Large sharks hunting each other 'may be more common than thought'

LONDON - Large sharks hunting each other may be more common than previously thought, scientists believe, reported German news agency (dpa).

Researchers said they have the first documented evidence of a female porbeagle, large sharks found in the waters around Cornwall and the UK coast, being eaten by possibly an even bigger shark.

The team said the findings, published in the journal Frontiers In Marine Science, suggest predators eating each other may be more widespread than thought.

Porbeagle sharks are listed as "vulnerable" in the IUCN (International Union for Conservation of Nature) Red List.

But their North Atlantic population is classed as "critically endangered" due to overfishing and they have almost disappeared from Mediterranean waters.

Study author Brooke Anderson, a former graduate student at Arizona State University, said: "There could be major impacts for the porbeagle shark population that is already suffering due to historic overfishing."

The two metre-long porbeagle was pregnant when she was first tagged four years ago by Anderson and colleagues at Cape Cod in Massachusetts, US.

A device known as PSAT, which stands for pop-off satellite archival tag, was mounted on her fin.

These tags transmit a shark's location when its fin rises above the surface.

When submerged, the tags continuously measure depth and temperature, and store these data until they fall off.

The hope was that this porbeagle would help scientists identify where shark mothers raised their newborns.

But just 22 weeks after she was tagged, her PSAT started to transmit from near Bermuda.

This implied that the PSAT had popped off and was now floating at the surface, scientists said.

Analysing the data, the researchers found that the female had been cruising underwater for five months some 100 to 200 metres below surface at night and between 600 and 800 metres during the day, where temperatures ranged between 6.4 and 23.5 degrees Celsius.

Then the PSAT data suddenly changed, showing the temperature to be a constant 22 degrees Celsius, at a depth between 150 and 600 metres, the researchers said.

Scientists believe that a great white shark - the world's largest known predatory fish - may have killed the pregnant porbeagle.

These sharks have 300 teeth, which they use to rip their prey into pieces which are swallowed whole.

Anderson said: "In one event, the (porbeagle) population not only lost a reproductive female that could contribute to population growth, but it also lost all her developing babies."

She added: "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." - BERNAMA-DPA