“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.

(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