Phocaena phocaena is well known as the dolphin’s poor relation, the harbour porpoise. They have often been found stranding in large numbers in the North Sea, but recent sharp-edged mutilation on these bodies have aroused suspicion about the perpetrators. Halichoerus grypus, the grey seal is a large predator of fish but are they attacking to cetaceans and causing their deaths?
721 bodies, investigated between 2003 and 2013 were investigated to prove this theory correct or otherwise. 25% proved to have salivary DNA evidence of seal predation on healthy, recently-feeding juvenile porpoise, whose thick blubber made them attractive to the seals (blubber was absent in 62% of attacks.) In the Netherlands this scientfic work has proved that grey seal attacks seem to be one of the predominant causes of death of porpoises.
Ducted propellers, fishermen hiding their bycatch, a small number (6) suffering from an infectious disease and other predators/scavengers have all been suggested as the causes of death. But active killing by the seal is now the proven verdict on these deaths. The porpoises bled just before their deaths and probably died from the effects of 5 wound-types. Small repetitive bites on the head, the tail and the flippers, created substantial loss of blood. The biggest bites passed through the epidermis into the thick blubber. Large amounts of blubber were missing in places where the large seal bite area of a minimum 5x10cm area existed.
Where the head or tail had not been affected, the corpse was still assumed to be a possible seal attack, when the blubber was removed in the same way as the rest of the 62% attacked. Possible escapees were those porpoises that had head and tail injuries but no blubber missing. They still died of these injuries.
It is likely but not proved that this type of behaviour was uncommon before 2003. The numbers of mutilated animals found has increased over the last decade. The increased number of dead porpoise in nets, caught by bycatch could well have influenced the seal behaviour, which rings alarm bells for those people who holiday often on the same beaches on which the porpoises are found. Could there be a new movie title from Holland-wood:
The Attack of the Vampire Seals?
Mardik F. Leopold of Wageningen IMARES in the Netherlands and his colleagues from Utrecht University and the Royal Netherlands Institute for Sea Research publish their incriminating paper today, entitled Exposing the grey seal as a major predator of harbour porpoises. Reading more on cetaceans? The records for 2013 in the North Sea show 59 porpoises here. A little disappointing, but grey seals breed all around the British Isles and this habit, if it is new, could be spreading from the south