© Chris FallowsScientists have reported multiple cases of cape fur seals attacking blue sharks and eating their guts off the coast of South Africa.
While most people will be familiar with images of seals being violently smashed out of the water snatched in the jaws of a shark, it appears some are exacting their revenge on the ocean predators.
Scientists have reported multiple cases of cape fur seals attacking blue sharks off the coast of South Africa.
The seals, which are normally prey for great white sharks, have been seen attacking and killing medium-sized blue sharks, devouring their guts before leaving the rest of the carcass.
Divers also report seeing the fur seals attacking other species of shark and rays.
The behaviour has left marine biologists baffled as to why the seals, which normally hunt smaller fish, have turned to attacking the torpedo-shaped sharks.
The blue sharks are around the same size as the seals and it is thought to be the first time a seal has been seen preying on such a large predator.
It was first observed by marine photographer Chris Fallows, who runs a dive boat operation from Cape Town in South Africa.
Dr Neil Hammerschlaf, a marine biologist at the Rosenstiel School of Marine and Astmospheric Science who was involved in the study, said the seals appear to be eating the guts of the sharks as they contain the most energy.
He said this behaviour may have serious implications for blue sharks, as their numbers are already in decline due to over fishing.
'Seals are generally considered prey of larger sharks,and blue sharks have been observed pursuing juvenile and adult male Cape fur seals, though no kills have been witnessed,' he continued.
'There are relatively few reports of pinnipeds (seals) preying on sharks in general and in all cases these involved smaller sharks.
'The consumption of large sharks by a Cape fur seal is a departure from the prevalent view of this species' diet, which is generally reported to consist of a diverse diet of small fish species, cephalopods and birds.
'The prevalence of the behaviour reported here is unknown - it could reflect opportunistic predation by a generalist predator or it might be a common tactic.
'These observations are important not just for understanding the interactions between these two species but more broadly for their implications in understanding the trophic ecology of pinnipeds - many populations of which have increased while numerous shark populations have declined.'
Fur seals have been known to mob great white sharks in large numbers but this is thought to be a defensive tactic to scare them away from breeding colonies.
In this case the fur seals have been observed attacking the blue sharks, which are open ocean predators that can grow to be about 9ft long (2.82 metres), on their own.
Mr Fallows, who runs Apex Expeditions, first noticed the behaviour in 2004 while on a dive off Cape Town.
In 2012 he again saw a large Cape fur seal attack five blue sharks from a shoal of ten over a two hour period.
The sharks were estimated to measure between 3.6ft (1.1 metres) and 4.5ft (1.4 metres) in length.
Mr Fallows said: 'They swim up to them and grab them. There is no protracted chase.
'It is almost as though the sharks are not expecting this to happen and do not overly react to the seals advances.
'We have seen them feeding on three other small species of sharks. We have also observed them feeding on rays.
'I would think it is possibly a learned behaviour based on limited normal food availability.'
A study on the behaviour is now published in the African Journal of Marine Science.
Scientists believe the seals could be attacking the sharks partly to remove competition for the fish they normally eat.
Dominic Tollit, a marine mammal consultant with the Sea Mammal Research Unit in Canada, told New Scientist that attacking such a large and heavily armed predator is a high risk strategy if simply looking for food.
He said: 'Would you attack a lion if you're hungry, or would you go for something smaller?'
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