When grey seals become killers

What predator has been attacking porpoises? The animals have increasingly been found washed up on North Sea beaches bearing unusual bite marks. The specialists who have been addressing this question for the last few years have identified the culprit as grey seals, which have never been known to attack these kinds of animals before. They were given away by their DNA which was detected in the bite marks as Thierry Jauniaux and Mutien-Marie Garigliany both of whom are researchers at the Animal Pathology Laboratory of the University of Liege have shown. But the reasons remain to be determined.

The wounds are reminiscent of bite marks from a large dog or slash wounds from an unscrupulous fisherman. Parallel strips of flesh were cut deep into the skin and fatty tissue (the sub-cutaneous layer of fat) of the porpoises. In a career spanning more than twenty years, Thierry Jauniaux has never seen anything like this despite the fact that he carries out autopsies on more than 150 marine mammals per year. “So evidently, I will notice any new phenomenon immediately".

But up to 2011, the bodies of these cetaceans he examined had died from an infectious disease, such as a virus, or a complications due to a parasite or simply because they had been caught in a fisherman’s net and had then been released.  

Then the first case of an animal bearing these somewhat unusual cutaneous bite marks occurred. Then another and another and so on. The theory of an aggressive dog roaming the beaches or a fisherman slashing the bodies of animals that were caught by accident in order to ensure that they sank into the ocean were not very convincing because these strange bite- marks, observed on the neck or tail of the animals were inflicted before death. “Also, the marks were not clear”, explains Thierry Jauniaux while cutting a piece of paper with a scissors and then with his fingers by way of demonstration. “In the first case, the edges are smooth, in the second case, they are slightly jagged. It is this second type of bite that we have observed on the porpoises so this could clearly not have been caused by a knife”.

In order to identify the author of the bites, the distance between the edges of the wounds was measured and compared to the interdental spaces of different species. A match was found with grey seals.

Grey seals !!?? These animals had never been known to attack porpoises which are sometimes called the ‘pigs of the sea’. The video of an attack, filmed by an ornithologist on observation duty in Cap Gris Nez (North of France) who had had the presence of mind to put his smartphone on long-view, confirmed the suspicions. But the distance of the observation was quite considerable and doubts still remained. In short, it was necessary to prove the phenomenon scientifically. 

“The Experts”

“We started to play the role of expert at this point except that it took us more than 45 minutes to solve the mystery”, says Thierry Jauniaux and Mutien-Marie Garigliany, researchers at the Animal Pathology Laboratory of ULg. The results of this study have just been published by the online magazine PLOS ONE (1).

As often demonstrated in American TV series, the identity of the culprit was confirmed largely thanks to DNA. During autopsies on five porpoises, swabs,-those long cotton strips that police scientists always use – were rubbed inside the lesions. Given the long time spent in sea-water by the carcasses before they reached the shore, the chances of detecting genomic DNA were slim.

On the other hand, mitochondrial DNA was seen to be more resistant. “It keeps for a much longer time thanks to the prolonged maintenance of the integrity of the mitochondrial membrane”, explains Mutien-Marie Garigliany. The analyses made it possible to detect some of this DNA, not on the edges of the lacerations, but where the tooth-marks were deepest. A genetic imprint left by the aggressive animal’s saliva although in limited abundance.  

“It was therefore necessary to perfect an amplification method which, by means of DNA traces, could lead to the detection of quantities in sufficient amounts to detect a sequence”, explains the researcher. “But nothing existed in the literature for this species; we needed to do everything from scratch”.  The use of DNA for the study of predation behaviour had already been applied to the ecosystem on land but never in the marine environment.

This totally new procedure yielded results: the grey seal was indeed the cause of these wounds. Their DNA was identified on two of the five carcasses. In order to be completely sure, the researchers from Liege conducted another test. Using the head of one of these large dead mammals, they simulated a bite-mark on the carcass. The result was a match again. 

A migration that raises a question

There was therefore no doubt as to the identity of this culprit or “murderer”. If these bite-marks were not the direct cause of death, they probably led to it. Four of the five ‘sea pigs’ autopsied died following a pulmonary edema resulting from asphyxiation. In other words, the grey seals tried to submerge the porpoises until they could no longer breathe.

Why did these attacks commence? And first of all, what are grey seals doing in the North Sea? Ten years ago they were completely absent from this zone preferring the waters of the North Atlantic and the Baltic Sea and more particularly, the coasts off Iceland, Norway and the United Kingdom. “Between 1991 and 2002, I did not have a single case to autopsy”, confirms Thierry Jauniaux. “From 2002, I have come across some cases. Today, there are more and more cases of the phenomenon”. The same applies to the porpoises: between 1991 and 2000, the researcher saw 55 cases. Currently, this number can be reached in six months.

While the numbers of these marine mammals is decreasing, there has been an explosion of grey seal numbers in the North Sea. Is this migration to the detriment of other species? Similar attacks have also been observed on common seals in Germany. When faced by other mammals, grey seals can count on their imposing physical stature. Weighing up to 320 kilos and capable of measuring up to two metres in length, only whales and sharks are their predators. There are no sharks along the coasts of Belgium or the North of France…Weighing a maximum of 100 kilos; the ‘sea pigs’ don’t stand a chance.  

Are the bites studied by the specialists at ULg the result of a battle for the same prey? The theory of competition for food could be envisaged. Although the grey seals eat mainly large fish and porpoises tend to content themselves with small crustaceans, some species are eaten by both. “We can imagine that these attacks are the result of overfishing, which diminishes the quantities of fish available and leads to greater competition”, explains Thierry Jauniaux.

It could also be thought that these attacks are not cases of simple aggression, but that the grey seals have started to feed on porpoises… This explanation is sometimes put forward, all the more so because the bites are more common in winter, a period when metabolic requirements are at their highest. But the step towards making this conclusion cannot so easily be made because following the autopsies, when all the pieces of skin and blubber are “reconstructed”, nothing seems to be missing. To establish the facts, it would be necessary to analyze the content of the digestive tracts of these new predators to try to find traces of the supposed new prey. But these predators transform and assimilate food very quickly: in barely one hour, their stomach is empty. A great deal of luck would be required to find them on a beach just after a meal!

Are nets to blame?

Another theory that has been suggested: an aggression provoked by too much proximity in a limited species. “Sometimes the marks of fishing nets are observed on the bodies of the porpoises. We suspect that the bite marks in this instance are the result of overcrowding”, he continues, adding that the attacks are generally localised in certain places, Boulogne-Sur-Mer and Dunkerque in France. The grey seal is a somewhat sedentary species even though, at some times of the year, they are known to make long journeys. “Could this be the work of isolated individuals or even an individual male adult seal”?

Here again genetic technology can shed light on the situation. “To establish the sex of the animal, we needed to use genomic DNA but, once again, we have little chance of detecting it given the long periods the carcass has spent in the water before beaching”, points out Mutien-Marie Garigliany. “On the other hand, thanks to mitochondrial DNA, we could determine whether we are dealing with different individuals but in order to do this, we need to accumulate more samples”.

The mystery around this new predatory behaviour is far from being solved. For the two specialists from Liege, the grey areas surrounding the problem must not be used as an excuse by lobbies to start a new seal hunt. Nature has reasons that mankind can try to understand, but against which he can only act in a very limited way…

(1)  Bite injuries of grey seals (Halichoerus grypus) on harbour porpoises (Phocoena phocoena), Thierry Jauniaux, Mutien-Marie Garigliany, Pauline Loos, Jean-Luc Bourgain, Thibaut Bouveroux, Freddy Coignoul, Jan Haelters, Jacky Karpouzopoulos, Sylvain Pezeril and Daniel Desmecht, PLOS ONE, 2014