The Power of long-term Stranding Scheme Data

The IWDG Cetacean Stranding Scheme has received a total of 122 reports between January and March of this year. The year 2021 was a peak year for strandings, with 200 animals reported during the same time period. Factors which may have contributed to 2021 being a peak year overall were the lockdowns which occurred as a result of COVID-19, as well as the launch of the new IWDG Reporting App. Although both of these events would have resulted in an increase in reporting, there has been an overall trend of increased strandings since the scheme began in 1990. If we were to view 2021 as an outlier year, 2023 would be the runner up for the highest number of strandings reported during the peak period – January to March. The majority of these records can be attributed to the common dolphin, most often juveniles around 1.5 m, which represented 60% of all reports so far this year.

Total number of strandings reported to IWDG from January to March 2018, 2019, 2020, 2021, 2022 and 2023.

Many mariners, including fishers, are commenting to the IWDG that they never used to see so many dolphins when they were at sea years ago. This perception is widespread and has merit. There is recent evidence of an increase in common dolphin abundance in inshore waters to the south of Ireland in the Bay of Biscay (Astarloa et al. 2021), which has been associated with changes in climate indices and prey. If a similar increase has occurred further north off Ireland, this may in some part explain increased stranding rates, as increased numbers inshore will inevitably lead to an increase in strandings, rather than due to increased mortality. It is important to note that this does not necessarily mean there are more animals, but increased strandings could be due to changes in distribution.

Becoming entangled in fishing gear was the primary cause of death found in common dolphins during the winter period in France according to researchers at the Pelagis Sea Mammal and Bird Observatory, after a record number of dolphins (n=370) were reported along the Bay of Biscay between 1 December 2022 and25 January 2023. When comparing trends from the IWDG StrandingScheme with schemes from neighboring countries, they are quite similar suggesting similar pressures are applying to a much wider area than just Ireland. However, without an established post-mortem scheme in Ireland, we are unable to investigate causes of death, and are therefore unable to identify the percentage of animals which are washing up as a result of bycatch, or other factors. During the recent three year post-mortem project, infectious diseases and malnourishment were also responsible for a high proportion of deaths, which may be linked to this increase in inshore waters as dolphin follow prey or have to travel further afield to obtain their daily requirements.

In 2021, we updated our Stranding Network Volunteer Protocol to include the collection of high quality images of specific areas on stranded carcasses where we would be most likely to see the subtle signs of bycatch. The probable/possible bycatch cases identified using this method are just that – probable or possible. Without the presence of actual fishing gear, we are unable to confirm the cause of death, even if it appears evident in the photographs, without a post-mortem examination. Although we are focusing on bycatch here, there are other, both anthropogenic and natural, causes of death which need to be identified and monitored.

Images collected by IWDG Stranding Network Volunteers. Image a. shows a dorsal fin slice, and image b. shows encircling marks around the beak of a common dolphin. Both are commonly associated with bycatch.

In 2022, IWDG were able to secure partial funding from the National Parks and Wildlife Service to support our Deep Diving and Rare Species Investigation Programme (DDRIP), which, as the name suggests, targets rarer species such as beaked whales for post-mortem examination. We hope that taking the lead on this project will result in the establishment of a government supported, long-term post-mortem scheme covering all cetacean species which strand along the Irish coast. This will bring us in line with other EU and UK countries in the NE Atlantic with such schemes, allowing the data to be comparable across countries.

First ever record of a dwarf sperm whale in Ireland, May 2022. This female was recovered from Glengarriff, Co. Cork to the Regional Vet Lab in Cork City to be examined under the DDRIP project.

Although the stranding scheme would be a more powerful tool alongside a post-mortem scheme, there is still significant value in recording the date, location, and species (where possible) of each stranding, in order to identify and monitor any trends.

A recent publication suggested using long term stranding scheme data as a tool for informing climate change policy in the UK (Williamson et al. 2021). Researchers examined data from the UK’s Cetacean Stranding Investigation Programme (CSIP) and reported an increase in warm water adapted species (common and striped dolphins), and a decrease in cold water adapted species (white-beaked and Atlantic white-sided dolphins). The IWDG are seeing a similar trend with regards to the common and Atlantic white-sided dolphins (see Figures 2 and 3 below). Williamson et al. (2021) identify one of the main drivers of this change to be shifts in the abundance and distribution of prey due to climate change, and refer to documented cases of this occurring in the North Sea, the Irish Sea, and the wider N. Atlantic.

Common dolphin strandings in Ireland have been increasing significantly over time, however, the number of stranded Atlantic white-sided dolphins have been decreasing. These trends are consistent with records reported to the IWDG Cetacean Sighting Scheme.

Atlantic white-sided dolphin

IWDG Atlantic white-sided dolphin stranding records from 2000 to 2022.

Common dolphin

IWDG common dolphin stranding records from 2000 to 2022.

In conclusion, long-term datasets such as the IWDG Cetacean Stranding and Sighting Schemes are extremely valuable and must be protected and supported. Ireland would have very little data on the occurrence of cetaceans in Ireland were it not for these recording schemes, which would not be possible without the continued reports and support from the public.

As top predators, cetaceans (whales, dolphins and porpoises) have long been identified as important indicators of ocean health, and have now being suggested as a tool for monitoring the changing conditions of the oceans and to inform government policies.

The number of stranding records IWDG has received in 2023 is alarming, and is the second highest year on record for the peak stranding period (January – March). We will continue to monitor and report on what we are seeing as the year goes on. We must continue to push for a cetacean post-mortem scheme in Ireland, as valuable data is being missed from these stranded animals. From our volunteer photographs, we are able to see on a small scale the number of animals which are washing up with signs of fisheries bycatch, signs which would have otherwise gone unnoticed had it not been for our dedicated volunteers.


Thank you to all who volunteer with our Stranding Scheme Network, and to all of you who continue to report your sightings and strandings to IWDG.

IWDG is a registered charity and we rely heavily on outside funding to run our various research projects. You can help IWDG continue our DDRIP Project in 2023 by donating to our Go Fund Me page :


Stephanie Levesque,

IWDG Stranding Officer


Astarloa, A., Louzao, M., Andrade, J., Babey, L., Berrow, S., Boisseau, O., Brereton, T., Dorémus, G., Evans, P.G., Hodgins, N.K. and Lewis, M.(2021) The role of climate, oceanography, and prey in driving decadal spatio-temporal patterns of a highly mobile top predator. Frontiers in Marine Science, p.1463.

Williamson, M.J., ten Doeschate, M.T., Deaville, R., Brownlow, A.C. and Taylor, N.L.(2021) Cetaceans as sentinels for informing climate change policy in UK waters. Marine Policy131, p.104634.