In the chilly waters of the North Sea, off the Dutch and Belgian coasts, a spate of vicious attacks has been baffling biologists. The mutilated bodies of harbour porpoises have washed ashore, their throats ripped out and skin covered in puncture wounds. Now DNA evidence has nailed the culprit 鈥� not sharks, propellers or fishing nets, as previously suspected, but grey seals.
鈥淚t鈥檚 hard to believe that a nice cuddly looking animal could do this,鈥� says Mardik Leopold of Wageningen University and Research Centre in the Netherlands, who led the work. 鈥淲e were very skeptical ourselves at first. We thought it was suction dredgers, but we鈥檝e been on these ships and observed and it鈥檚 impossible. DNA is really the smoking gun.鈥�
Leopold wasn鈥檛 the first to suspect that grey seals are behind the increase in porpoise attacks. In 2012, Jan Haelters of the Royal Belgian Institute of Natural Sciences ran an analysis of the wounds of beached porpoises that implicated the seals. At the time it was not thought possible that conclusive evidence of seal DNA would ever be found on the porpoises.
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Unexpected evidence
鈥淵ou have to sample the right wounds,鈥� says Leopold. 鈥淚t鈥檚 tempting to sample the big lacerations, but these are all washed clean. Next to these are tooth imprints that are smaller and pressed closed quickly. It鈥檚 at the bottom of these puncture wounds that we found DNA.鈥�
Leopold鈥檚 team covered all their bases, testing for orca DNA as well as dog, fox and wolf to rule out post mortem scavenging. All came back negative. The only positive match was for grey seals. The analysis showed that three different seals had attacked and killed three separate porpoises at different points along the Dutch coast.
鈥淭he hardest thing was the pathology,鈥� Leopold says. 鈥淭hese bodies are really messy when you find them. They鈥檝e been torn up and parts are missing and you simply don鈥檛 believe your eyes.鈥�
With DNA matches in hand, Lineke Begeman 鈥� the pathologist on Leopold鈥檚 team 鈥� started to compare the wounds and bite marks on the three confirmed seal victims with photographs of 1081 other porpoises beached between 2003 and 2013. Of the 721 porpoises whose carcasses were fresh enough to be properly examined, Begeman found that 17 per cent bore wounds likely to have been inflicted by grey seals. Another six per cent had puncture wounds but no torn flesh 鈥� probably those that escaped and died later. The seals are attacking healthy porpoises too, not the sick or weak. 鈥淭hey take the young, naive, fat ones,鈥� Leopold says.
Haelters, who was not involved in the research, believes this analysis is correct. The puncture wounds that DNA assigned to the seals are a match for those he and his team examined in 2012. The mystery is why grey seals have started attacking and eating porpoises.
Search for motive
鈥淚 believe the phenomenon is increasing,鈥� Haelters says. 鈥淚 have been collecting porpoises from Belgian beaches since 1995 [and] I am pretty sure we had no cases in Belgium prior to 2011. We now have cases every year.鈥�
Haelters and Leopold agree that an increase in fishing that accidentally traps porpoises in its nets is one possible explanation. Grey seals are known to scavenge nets, and may have been introduced to fat-rich porpoise that way. It may have just been a lucky bite. 鈥淚t is not unknown for pinnipeds to attack other marine mammals, so it could be that they developed a 鈥榯aste鈥� for the porpoises,鈥� says Tom Jefferson of the National Oceanic and Atmospheric Administration in the US.
While human swimmers need not panic, Leopold and Haelters say they should at least be aware of grey seal鈥檚 newfound taste for mammal. There鈥檚 no reason to think seals wouldn鈥檛 attack humans. 鈥淭his is, at least for Belgium and the Netherlands, our largest predator, and the images illustrating what they can do with a porpoise speak for themselves,鈥� Haelters says.
Proceedings of the Royal Society B DOI: 10.1098/rspb.2014.2429
