Boats caused 'corkscrew' seal deaths, say researchers

The team's initial findings have ruled out sharks being responsible for the seals' injuries

Scientists probing a mysterious spate of fatal "corkscrew" injuries to seals in the UK believe boats were to blame.

Experts at the University of St Andrews have ruled out claims that sharks were responsible for spiral-shaped cuts found on about 50 seals washed up along the UK's coast in August.

They said the injuries were most likely caused by the ducted propeller systems on ships operating in shallow waters.

Many of the seals were found in the Tay and Forth estuaries and north Norfolk.

Researchers at St Andrews' Sea Mammal Research Unit (SMRU) worked with the RSPCA and the Scottish Agricultural College to try to establish the cause of their deaths.

Their initial findings have eliminated most possibilities - including the effects of fisheries, deliberate mutilation, the effects of illegal traps and attacks by killer whales or sharks.

The team studied more than 30 carcasses of grey and harbour seals washed up with a single, smooth-edged cut starting at the head and spiralling around the body.

The researcher leading the investigation, Dr David Thompson, said: "Investigations have revealed a number of features that show the injuries are entirely consistent with the animals being sucked through large ducted propellers."

He said the team used scaled simulations with models to establish how the spiral injuries could be created.

'Rope cutter'

They also examined the injuries and concluded that the lacerations were made by the seals rotating against a smooth-edged blade while at the same time being dragged past the blade by a powerful force.

"Most diagnostic of all has been the imprint on some animals of the serrated 'rope cutter' that is present on most of these types of ducted propellers to stop ropes getting entangled in the propellers," he added.

However, the team said it had yet to understand why the seals would place themselves at risk of being sucked into these propellers in the first place.

Dr Thompson said: "There has been a lot of confusion as a result of a television documentary screened recently that claimed to show that Greenland sharks were responsible for similar types of injuries to seals in Canada.

"We are certain that this is not the cause of the injuries here and are also of a view that many of the cases from Canada are unlikely to be caused by sharks."

Professor Ian Boyd, director of SMRU and the Scottish Oceans Institute, said he hoped the current cases were isolated, "reflecting very specific circumstances that, once understood, can be avoided in future through simple changes to the operational procedures of the vessels involved".