Clue to mysterious 'corkscrew' seal deaths
- 25 August 2010
- From the section Scotland
A researcher stationed on a small Canadian island said she may have solved the mystery deaths of about 50 seals washed up along the UK's coast.
Experts were left baffled by the cause of the carcasses left with a single, smooth-edged cut that spiralled around the body.
Zoe Lucas who works on Sable Island said she had conducted an eight-year study of "corkscrew" seal injuries.
She claimed the Greenland Shark was capable of inflicting such wounds.
However, scientists at the Sea Mammal Research Unit (SMRU) at the University of St Andrews have ruled out the possibility of the deaths being caused by sharks.
Scotland's Environment Secretary Richard Lochhead asked the seal unit to investigate the cause of the injuries, seen in the coastal areas around Norfolk and the Tay and Forth estuaries.
It has said the most likely cause was seals getting sucked into a ducted boat propeller - used on vessels that need to move slowly or remain stationary.
Canadian researcher Ms Lucas told the BBC Scotland news website that the same seal injuries were seen on Sable Island, which is a highly restricted area.
She said: "There are no ships operating close to the beach. Here on Sable Island we can actually pin it down to a marine predator."
But SMRU director Prof Ian Boyd insisted the pattern of the cuts was not characteristic of a shark injury.
However, Ms Lucas said the Greenland Shark, which is a member of the dog-fish family, used a very different mechanism for killing than other sharks.
She explained: "Its teeth aren't the big cutting teeth of White Sharks, Tiger Sharks and Mako.
"The lower jaw is a band of interlocking teeth and the upper teeth are very small and pointy like little thorns and they're for grasping, not cutting.
"The shark likely gets a grasp on the seal tissue and just with a lot of violent head shaking it manages to get a grip on this tissue and then tear it and just rip it off.
"The reason you get the corkscrew is because it's quite likely that the tear itself just runs along the line of least resistance, which could be along the collagen fibres which wind around the body of marine mammals in a diagonal pattern."
Prof Boyd said it was strange that the injuries seen on Sable Island seals were similar to those in the UK.
However, he said the more likely common factor was that there were gas rigs near Sable Island.
He said: "It's not quite true that they don't have vessels at Sable Island similar to the ones we have here.
"The most likely cause of this is the animals being caught in ducted propellers.
"In fact some of the marks on the animals themselves have tell-tale signs of these particular propellers.
"Each of these propellers has what's called a rope cutter and you can actually see signs of a rope cutter having actually hit the animal in various places."
Ms Lucas argued: "I don't think they understand the mechanism in which these wounds could have been caused.
"The shark isn't cutting that tissue. It's tearing the tissue of the seal.
"So that clean-edged wound isn't caused by teeth, it's caused by the tissue tearing and you can duplicate that."
A paper on Ms Lucas's study, which included analysis of about 4,000 seal corpses, has been submitted to the journal, Proceedings of the Nova Scotian Institute of Science.