Scientists have discovered through DNA analysis that harbour porpoises (Phocoena phocoena) that have washed up severely mutilated and dead along the Dutch as well as the Belgian and northern France coastline beaches with an unknown cause of death and mutilation were actually attacked by grey seals (Halichoerus grypus). This is a new behaviour for the normally fish eating mammals.
Two different papers came out this past month (Leopold et al and Jauniaux et al) confirming this behaviour in the North Sea though DNA analysis (as well as a Note in Marine Ecology Progress Series on the ability to extract usable DNA from bite wounds). By collecting swab samples from bite marks on washed up harbour porpoises scientist were able to confirm that the injuries and death had been inflicted by Grey Seals. Along the Dutch coastline 721 of 1,081 stranded porpoises were examined by Leopold et al between 2003 and 2013 of these at least 17% were subject to attack by grey seals. If we take into account that individuals with larger wounds would have sunk without recovery and that some porpoises may have escaped attack and later died from wounds this number may be much higher and mean that seal attacks are a significant contributor to harbour porpoise death in the North Sea alongside fisheries bycatch (approx. 20%), infectious disease (approx. 18%) and emaciation (approx. 14%).
The Jauniaux et al study along the French and Belgian coastline the scientists instead looked at five stranded porpoises with bite-like skin lesions which they swabbed for genetic material. They used these to confirm that even after several days in seawater genetic material of grey seals can be recovered from wounds and that bite-like skin lesions found on the dead porpoises is definitely the result of grey seals attacks. It was also found that some wounds (puncture) from grey seal attacks more readily retain genetic material than others (lacerations).
Scroll down to the bottom to see some of the rather graphic images of mutilated porpoises.
So how, when, why, where did this all start?!?
These are all very good questions and it will be hard for scientists to come up with a definitive answer for all of them. Leopold et al speculate that grey seals could have progressed from opportunistically feeding on harbour porpoise that had become entangled in fishing gear to full on attacking them as prey. The first confirmed victim of a grey seal attack washed up in 2003 and between then and 2013 the number of mutilated carcasses increased the authors states that there would
have to have been the perfect set of prerequisite in place for this to have come to be:
“These include sympatry of predator and prey, and possibly a high incidence of fisheries bycatch of the prey in static fishing nets to induce this behaviour.”
The study by Jauniaux et al even used the head of a recently dead grey seal to mimic bite-like skin injuries on a porpoise carcass to confirm seal DNA transfer. They believe that the injuries are likely from predation although cases of aggressive behaviour can not be ruled out for all attacks.
So what does this mean for harbour porpoises?