Who dunnit?

11th Feb 2010 My usual morning walk on Saturday 13th January along the seafront at Whitehead was made all the more interesting by the sighting of 2 Bottlenose Dolphin quite close inshore. There had been a recent run of records of this charismatic species through January with sightings mainly along the east Antrim coast. This follows a pattern of increasing sightings of this formerly scarce species in this part of the Irish Sea. One group, seen off Ballygalley, Co. Antrim (a few miles north of Larne) was even filmed, the resulting footage clearly showing the dolphins ‘playing with their food' with fish being repeatedly thrown and retrieved.

And so to that afternoon when I received a call from staff at the Exploris Aquarium to say that a ‘stranded dolphin' had been seen around Whitehead. A quick search in failing light revealed nothing but luckily the following day a local kayaker, Bob Nixon, called round to say he had found a dead Harbour Porpoise towards Blackhead which was confirmed when I found it later that day. The corpse of a 1.45m long male was on the rocks, looking rather forlorn with no obvious sign of injury and in very good condition, so much so that I did check for any signs of life – but this was one very dead Harbour Porpoise.

And that, as far as I was concerned, was the end of the story. More exciting than some weekends but really just amounting to 2 Bottlenose Dolphin sighted and one Harbour Porpoise deceased.

Northern Ireland Environment Agency now has a standing arrangement with the AFBI Veterinary Sciences Division to carry out post mortem examinations on all suitable cetaceans. Gary Burrows of NIEA arranged with colleagues Andy Black and Tracy Platt for the porpoise to be collected and delivered for examination by pathologist Tony Patterson. The post mortem results, received last week, made for a very interesting tale indeed. The porpoise was recorded as having amongst other injuries multiple rib fractures and the blubber separated from the underlying muscle mass all consistent with a massive impact. The absence of external injuries ruled out a collision with a boat for example. The veterinarian's conclusion was that the pattern of injuries was consistent with a Bottlenose dolphin attack! So in the space of 24 hours I had seen my very first murder victim and indeed probable murderer.

While this behaviour has been recorded elsewhere in the world, notably in California and, nearer to home, in the Moray Firth, Wales and off Cork this appears to be the first report in Northern Ireland.

Adult Bottlenose Dolphin are substantial in size, weighing over 4 times more than Harbour Porpoise and capable of tremendous bursts of speed and hence, of delivering a substantial impact. Quite why this aggressive interaction with porpoise occurs is unclear. Suggestions include competition for food, territorial conflict or sexual frustration. A study of Bottlenose Dolphin calve corpses in north-east Scotland has shown that many of these also show evidence of being attacked and killed by adults of the same species – yes some Bottlenose Dolphin populations practice infanticide. This has led to a suggestion that, given the similarity in size between these calves and Harbour Porpoise, the adult dolphins are actually attacking the porpoise mistaking them for Bottlenose calves. Interestingly the pathologist who first recorded infanticide among cetaceans was the same Tony Patterson who carried out the examination of this unfortunate animal.

Attacks on Harbour Porpoise by Bottlenose Dolphin have been filmed in the Moray Firth. One observer described seeing the dolphins “flipping up a porpoise with their beaks and battering it when it landed on its back on the water" with another noting that "the dolphins pack one hell of a punch."

Many thanks indeed to Bob Nixon for letting me know about the porpoise, NIEA staff (Tracey Platt, Andy Black and Gary Burrows) for recovering it and organising the post mortem and to Tony Patterson for this fascinating post mortem record.

Ian Enlander