Investigation in the Marine Park!

  • Le matin du 10 juillet, des goélands picorent quelque chose de difficile à définir, c'est un "bout" de marsouin! / On the morning of July 10, 2017, a handfull of gulls are picking at something hard to define, it's a "chunk" of porpoise! © GREMM
    27 / 07 / 2017 Par Mathieu Marzelière

    It all begins on the morning of July 10, 2017, a rather ordinary day. The weather is fairly mild, visibility is at its max, and the St. Lawrence looks like a sheet of glass. In short, we had the perfect conditions to find our giants. But upon leaving the harbour, a handful of gulls attract the trained eye of our captain. The birds are very active and are pecking at something hard to define that resembled a piece of plastic or rubber. With the approach of the first bird, the captain’s hypothesis is confirmed: it’s a “chunk” of porpoise!

    All in all, we collect no fewer than three pieces, one of which warrants a little more attention: we can make out a clear and distinct cut in the animal’s hide. Moreover, the animal’s flesh was completely absent… bizarre. But who (or what) might be behind this grisly encounter? These couple of clues lead me to believe that it was the result of poaching. But the captain leads us down a different trail. In his view, suspect number 1 would be… a gray seal!

    Is there predation of porpoises by grey seals? © GREMM

     

    Back in our port of call, I decided to do a little investigating. I’ve gone from research assistant to detective!

     

    Are porpoises targets for poaching? © Fishermen’s Voice

    The first thing I do is contact the Saguenay-St. Lawrence Marine Park team, which receives the carcasses for analysis. For years, park wardens have questioned the origin of these mutilations and no theory has been ruled out, particularly poaching, as historically porpoises taken accidentally were consumed by locals. Perhaps this tradition still lives on today?

     

    Let’s go back to the captain’s theory. There are a few studies from Europe on predation of porpoises by gray seals. Researchers from the University of Lièges in Belgium have speculated on the origin of the mutilations observable on the porpoise carcasses stranded on their beaches. After identifying the jaw potentially behind these acts, a DNA test in the wounds confirmed the identity of the attacker: it was indeed a gray seal. There is thus a form of predation by the latter. It must be said that at 350 kg (wet), a large adult gray seal can easily attack a small cetacean weighing 50 kg. Is it predation for feeding purposes? A confrontation related to competition for the same food resource? A form of play or sparring between males?

    There are few studies from Europe on predation of purpoises by grey seals. © GREMM

    I therefore got in touch with researcher Anik Boileau of the Sept-Îles Education and Research Centre (CERSI).  Anik is a specialist in harbour porpoises and in recent years has received and analyzed carcasses of this species whenever they are reported to the Quebec Marine Mammal Emergency Response Network. Although researchers have documented such predation in Great Britain, it has not been confirmed in the St. Lawrence; for the researcher, however, it could very well exist here as well. A number of professional seamen in the St. Lawrence report having witnessed with their own eyes an incident of predation by a gray seal on a harbour porpoise. But unfortunately, no photo or video evidence has been produced to document the scene.

    Besides the gray seal, Anik Boileau mentions a new suspect, a close cousin of the porpoise… dolphins. There is believed to be one documented case of such an attack on harbour porpoises. Similarly, the researcher currently has a carcass that shows traces of bites similar to those of a white-sided dolphin. These aggressions between different species might thus suggest interspecific competition.

    Is there interspecific competition between Atlantic white-sided dolphins and porpoises? © GREMM

    Of course, other species that frequent our waters such as the great white shark or the killer whale are also possible suspects. However, in the St. Lawrence Estuary, it seems unlikely that either of these two predators attacked the porpoise we discovered.  It is also noted that the main cause of mortality found in porpoise carcasses is infectious diseases.

    Unfortunately, there is no designated culprit at this time, but an investigation is under way. So, if you’re walking along the beach or out at sea and you find a piece of flesh resembling that of a porpoise, please contact the Quebec Marine Mammal Emergency Response Network directly at 1-877-7baleine. With this small gesture, you can help advance research and enable us to bring this investigation to a close.

    Story to be continued…

     

    Sources

    Exposing the grey seal as a major predator of harbour porpoises

    Les problèmes de voisinage des Marsouins

    Short Note: Predation of Harbour Porpoises (Phocoena phocoena) by Grey Seals (Halichoerus grypus) in Wales

    Centre de recherche et d’éducation de Sept-Îles

    Les phoques gris, tueurs de marsouins?


     Mathieu Marzelière joined the GREMM team this year as a volunteer research assistant. As part of the photo-census program of large rorquals in the Marine Park, he collects photos and data on board tour boats. He also shares this information with the editorial team of Whales Online.