Custom Search

Friday, August 26, 2011

Mystery 'corkscrew' deaths of seals revealed as fatal attraction to sounds of ship's propellers

-Females may be mistaking noise of ships for mating calls

By Daily Mail Reporter

Scientists investigating the death of seals washed up with mystery 'corkscrew' injuries now believe that females may have a fatal attraction for some ship's propellers.

Experts suspect that female harbour seals - also known as common seals - are lured into the killer ducted blades because they produce the same accoustics as mating calls of male seals.

Of seven harbour seals found along the Fife coast with the horrific trademark injuries this summer - five turned out to be females.

The UK is host to populations of grey as well as common seals. Scientists were left baffled when dozens of corpses were found with horrific injuries

Scientists at the Sea Mammal Research Unit at St Andrews University have now traced twice as many seals killed by the corkscrew injuries - first reported last year - and over a wider area and period.

They believe off-shore wind farms may have increased boat traffic.

When they analysed the deaths they found nearly all the deaths in the summer were adult females but the deaths in winter were mostly juveniles.

At first the phenomenon baffled scientists until they struck on the theory that the sound of certain ship's propellers may be attracting the female seals.

They have now carried out acoustic tests on both captive and wild seals - playing recordings of propellers - which seems to initially back-up the theory.

However, they cannot yet explain what is attracting the young.

The scientists are now convinced that the corkscrew wounds are caused by ducted propellers from coastal inshore boats - and not sharks.

The corpses of at least 50 seals confused experts at first because they had all suffered a single smooth-edged cut spiralling the length of their body.

Since last year the total of 'corkscrew' deaths has now risen to 90.

It is thought the horrific single-cut injuries have been caused by boat propellers

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

'The number and range of deaths is more extensive than we first thought. They are more extensive in the UK - we now have confirmed cases in the Firth of Forth, Aberdeenshire, Ardrossan in Ayrshire, Orkney, Northumberland, Strangford Lough in Ireland, as well as the previous ones in Fife and Norfolk.

'Seals with similar characteristic spiral or corkscrew injuries have been reported from Atlantic Canada for at least the last 15 years at Sable Island off Nova Scotia and the Gulf of St Lawrence.

'What we have found is that in the summer the deaths are almost exclusively adult female harbour seals and in the winter mostly juvenile grey seals.

'The possibility is that the females are being attracted by the sound of the motors. Seals communicate at a similar low frequency roar during the breeding season.

'It is clear that the seals are responding inappropriately to some aspect of the operation of these devices. The localisation in space and time of these events makes it unlikely that the seals are being hit as a result of random coming together of swimming animals and fast moving vessels.

'The concentration of carcasses in each locality suggests that the vessels must be either stationary or slow moving but operating their propellers, such as when using motors for dynamic positioning.

'This suggests that some aspect of the operation of these devices is attracting the seals to within a danger zone from which they do not appear to be able to escape.

Developing any mitigation measure will require that we identify and understand the attractive mechanism.

Since cases were uncovered in Fife and Norfolk (see map) there have been confirmed cases in Aberdeenshire, Ardrossan in Ayrshire, Orkney, Northumberland and Strangford Lough in Ireland

'An acoustic cue is suggested by the fact that all seals killed during summer month have been female harbour seals which are thought to be attracted by underwater calls of breeding males.

'Juvenile grey seals which are the main victims during winter months in Norfolk and Scotland have also been shown to be attracted by conspecific calls with a pulsing rhythmic pattern.'

Trials have been taking place looking at matching the sounds of boats and the mating calls of seals.

'We have not identified the propeller types yet, but if we do we have the potential to change the sound and possibly avoid these types of deaths,' said Dr Thompson.

'But we believe that workboats are highly likely to be involved - and in recent years there are more and more of these type of boats working in coastal waters. That is why the problem appears to be increasing.

'We think these corkscrew deaths have now been around in the UK since the 1980s. We do not know the extent of the problem. We know that only a tiny proportion of the 30,000 plus seals that die each year in UK waters are washed ashore.

'Our methods included scaled simulations using models that show how the spiral injuries can be created, as well as fine-scale observation of the injuries themselves that show 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.'

The SMRU's preliminary report will be presented to Marine Scotland by the end of the year.



Post a Comment

Note: Only a member of this blog may post a comment.

Twitter Delicious Facebook Digg Stumbleupon Favorites More

Powered by Blogger