By Ellie White
Following questions from the audience who attended a series of talks on cetaceans hosted by Conservation Chat UK, a member of our research team at Operation Cetacean has been diving into the literature surrounding conflicts between dolphins and porpoises.
Attacks on Harbour Porpoises (Phocoena phocoena) perpetrated by both Common Bottlenose Dolphins (Tursiops truncates) and Short-Beaked Common Dolphins (Delphinus delphis) are a phenomenon that has been reported by budding wildlife spotters along the UK coastline. However, little research into this area has been conducted as to how often and why these attacks occur.
But what do we know?
Acoustic studies have shown that in areas where species of dolphins and Harbour Porpoises coexist, the porpoises often avoid interactions with the dolphins.
It is generally the larger species of Bottlenose Dolphins who have been reported as attacking Harbour Porpoises.
Attacks are nearly always conducted by adult males, sometimes acting alone but also sightings of groups of 3 adult males have been recorded.
There are some incredible images of Bottlenose Dolphins in Cardigan Bay, South Wales, tossing the Harbour Porpoises up in the air caught by wildlife photographers Jamie Muny and Alister Kemp. They stated that the attacks lasted between 5 and 10 minutes, which may not seem like a long time, but it is plenty long enough to induce fatal injuries to the Harbour Porpoises.
Similar behaviour between Bottlenose Dolphins and Harbour Porpoises has been recorded elsewhere in the world, like in the German Baltic Sea. A 2020 research paper conducted necropsies on six stranded Harbour Porpoises and recorded massive spinal and abdominal haemorrhages, multiple rib fractures and in one case a punctured lung. These findings correlated to public sighting reports of a solitary male Bottlenose Dolphin, both in time and location leading the study to confidently conclude these fatal injuries were inflicted by the dolphin.
Though shocking this new paper offers little new information. In 1996, 63% of stranded Harbour Porpoises in Moray Firth, Scotland, died from similar traumatic injuries to the spine and internal organs. The skins also exhibited cuts, the spacings of which matched… yep you guessed it… the dental structure of Bottlenose Dolphins!
So WHY is this happening? Perhaps it is competition over food or habitats? Or simply rough object-orientated play? More sinister is the suggestion that they are practicing their infanticide skills… Personally, I would like to see more theories which encompass the fact that it is mainly solitary males conducting these attacks. Perhaps they are just lonely and overly boisterous mammals who forget their size and weight? Either way, greater documentation of these events and further study into the behaviour and interactions of both species would greatly aid our understanding of these conflicts and I cannot wait for further research to be done!