The deaths are cruel, violent and gruesome, and they are threatening the survival of some of Scotland's rarest wildlife.

Seals and porpoises around the coast are being killed in large numbers because they get trapped between ships' propellers and their covers, according to the latest scientific research for the Scottish Government. The animals die from distinctive "corkscrew" cuts spiralling around their bodies.

Scientists have concluded the slaughter could have contributed to dramatic declines in harbour seal populations on the east coast and is "unsustainable". But they are still investigating exactly which offshore industries might be to blame.

This weekend a powerful coalition of 10 environmental and animal welfare groups is demanding an end to the killing. They are calling on ministers to ban the use of the propellers in new offshore developments, and to restrict their use by ships in areas where seals are at risk.

"Hundreds of seals and porpoises have died needless and horrific deaths all around our coasts," said Sarah Dolman, Northeast Atlantic programme manager for Whale and Dolphin Conservation.

As many as 80 dead seals have been washed up around Scotland's shores, confirmed killed by corkscrew injuries, as well as four porpoises. Most of the bodies have been found in the last five years, and scientists say they may be only a small proportion of those that have died, as many carcasses could have been lost at sea.

The animals all suffered a deep gash encircling their bodies. The majority of the seals that died so painfully have been found along the east coast between North Berwick and Dundee, with another cluster around Orkney. Corkscrew deaths have also been recorded in a few other locations on the east and west coasts (see map).

The victims were often adult female seals, sometimes pregnant. Of the 80 confirmed corkscrew deaths, 32 were harbour seals, some populations of which have been in sharp decline, and 48 were grey seals, which are more common.

A wide variety of culprits has been blamed for causing the injuries, including sharks, killer whales, fishing equipment, naval submarines, tidal turbines, dredgers and even malevolent people with sharp knives. But these have all been ruled out by scientists from the Sea Mammal Research Unit at St Andrews University. In its latest report for Marine Scotland, part of the Scottish Government, it has pointed the finger at particular types of ducted propellers, widely used in marine vessels for many offshore industries.

"It's basically a shipping issue," said Dr Dave Thompson, the unit's seal biologist. ''Many vessels with ducted propellers work close to shore and all of them need to enter and leave ports and may then come into areas with large numbers of seals."

Dr Andrew Brownlow, a veterinary ­investigation officer at Scotland's Rural College in Inverness, said the corkscrew deaths were having "a serious detrimental effect" on local populations of harbour seals. "This is most notable in the Firth of Tay where seal numbers have declined 90% since the early 1990s," he added.

Given harbour seal numbers in this area are already critically low, any additional mortality, especially of breeding females, is a serious problem.

"We have no reliable way of estimating what proportion of the casualties we are seeing, but given cases have to float, make landfall, be spotted and finally reported, it is probable a significant number of animals are being missed," Brownlow told the Sunday Herald.

The 10 groups that have written to Marine Scotland demanding action to stop the killing include Whale and Dolphin Conservation, Wildlife Trusts, Marine Conservation Society and OneKind, the Edinburgh animal rights group. They are supported by the National Trust in England, which has recorded 45 seals killed by corkscrew injuries washed up in or around its reserve on the north Norfolk coast.

The groups' joint letter argues that the best way to prevent more deaths is to prohibit the use of ducted propellers as part of the licence conditions for new developments. It also suggests offshore industries should been asked to report and restrict the use of the propellers close to seal populations.

Offshore industries expressed concern about the deaths, but none accepted responsibility. "The death of any sentient marine mammal, particularly after sustaining such horrific injuries, is something that no mariner would wish to see," said David Balston, the safety and environment director for the UK Chamber of Shipping.

"The UK Chamber has, over the last two years, tried therefore to engage with the lead researchers working in this area, ­offering our expertise and that of our members to find a practical solution, but without success."

REACTION

The offshore oil and gas industry has promised to work with others to develop mitigation techniques, if they are deemed appropriate. "There is insufficient data to enable a definitive conclusion to be drawn on the reasons for these injuries," said a spokesman for Oil & Gas UK.

Seafish, the fishing industry body, stressed fishing methods were not to blame, and pointed out there were no large-scale fishing operations in the areas where most seals had died.

Scottish Renewables, which represents companies developing offshore wind, wave and tidal technologies, said some of the seal deaths pre-dated its activities, and that work should continue to identify the causes and "any changes in working practices or design required to protect marine wildlife".

The Scottish Government highlighted other reasons for the decline of harbour seal populations, but agreed it was important to find out exactly what was causing corkscrew deaths and to consider possible mitigation measures once researchers had completed their investigations.