islands. This meant that the PSAT had broken free and was now floating on the surface of the Atlantic Ocean, some 1,000 kilometres off the eastern coast of the United States.
Swallow, swallow, gone
The data showed that for five months, this shark swam at depths of between 100 and 200 meters at night and swam between 600 and 800 meters during the day, in waters with a minimum temperature of 6.4 and a maximum of 23.5 degrees. Suddenly, starting on March 24, 2021, the temperature remained constant at around 22 degrees for four days, at depths of between 150 and 600 meters. The only logical explanation? The shark had fallen prey to a larger, warm-blooded predator, and the PSAT was pooped out by the apex predator about four days later.
Two suspects
“There are two warm-blooded predators large enough to hunt adult porbeagles: the white shark and the mako shark,” the shark experts write. The white shark, also known as the great white shark, is the largest predatory fish in the world. With a length of up to 4.9 meters for females and a maximum of 4 meters for males, and a weight of 680 to 1100 kilos (adult female), the white shark can rightly be called a sea monster.
Mako or man shark?
The mako shark, also known as the mako shark, can grow up to 4.5 meters long and weigh around 680 kilograms. Its top speed is a whopping 50 kilometers per hour, and it feeds on squid, bony fish, small sharks and sea turtles, among other things. White sharks also eat whales, dolphins, seals and rays. Of the two, the white shark is the most likely culprit, as mako sharks often make rapid, up-and-down movements during the day – diving behavior that was not recorded by the PSAT.
Unexpected discovery
“The predation of one of our pregnant porbeagles was an unexpected discovery,” Anderson said. “We often think of large sharks as apex predators, at the top of the food chain, with nothing to fear from other predators except humans. But through the development and application of new technology, we are beginning to discover that interactions between large predators are much more complex than we thought.”
Anderson plans to do much more research. “It is imperative that we continue to study how often large sharks prey on each other and what impacts these interactions have on the ecosystems in which they operate.”
You can see Anderson in action with her colleagues and many different sharks at her X account.