Between June 2008 and December 2010, more than 76 seals washed up dead on the shores of the UK: harbor and grey seals, male and female, juvenile and adult, with one similarity – an identical curvilinear skin laceration spiraling along the length of the body.
Similar sightings had been reported in Canada, and were attributed to Greenland sharks, but the lack of Greenland sharks in UK waters and additional post-mortem examinations led a team of scientists at Scotland’s Sea Mammal Research Unit to a different conclusion.
Researchers closely examined the occurrence of corkscrew seals and found the majority of cases to be harbor seals, 85% of which were female. The team also found all seals were healthy prior to the injury, which was fatal, along with some accompanying head injuries.
So what could be causing these strange deaths? After ruling out the shark theory, the researchers examined boat propellers, although most vessels with screw propellers would leave chop wounds rather than spirals. However, they discovered that vessels such as tug boats contain a certain type of propeller housing, different than typical boat traffic. These types of vessels were also frequenting Nova Scotia during the time of the corkscrew seal episode in Canada as well as in the UK. It was then concluded the work vessel-housed propellers were the culprit.
Mystery solved? No!
All of the affected seals seemed to be healthy at the time of death. Why would a healthy seal get dragged into a propeller, assuming it has normal sensory function and agility? After some preliminary analysis, researchers at SMRU suggested that the sounds produced by these specific propellers are different from a standard propeller noise. Not only that, but it is thought that the sounds are alarmingly similar to harbor seal mating calls. This “fatal attraction” also explains the disparity in male to female ratio in corkscrew seals.
Unfortunately, these propellers are almost exclusively used in the offshore energy construction underway in Scotland, which is limited to shallow coastal areas also inhabited my harbor seals. The team at the Sea Mammal Research Unit is currently conducting more research to determine the best way to save harbor seals from these grisly interactions, while expanding the scope of offshore renewable energy in the UK. Click here to learn more about their work and further developments of the case of the corkscrew seals.