Most shark species are cold-blooded, but porbeagle sharks have the ability to generate and conserve heat, and because of this ability, they can withstand the cold of the North Atlantic.
Anderson and his colleagues have spent the past few years studying the long migrations of porbeagle sharks, with a particular focus on pregnant females. By tracking the movements of pregnant females, Anderson and his colleagues hope to discover which porbeagle shark habitats are most in need of protection.
Although porbeagle sharks can live up to 65 years, females do not begin to reproduce until about 13 years of age and give birth to a small number of offspring at each birth. This issue makes it difficult for their population to recover from threats such as overfishing and habitat loss. As a result, porbeagle sharks in the Northwest Atlantic are listed as “vulnerable” by the International Union for Conservation of Nature, while populations of the species elsewhere are considered “critically endangered.”
On October 28, 2020, researchers captured a 2-meter female porbeagle shark off the coast of Cape Cod in the northeastern United States. After confirming the shark’s pregnancy using ultrasound, they attached a GPS transmitter and a detachable satellite tag. The tag, which is designed to collect information about water depth and temperature, is detached after a year and comes to the surface of the sea, where it sends its data to researchers via satellite.
After attaching the tracker, the researchers carefully returned the shark to the water. For the first five months, everything was normal, according to data collected by the label. During the day, the pregnant shark was moving at a depth of 600 to 800 meters and was closer to the surface at night. The GPS tag on his fin only sent a signal once, in early November, and it was picked up close to where he was caught.
Video description: The latest video of a two-meter pregnant porbeagle shark that fell prey to another animal 5 months after being tagged near the coast of Cape Cod.
On April 3, 2021, the detachable tag unexpectedly began transmitting data from a point southwest of Bermuda in what appeared to be a surfacing event. However, the information the tag collected seemed strange.
About a week before the early data transmission began, the tag recorded a gentle and steady increase in temperature and remained several hundred meters below the water surface. According to Anderson, this only happens when the tag is in the predator’s stomach, which can maintain its body temperature above that of the ocean. Then the tag passes through the digestive system of the predatory animal and after 4 days it is excreted and comes to the surface of the sea where it starts sending data. The data all pointed to the conclusion that an animal had eaten the pregnant porbeagle shark.
But who was the killer? Anderson says the only creatures that can prey on such a large predator are large marine mammals, such as orcas or other larger sharks. Researchers rejected the theory of orca whales. If the detachable tag were placed in the stomach of a warm-blooded mammal, it would register a much larger temperature rise.
Instead, the relatively modest increase pointed to two other suspects capable of warming their bodies like porbeagles: the great white shark and the shortfin mako shark. But the depth data recorded by the tag shows a diving pattern that more closely resembles that of a great white shark.
Losing a pregnant porbeagle shark is two losses at once, says Anderson. “Within minutes, the population of the species lost not only a pregnant female that played a reproductive role, but also all the babies in her womb,” he says.
Neil Hammerschlag, a marine ecologist and conservation biologist who was not involved in the new research, said: “The findings highlight the threats currently facing porbeagle sharks and highlight the importance of protecting porbeagle sharks. “What happens naturally is out of our control, but we can control anthropogenic threats,” he says.
The researchers’ findings have been published in the journal Frontiers in Marine Science
