Are we Experiencing an Unusual Mortality Event (UME) in Ireland ?

30th Nov 2013

In the US elevated numbers of cetacean strandings, which are unexpected and involve a significant die-off is declared as an Unexpected Mortality Event (UME). Under US law this requires the state authorities to investigate the event and test the animals to determine what killed them.

Currently there is a UME off the east coast of the US where large numbers of bottlenose dolphins are being washed up. The suspected cause is a morbillivirus, with secondary infections of brucella.

There has been an increase in cetacean strandings year on year over the last decade in Ireland with a record number reported last year (2012). IWDG have already noted that this year will exceed last years’ record with 175 records already received to date (see Mick O’Connells posts). The species stranded are many and varied, as is the case in most years, but this year the records are dominated by common dolphins.

Over the last year numbers of stranded common dolphins has increased from 20-30 per annum to 50-60 per annum. This year we have recorded 85 already and more are being reported each week. There are also another 15 strandings which are either common or striped dolphin. The number of striped dolphins stranded is consistent, or even a little lower, than previous years so it is likely many of these unidentified dolphins are common, resulting in a potential maximum 100 common dolphin strandings this year so far. What is causing this increase ? Is it better recording ? Is it increasing population size ?  Is it increased mortality ? We cannot answer any of these questions, but increased recording effort is not thought to be a major factor as the IWDG Stranding Scheme is now well-established and we frequently have multiple reports of the same event, suggesting coverage is good. The IWDG Cetacean Stranding Scheme is recognised as being effective at identifying unusual mortality events. So is this an “unusual mortality event” ?

Usually with an unusual stranding (mortality) event, there is a pattern in the timing or distribution, or the state of the carcass, or whether animals are dead or alive or in the pattern of stranding to assist in speculating as to what may be the cause. No pattern is obvious in the common dolphin strandings this year. There was a peak in the spring especially off the northwest, which was attributed through post-mortem examination to fisheries bycatch by pelagic trawlers. Since this stranding peak, numbers of common dolphin strandings have still been high with a second peak in the autumn. Strandings have been recorded from all along the western seaboard and south coast, with few on the east and north coasts, which is consistent with previous years. After the peak in January and February there has been a second peak in September and a number of mass strandings (more than one animal involved) in May and November. This year there has been an increase in live strandings, up to 13 events involving 31 animals in 2013 compared to a high of nine in 2012 when 6-7 would have been more typical in previous years. If there is a real increase in mortality what could be the cause ? what could result in, common dolphins in good nutritive condition, often with food in their stomachs live stranding on our coasts ? Could it be something that is occurring over a wide geographical and temporal space.  Is it offshore, where potentially negative activities are harder to witness ? Is it related to the unusually high sea temperatures we have experienced this year ?

Now the numbers we are playing with are relatively small and only a few extra records can inflate the overall numbers, but at what point do we acknowledge that something significant is happening.  Should we have a trigger to invoke action ?  What action should we take and who should initiate a reaction ? Can a number of dolphins stranded in good condition be post-mortemed to identify cause of death ? The IWDG Cetacean Stranding scheme is a good scheme for monitoring cetaceans in Ireland, but a monitoring scheme must lead to action if something unusual or significant is occurring.

Dr Simon Berrow

IWDG Executive Officer

Fergus Sweeney is a cameraman based in Belmullet shot some footage of this stranding.

Watch the video at this link  

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