Dozens of heavily mutilated harbour porpoises that wash ashore every year on Dutch beaches are the victims of attacks by grey seals. This was the conclusion of a joint study conducted by the Faculty of Veterinary Medicine (University of Utrecht), IMARES Wageningen UR, and the NIOZ Royal Netherlands Institute for Sea Research. DNA from three different grey seals was found in fresh bite wounds on three dead porpoises that washed ashore in August, October, and December 2013. It was a landmark discovery. Never before has forensic examination of DNA in washed-up cadavers proven successful.
Microscopic examination revealed that the wounds had been inflicted on living porpoises, and clearly pointed to fatal attacks and not to carrion eaters.
Various theories were mooted about the reason for the mutilation. Belgian researchers suggested in 2012 that grey seals might be preying on porpoises. Then, in 2013, a French research group published images of three cases in which a swimming porpoise appeared to be under attack from a grey seal. However, there was still no irrefutable evidence that grey seals were actually killing porpoises. The situation prompted the Ministry of Economic Affairs to order an investigation to determine the cause of death of the washed-up porpoises and make funding available for DNA analyses and autopsies. Questions were asked about how these wounds had been sustained. And, more to the point, could they have been inflicted unintentionally by fishing operations or shipping? The findings of this research project show that this is not the case.
Autopsies were performed on more than 1,000 washed-up harbour porpoises between 2003 and 2013. Traces of bite and claw wounds were found on around 25% of those that still allowed a full investigation. This suggests that these individuals had been attacked. Harbour porpoises may be regarded as prey by grey seals. Indeed, chunks of nutritious blubber had been ripped away from the mutilated porpoises, mostly youngsters in excellent nutritious condition.
The populations of harbour porpoises and grey seals in Dutch waters have increased considerably in recent decades. At present, the number of grey seals is rising by around 15% a year. At the same time, the number of harbour porpoises has been rising. Grey seals feed mainly on fish, but apparently have found a new and rewarding food source: porpoise blubber. How exactly the seals manage to overwhelm the porpoises is still unclear.
The project was carried out by three institutes and involved three different disciplines. It relied on a small army of volunteers who patrolled Dutch beaches, picking up the cadavers of washed-up porpoises for further examination. The autopsies were performed at the Faculty of Veterinary Science at the University of Utrecht. The DNA of grey seals in the wounds was discovered in the Laboratory of Molecular Biology at the NIOZ. The wounds on the porpoises were jointly assessed by the Faculty of Veterinary Science at the University of Utrecht and IMARES Wageningen UR. IMARES also took charge of the dietary and biological aspects of the research.