The surprise killing of a pregnant porbeagle (Lamna nasus) shark being tracked near Bermuda left researchers trying to solve an oceanic mystery. From the transmitted data, they were shocked to discover that it was likely eaten by an even larger shark, in what is the first recorded case of its kind
Who killed the pregnant porbeagle?
In a marine science version of the game Clue, researchers from the U.S. have now accused a larger shark, with its deciduous triangular teeth, in the open sea southwest of Bermuda. This scientific whodunit is published in the peer-reviewed journal Frontiers in Marine Science.
Lead author Brooke Anderson, a former graduate student at Arizona State University, explained why this mystery matters:
This is the first documented predation event of a porbeagle shark anywhere in the world.
In one event, the population not only lost a reproductive female that could contribute to population growth, but it also lost all her developing babies. If predation is more widespread than previously thought, there could be major impacts for the porbeagle shark population that is already suffering due to historic overfishing.
Porbeagles are sharks from the Atlantic and South Pacific Ocean and the Mediterranean. They are large, active, powerfully built – up to 12 feet (3.7 meters) long and weighing up to 500 pounds (230 kg) – and long-lived, living up to 30 or even 65 years. Females don’t reproduce until they are about 13 years old, and then give birth to an average of four pups every one or two years, born live after a gestation period between eight and nine months.
Because of their slow reproductive cycle, porbeagle populations cannot recover quickly from persecution, recreational fishing, bycatch, and habitat loss and degradation to which they currently are exposed. Indeed, Northwest Atlantic porbeagles are listed as endangered on the International Union for Conservation of Nature (IUCN) Red List of Threatened Species, while the Northeast Atlantic and Mediterranean populations are critically endangered.
As part of their research into shark migration, Anderson and colleagues captured porbeagles off Cape Cod in Massachusetts in 2020 and 2022. Each porbeagle was equipped with two satellite tags, a fin-mount satellite transmitter and a pop-off satellite archival tag (PSAT), before being released. Fin-mount tags send the current location to satellites whenever the shark’s fin rises above the surface. PSATs continuously measure depth and temperature and store these data until the tag falls off, typically after a predetermined period, after which they float to the surface and transmit their stored data to satellites.
Among the tagged porbeagles was a pregnant female, 7 feet (2.1 meters) long. Anderson and her coauthors hoped to obtain data from this female to help identify important habitats for porbeagle mothers and their newborns.
But fate intervened. Unexpectedly, this female’s PSAT started to transmit off Bermuda 158 days after its release. This implied that the PSAT had popped off and was now floating at the surface.
Data then transmitted showed that this female had been cruising for five months at a depth between 100 and 200 meters at night and between 600 and 800 meters during the day, in waters with a temperature between 44 and 74 degrees F (6.6 and 23 degrees C). During this time, the fin-mount tag had transmitted only once, confirming that she remained underwater most of the time.
But suddenly, over the period of four days starting March 24, 2021, the temperature measured by the PSAT remained at an approximately constant 72° F (22 °C), at a depth between 500 and 2,000 feet (150 and 600 meters). Only one explanation was possible: that day, the unfortunate porbeagle had been hunted and eaten by a larger predator. The PSAT must then have been excreted about four days later, starting to transmit.
The paper’s authors had a short list of animals-of-interest as suspects. They said they were likely dealing with one of two known killers:
Two endothermic predator candidates large enough to predate upon mature porbeagles and located within the vicinity and at the time of year of the predation event include the great white shark Carcharodon carcharias and shortfin mako Isurus oxyrhinchus.
Shortfin mako sharks are known to feed on cephalopods, bony fish, small sharks, porpoises, sea turtles and seabirds, while great whites also feast on whales, dolphins, seals and rays. Of the two candidates, a great white shark was the more likely culprit, as shortfin mako typically make rapid oscillatory dives between the sea surface and deeper depths during the day while in the open ocean – a behavior not registered by the PSAT.
Anderson said it was a classic case of there always being a bigger fish:
The predation of one of our pregnant porbeagles was an unexpected discovery. We often think of large sharks as being apex predators. But with technological advancements, we have started to discover that large predator interactions could be even more complex than previously thought.
We need to continue studying predator interactions, to estimate how often large sharks hunt each other. This will help us uncover what cascading impacts these interactions could have on the ecosystem.
Bottom line: Researchers uncovered a rare instance of large sharks preying on one another. The revelation demonstrates the complexity of predator interaction and resulting ecological impacts.
Read more: Female sharks gather under sea to … sleep?
The post Sharks may be hunting other sharks first appeared on EarthSky.
