Pregnant shark helped scientists understand behavior of large predators, study shows

When scientists attached a tracker to a pregnant female porbeagle shark in October 2020 to learn more about the creature’s habitat, they didn’t expect the device to capture evidence of how larger sharks hunt each other .

But when the tracker registered unexpected activity in March 2021, the scientists realized that a larger predator of the same species had eaten the research animal . The team shared these unexpected findings in a new study published last Tuesday (3) in the journal Frontiers in Marine Science.

“This is the first documented event of porbeagle predation anywhere in the world,” said Brooke Anderso, lead author of the study and a marine fisheries biologist with the North Carolina Department of Environmental Quality.

Porbeagle sharks, found in the Atlantic and South Pacific oceans, as well as the Mediterranean Sea, can reach just over 3.7 meters in length and weigh up to 230 kilograms.

These large, elusive animals can live between 30 and 65 years, but females can only reproduce after the age of 13. They give birth to four young every one to two years.

Habitat loss, overfishing and incidental capture in fishing nets are threatening porbeagle populations. The northwestern Atlantic species is listed as vulnerable on the International Union for Conservation of Nature’s Red List of Threatened Species.

“In a single event, the population not only lost a breeding female that could contribute to population growth, but also lost all of her developing young,” Anderson said. “If predation is more widespread than previously thought, this could have major impacts on the porbeagle population, which is already suffering due to historical overfishing.”

Now, researchers believe they have identified two suspects — a great white shark and a short-finned mako shark — in this scientific mystery, which could change the way scientists think about how these large fish interact.

Tracking the sharks

When Anderson and his colleagues were tagging porbeagle sharks near Cape Cod, Massachusetts, in 2020 and 2022, the intention was to track where pregnant females went, in order to identify areas where the fish and their newborn pups need conservation efforts and protection.

She and her team have been studying porbeagle sharks for more than a decade and have become experts at tagging them for research.

The researchers used fishing rods and reels to catch them and bring them aboard the boat. Saltwater pumps were placed in the fish’s mouths to allow them to breathe.

“They are actually very calm, which makes the tagging process easier,” Anderson said. “We have tagged dozens of porbeagle sharks over the past decade and are currently analyzing the data to determine the most important habitats for the population, which can be prioritized for conservation and management directions.”

Each shark was equipped with two satellite trackers: a fin-mounted transmitter and a removable satellite archiving tag.

The fin-mounted equipment transmits the shark’s current location to satellites when they are above the ocean’s surface. Removable tags measure the depth and temperature of the ocean, storing the data until they detach, float to the surface and transmit their information.

The fact that the pregnant porbeagle was prey to a larger shark turned out to be an additional scientific discovery, Anderson said.

The fish tagged by the team, which was 2.2 metres long, remained mostly submerged for five months, swimming at depths of 100 to 200 metres at night and 600 to 800 metres during the day. The ocean temperature varied between 6.4 and 23.5 degrees Celsius.

But 158 ​​days after the shark was tagged and released, the removable tag began transmitting data from an area southwest of Bermuda, suggesting it had detached from the shark and was floating on the ocean’s surface.

For four days in March 2021, the tag recorded a constant temperature of 22 degrees Celsius at a depth of between 150 and 600 meters. Then the tag floated upward.

The team pieced together several factors that indicated the shark had been eaten, and the tag was excreted by a larger predator that devoured the animal, Anderson explained.

“The first and most important finding was the sudden increase in temperature recorded by the tag, even at 600 meters depth,” she said. “This immediately indicated that the tag was now inside the stomach of a warm-blooded predator, such as a lamnid shark. There was also a slight change in the dive pattern recorded by the tag, which indicated that the tag was now tracking another animal (the predator).”

The removable tag came off eight months earlier than expected, and the device mounted on the shark’s fin never transmitted any data again.

“If the pregnant porbeagle was still alive, we would expect her to have returned to the surface of the sea and the fin tag would have transmitted her location,” Anderson said.

Unusual suspects

Porbeagle sharks belong to a family called lamnids, which also includes the great white shark and the mako shark. Unlike other fish, most lamnids are endothermic, meaning they can keep their bodies warmer than the water temperature.

“The porbeagle does this better than almost any of its relatives, and it loves the cold waters of Canada and New England year-round,” Anderson explained.

To determine what might have eaten the porbeagle as it swam near Bermuda, the team narrowed down the list of large predators that swam in the same region and were large enough to prey on it — including its relatives, the great white shark (Carcharodon carcharias) and the short-finned mako shark (Isurus oxyrinchus).

Short-finned makos are known to hunt small sharks, porpoises, sea turtles, seabirds, bony fish, and cephalopods, while great whites feed on whales, dolphins, seals, and rays.

Anderson’s team suspects the great white is the main culprit, considering that short-finned makos often make rapid dives between the surface and depths during the day, something the removable tag didn’t capture.

“We often think of large fish as apex predators, but with technological advances, we’re starting to discover that interactions between large predators can be even more complex than previously thought,” Anderson said. “It’s clear that we need to continue studying these interactions, for example to estimate how often large sharks hunt each other and to figure out what cascading impacts these interactions might have on the ecosystem.”

It is not the first time that a large animal has been devoured by another, but documentation of such events is rare.

Sharks hunting sharks in the open seas

Some of the largest shark species are not afraid to hunt other sharks, which is a fascinating part of the shark world that often goes unnoticed, said shark biologist Dr. Adrian Gutteridge, fisheries assessment manager for the Marine Stewardship Council, an international nonprofit, and a member of the International Union for Conservation of Nature’s Shark Specialist Group.

Gutteridge, who was not involved in the study, agrees that the great white shark is the most likely culprit.

“This porbeagle shark, at 7ft 2in, may have looked formidable, but great whites are only about 5ft long at birth,” said Gutteridge. “By the time they reach their full size of 14ft to 16ft, they are fully capable of taking down other fish. So for this porbeagle shark to meet its end in the jaws of a larger great white is not so much a surprise as it is a reminder that great whites are at the top of the food chain.”

Satellite tagging is helping researchers track and discover these fish’s nurseries, seasonal movements and behavioral patterns, which is especially crucial for protecting vulnerable populations, Gutteridge said.

For decades, Northwest Atlantic porbeagle have been hunted for food. Fortunately, this population is stabilizing and increasing, but continued protection is vital to allow this recovery to continue, Anderson said.

Now, the team wants to find out how often other sharks prey on porbeagle sharks. “Unraveling the mysteries of the open ocean has always been a challenge,” Anderson said. “It’s possible that the more large fish we can tag and track, the more behaviors like this will be revealed.”

This content was originally published in Pregnant shark helped scientists understand the behavior of large predators, study shows on the CNN Brasil website.

Source: CNN Brasil