Common Dolphins Dying in Ever Increasing Numbers in Ireland19th Feb 2018
For the last seven years or so, the news from Ireland's seashore has become something of a broken record in late winter/early spring. Each year, the message is the same - ever increasing numbers of stranded cetaceans recorded on the Irish coast. Each year thinking, wow, we must have reached a peak now. And each year, the total increase in strandings of all species is down to a massive increase in one species - the common dolphin Delphinus delphis.
Photo: Common dolphin Derrynane, Co. Kerry 16th Feb 2018. Vinny Hyland/Wild Derrynane
Ten years ago, a typical year for strandings (of all species) would be around 130 records per year. In 2017, IWDG received and validated 263 strandings. Ten years ago, an average year would have had an average of around 30 common dolphin strandings. In 2017 IWDG received and validated 107 common dolphin strandings - an increase of around 350% in strandings of a protected species in a Whale and Dolphin Sanctuary. Last years total number for common dolphins excludes c.40 unidentified species, of which a large proportion were allso likely to be common dolphins. I should also point out that 'strandings' refers mainly to animals which die at sea and wash ashore, only around 10 - 15% of strandings are 'live strandings'. It is also worth noting that a recent French study suggests that as little as 8% of dead animals at sea actually get washed ashore and recorded - if that were to apply to the 107 stranded common dolphins in 2017, that would raise the total number of dead animals to over 1000. As yet, there are no definitive answers to the cause(s) of this ongoing unusual mortality event but an excellent step forward was taken in this country last June when the Marine Institute and National Parks and Wildlife Service put a post mortem scheme out to tender. This was successfully won by a joint effort by IWDG, GMIT and Cork Regional Vet Lab, so hopefully some questions will finally get answered.
There are many theories on social media about just what has been happening these last few years and while the following is mainly speculation, it is based on quite a few years now looking at photos and reports of dead cetaceans and is just an attempt to try and clear the air a little;
- Changes in the earth's magnetic field: May affect cetaceans ability to navigate but would likely affect more than one species and would only affect the relatively small proportion of strandings which are 'live'
- Submarines: Presumably the inference here are the effects of sonar. Again, would not really apply to just common dolphins but more likely an issue with deep diving species such as beaked whales.
- Plastics (micro, bags etc): Probably is having some effect on common dolphins but not necessarily more than in other species.
- Species specific disease: Quite possible but no indication of same discovered as yet.
- Malnutrition: Yes, signs of this in a small number of live and dead stranded animals.
- Increase in strandings due to increase in common dolphin population: Possible, but if so, a gradual increase would be expected yet common dolphin strandings went from 22 in 2010 to 59 in 2011 and 74 in 2013.
- Bycatch: Several photos of stranded common dolphins show visual signs of being bycaught in fishing net eg broken beaks, cuts to the body, rope on tail etc and in 2013, a small number of post mortems were carried out by NPWS after unusual strandings of common dolphins in Mayo. Cause of death was consistent with bycatch and at least one of the dolphins had been feeding on horse mackerel when it died, suggesting a possible link to the pelagic trawl fishery for this species. There does also to appear to be some coincidental geographic connection between stranding locations and position of the offshore pelagic fleet in 2018, but again this is purely speculation On the other side of the coin, observers on EU pelagic trawlers have never reported common dolphin bycatch in this fishery. Hopefully the post mortem scheme will help clarify this one way or the other. If bycatch does prove to be an issue, the question then to be asked is what changed in Irelands offshore waters since 2011?
- Increased observer effort: Are people just better at reporting strandings now than they were 10 years ago? Possible of course, but again, why one species in particular? Also, in many ways, IWDG was more active at promoting the sightings and strandings schemes at shows, in schools etc long before the current peaks.
So, plenty to think about there. The reality is that these dolphins are dying from various causes including old age and disease but it is the cause or causes of the large increase in strandings that needs to be addressed. It may be one or more of the ones mentioned above, or maybe something we haven't considered, but here's hoping we get some answers to this ongoing saga.
IWDG Strandings Co-ordinator
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