On Thursday 13th May 2010, a group of five to ten bottlenose dolphins (Tursiops truncatus) chased and attacked a harbour porpoise (Phoconea phoconea) approximately 600 metres off shore from New Quay Head, Ceredigion in Cardigan Bay. The attack was witnessed by volunteers of Sea Watch Foundation and PhD research assistants from Bangor University as they carried out monitoring of marine mammal populations from the cliff top.
At first the witnesses thought that the dolphins were feeding, chasing fish, but it soon became clear that they had a more sinister motive in mind when the harbour porpoise was seen being tossed between two dolphins. The attack is estimated to have lasted between ten and twenty minutes. Volunteers lost track of the porpoise, but it is unlikely that it survived such an attack. SWF travelled by boat to the attack site to recover the porpoise body for post mortem, but sadly the boat reached the site too late and the body could not be found.
A large proportion of harbour porpoise strandings in Cardigan Bay are due to bottlenose dolphin kills. Classic injuries include broken ribs, haemorrhaging and teeth marks on the skin. The first case of a bottlenose dolphin attack on a porpoise was confirmed in the early nineties in the Moray Firth, NE Scotland and these attacks are the cause of death for the majority of harbour porpoise strandings in the area today. It is unknown why dolphins attack their smaller cousins; some researchers believe it may be a result of competition for food resources in an area or sexual aggravation.
Despite the apparent high number of attacks, the event is rarely witnessed. The first eye-witness account of this violent dolphin-porpoise interaction in Wales occurred in 2004, ten years after these attacks were identified through pathology. There have been just two more confirmed observations in Wales, once from SWF Director Dr. Peter Evans in February 2008 and in May 2009 by a SWF volunteer whilst undertaking a survey on wildlife watching trip boat, Ermol V.
Hanna Nuuttila and Bangor University are currently undertaking a PhD project in collaboration with Sea Watch to further assess the effectiveness of passive acoustic click detectors (PODS) as a monitoring tool for cetaceans. These PODS record echo-location clicks produced by the dolphins and porpoises and Sea Watch has found using the PODS that habitat partitioning exists between bottlenose dolphins and porpoise in Cardigan Bay, i.e. the harbour porpoise may be avoiding the dolphins. For further information see Simon et al. 2010.
This new POD project has involved the placement of 42 acoustic C-PODS recording clicks in a 1km grid off shore from New Quay harbour. The attack occurred within this experimental grid and it is anticipated that the PODS will have recorded the clicks produced during the event. Hopefully valuable information will be gained to learn more about the social interactions of bottlenose dolphins and harbour porpoises.