Who killed the shark of the mackerels? Researchers in the United States have identified a larger shark as the culprit offshore southwest of Bermuda, realizing that the large sharks are hunting each other.
This scientific investigation was published in the journal Frontiers in Marine Science.
“This is the first documented case of a shark predating a thresher anywhere in the world,” said lead study author Dr. Brooke Anderson, a former student at Arizona State University.
“In a single incident, the population lost not only a reproductive female that could have contributed to the population growth, but also all of her developing young. If large sharks are hunting each other more often than previously thought, this could have a major impact on the thresher shark population, which is already suffering from past overfishing,” says the researcher.
How do large sharks live?
Mackerel sharks live in the Atlantic and South Pacific Oceans, as well as the Mediterranean Sea. They are large, active, with a strong body, reaching up to 3.7 meters in length and weighing up to 230 kg, and long-lived, living up to 30 or even 65 years. Females do not breed until about 13 years of age and give birth to an average of four cubs every one to two years, with the cubs being born alive after a gestation period of eight to nine months.
Because of their slow reproductive cycle, thresher shark populations cannot recover quickly from persecution, recreational fishing, bycatch, and habitat degradation. Therefore, sharks in the Northwest Atlantic are listed as Endangered on the IUCN Red List, while populations in the Northeast Atlantic and Mediterranean are considered Critically Endangered, notes Eurek Alert.
A pregnant female
As part of their research on shark migration, Anderson and his colleagues captured sharks off Cape Cod, Massachusetts, in 2020 and 2022. Each shark was fitted with two satellite tags: a fin-mounted transmitter and an archival satellite tag which detaches (PSAT). Fin-mounted transmitters send their current location to satellites every time the shark’s fin surfaces. PSATs continuously measure depth and temperature and store this data until the tag detaches, usually after a predetermined period, after which it floats to the surface and transmits the stored data to satellites.
Among the tagged sharks was a pregnant female, 2.2 meters long. Anderson and his team hoped to obtain data from this female to identify important habitats for mother sharks and their newborn pups.
But fate intervened. Unexpectedly, this female’s PSAT began transmitting data near Bermuda 158 days after its release. This suggested that the PSAT had detached and was now floating on the surface.
What did the researchers find in the data from the large sharks?
The data submitted showed that this female swam for five months at a depth of between 100 and 200 meters at night and between 600 and 800 meters during the day, in waters with a temperature of between 6.4 and 23.5 °C. During this time, the fin-mounted transmitter transmitted only once, confirming that the female remained underwater most of the time.
But suddenly, starting on March 24, 2021 and over a period of four days, the temperature measured by PSAT remained constant at about 22 °C at a depth between 150 and 600 meters. The only possible explanation was that on that day, the unfortunate shark was hunted and eaten by a larger predator. The PSAT was then excreted after about four days, beginning to transmit the stored data.
Who killed the female shark?
“Two endothermic predators large enough to prey on a mature thresher shark that were nearby and at the time of year of the predation event include the great white shark (Carcharodon carcharias) and the shortfin mako shark (Isurus oxyrinchus)”, wrote the authors.
Shortfin mako sharks are known to feed on cephalopods, bony fish, small sharks, porpoises, sea turtles and seabirds, while great white sharks also feed on whales, dolphins, seals and catfish. Of the two candidates, the white shark was more likely to be the culprit, as mako typically make rapid, swinging dives between the sea surface and greater depths during the day, a behavior that was not recorded by the PSAT.
“The predation of one of our pregnant females was an unexpected discovery. We often think of large sharks as apex predators. But with advances in technology, we’re beginning to discover that interactions between large predators may be much more complex than previously thought,” Anderson said.
“We need to continue to study predator interactions to estimate how often large sharks prey on each other. This will help us discover what cascading impacts these interactions might have on the ecosystem,” she concluded.
We recommend you also read:
General culture test. Which insect can live 30 years?
The first evidence that butterflies can fly more than 4,000 kilometers
Gulf of Mexico sharks have learned to “steal” from fishing nets
The world’s largest land mammal migration has been confirmed
Source: www.descopera.ro
