Unfortunately, we are continuing to see an increase in stranding rates of some cetacean species along the Irish coast compared to previous years. We currently have a total of 295 stranding records logged in our database from 1 December to 18 September 2023, which includes one basking shark and four loggerhead turtles. This figure was only surpassed during this time frame in 2021, when 378 animals were reported.
Common dolphins continue to be the most commonly stranded species in 2023, with 141 animals reported to date. If we were to exclude the year 2021, this would be the highest number of common dolphins reported to the stranding scheme during this time period. We are also seeing a continuous increase in the number of harbour porpoise strandings, primarily along the east/southeast coasts (Figure 1).
Figure 1. The location of harbour porpoise strandings reported to the IWDG between 1 December and 18 September 2023.
In 2022, as expected, harbour porpoises were the second most frequently reported species (n=57). This was up from 51 records in 2021, though slightly down from the peak of 58 records in 2020. As with common dolphins, harbour porpoise strandings have shown an upward trend starting in 2011; rising from an average of 28.6 per year between 2000 and 2010, to an average of 44.4 per year between 2011 and 2022 (Figure 2). We have already reached a total of 55 records this year and are only in mid-September; therefore, it is almost certain the peak of 58 animals from 2020 will be surpassed in 2023.
Figure 2. IWDG Harbour Porpoise stranding records 2000 to 2022.
The majority of harbour porpoise strandings during 2023 occurred in March (n=8), and over the summer months of June (n=7), July (n=8) and August (n=9). As there is currently no post mortem scheme covering this species in Ireland, it is not possible to investigate the cause of death. All we can do is flag these stranding trends and do our best to investigate with the resources we have. Our amazing Stranding Network Volunteers visit as many stranding sites as they can in order to gather additional data such as length and gender, and collect skin samples for the Irish Cetacean Genetic Tissue Bank to support future research projects. In addition, they take a series of high quality images which allow us to assess the carcasses (as best as we can from photographs) to note on the nutritional condition, as well as identify any potential signs of anthropogenic interaction (i.e. bycatch, boat strike etc.)
At times, volunteers may head over to a stranding and not locate the animal due to the tide having taken it away. Thirty-six percent of porpoises were visited and located by volunteers in 2023 (n=20). Of those 20 animals, ten were classed as being in ‘fresh’ condition, and four as being in ‘very fresh’ condition. Six of the ‘fresh’ and ‘very fresh’ animals were noted to be in poor nutritional condition based on photographic evidence. Of course, this in no way means all of the others were in good health, as not all animals were fresh enough or had the appropriate associated images to determine the nutritional condition. The fresher the carcass, the more accurately we can assess nutritional states and identify signs of anthropogenic interactions (Figure 3).
Figure 3. Juvenile harbour porpoise in poor nutritional condition reported from Laytown Beach, Co. Louth on 18 September 2023. Photo by Orla Tuthill.
From 2017 to 2019, the IWDG delivered a post mortem scheme on behalf of the NPWS and the Marine Institute in partnership with the Cork Regional Vet Lab and the Atlantic Technological University in Galway. The scheme targeted common and striped dolphins, as well as harbour porpoises. Causes of death were established for 16 of the 19 harbour porpoises examined, with the number one cause of death being infectious disease (n=9). The second most common cause of death was bycatch (n=4). Once again, the take home message is this: if we are to ever understand why these animals are stranding, the only solution is to have a long-term post mortem scheme in Ireland covering all cetacean species. The full report from the post mortem scheme can be found online here: https://oar.marine.ie/bitstream/handle/10793/1782/Merged%20Necropsy%20Reports%20_2017-20182019.pdf?sequence=1
It is interesting to note that the IWDG have identified significant declines in harbour porpoise densities in all three Special Areas of Conservation (SAC) designated to protect this species (Roaringwater Bay and Islands, Blasket Islands and Rockabill to Dalkey Islands). Results from boat-based surveys carried out on behalf of the National Parks and Wildlife Service (NPWS) suggest that this may not be the result of population declines, but rather changes occurring in their distribution – changes which are likely driven by shifts in the distribution of their preferred prey. Changes in fish distributions, especially those which seek cooler waters, have been shown to be occurring throughout the North Atlantic driven by climate change. It is too early to correlate these two separate signals, but poor nutritional states in our harbour porpoises caused by changes in prey distributions could potentially lead to increased stranding rates.
We have had over 950 people report strandings to us this year. As usual, a huge thanks to all who report to the IWDG, and to all of those who volunteer with the scheme. We would not be able to flag changes in strandings trends as we have done here without your continued reports and support.
Stephanie Levesque
IWDG Stranding Officer