IT was a most unusual murder mystery which had left scientists baffled and ecologists perplexed.
Porpoises were being killed by a wounds infected with a distinct bacteria thought to only be found in dog bites, their carcasses washed up on shore displaying deep gouges and cuts.
But now it has emerged that the culprit is not some aggressive aquatic canine, but common grey seals which also carry the bug in their teeth.
The seals frequently attack porpoises, and those they didn't manage to kill were escaping only to succumb to the bacteria later.
The discovery was made after scientists from a wide range of backgrounds pooled their resources to get to the bottom of what was killing the harbour porpoises.
Grey seals have been revealed to be behind the porpoise deaths
In a study led by researchers at Scotland’s Rural College (SRUC), microbiologists, pathologists and ecologists took a forensic approach to identify the bacteria found in wounds of animals found stranded in Scotland, England, the Netherlands and Belgium.
READ MORE: Firth of Clyde dolphin found to communicate with porpoises
The mystery deepened when they identified it as Neisseria animaloris – previously linked to human infections following dog bites.
Lead researcher Geoff Foster, a microbiologist with SRUC Veterinary Services, said: “We realised it was unlikely that porpoises would be bitten by a dog.
“Subsequent investigations indicated that the porpoises had been attacked by grey seals. Such attacks can be fatal, but in the cases of the animals involved in this investigation, the porpoises appear to have survived the initial assaults, only to die later of infections introduced from the mouths of seals during the attacks.”
Previous research found that grey seals are the main predators of porpoises, with seal attacks among the most common causes of death in this small whale species.
The seals are effective hunters
This new study, published by Scientific Reports, has shown that porpoises which survive the initial attack may still die as a result of the bite injuries.
Co-author and lead researcher in the Netherlands, Lonneke IJsseldijk, said: “A predator never has a 100 per cent kill. There are always animals that escape. But for these porpoises this did not end well: after a long agony, these bitten porpoises still died.”
READ MORE: Seals and whales poisoned by sea's toxic legacy
The research also highlights a potential risk to humans, with Neisseria animaloris having a so-called zoonotic potential, which means that people can become infected with it.
Ms IJssedijk, who works in the Faculty of Veterinary Medicine at Utrecht University, said: “People should be careful if they come into contact with curious seals on the beach or while swimming.”
She added: “Grey seals have only been found in the southern North Sea since the 1980s, with their numbers rising to several thousand nowadays. Porpoise numbers in the southern North Sea have also risen in the last few decades, leading to increased encounters between the species – and a possible explanation for the rise in seal-related porpoise mortality recorded in this particular area.”
Porpoises ar the UK's smallest cetacean
Mr Foster added: “The number of porpoises killed directly or indirectly from these attacks will be higher than initially anticipated.
“This work highlights a previously unknown consequence of grey seal attacks on porpoises that is of relevance to scientists studying the threats to other wild populations, as grey seals also attack other marine mammals.”
READ MORE: Citizens encouraged to be 'eyes on the sea' and count whales and dolphins
The research was funded by the Scottish Government, UK Department of Environment, Farming and Rural Affairs (Defra) and the Dutch Ministry of Agriculture, Nature and Food Quality.
It was carried out by SRUC and Utrecht University in partnership with the Animal and Plant Health Agency.
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