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…