In Scottish Marine Animal Stranding Scheme, author Andrew Brownlow records that two seals found on the same stretch of beach in 2009 were juvenile harbour seals (Phoca vitulina).
Three seals examined in 2010 were pregnant adult harbour seals.
The key pathological findings were consistent in all five seals suggesting a common cause. All had corkscrew-shaped wounds originating on the left hand side of the mouth and spiraling in a single, clean-cut continuous wound around the head and neck to end midway down the body. In every case, the wound spiraled clockwise when viewed head-on.
Carcasses were relatively decomposed, however it is considered the seals were alive when the injuries were sustained. With this degree of trauma it is likely death was instantaneous.
Seal skin is tough and these animals showed a single unbroken clean cut, angled at about 35 degrees towards the tail.
The cut extended onto the rib cage but did not cause rib fractures or trauma to underlying harder body structures.
The juvenile seals examined by SMRU exhibited cuts through the bone of the skull. The blubber and connective tissue had been 'peeled' off the underlying skeleton, indicating powerful shear forces were involved in addition to the cutting process.
There was no evidence of any underlying disease process in any of the seals examined. Blubber thickness was normal for the time of year in all cases. The stomach of one animal had a large amount of partially digested fish showed it had recently fed.
The seals examined were either juvenile or heavily pregnant. It is possible animals in these groups have different feeding behaviors, or are slower than normal
Bob Reid, Scottish Strandings Coordinator (left)
"I am confident that we have to be looking for some type of ducted propeller/impeller that has not been used in this area previously, especially around the time of year when seals are heavily pregnant.
It is possible that pregnant seals and young seals are being attracted to the vicinity of these impellers by dead fish that are also being killed by the same devices or by shock waves caused by pile-driving by the vessel that is using the impellers. Freshly dead fish to eat without expending effort in catching and killing them might seem like an attractive proposition.
If we assume that these impeller/thrusters devices are linked to a GPS system and are used to keep a vessel in position then they would be constantly starting and stopping and if one was switched on while a seal was very close by, especially a heavily pregnant one it could easily be sucked into the mechanism causing the injuries described.
Dredging and Archimedes Screw sewage plant treatment have also been proposed.