The Shannon (Sionna) Approaches IMMA

Criterion B: Distribution and Abundance

Sub-criterion B1: Small and Resident Populations

The common bottlenose dolphins (Tursiops truncatus) in the Shannon Estuary comprise a small (c. 145 individuals; Baker et al. 2018), genetically discrete population (Mirimin et al. 2011; de Bonis 2023), which is thought to be largely resident in the Shannon Estuary. The population is considered to have remained stable in size, since the first abundance estimate using photo-identification and mark-recapture was carried out in 1997 (Blázquez et al. 2020). The most recent abundance estimate (Berrow et al. 2022), although in the range of previous estimates suggests the population may have declined by 17.1% during the last 16 years.  However the methodology is not sensitive enough to conclusively detect this slow decline. The decline in dolphins in the estuary is attributed to range expansion or permanent emigration rather than a decline in numbers.

Since 2008 the Irish Whale and Dolphin Group have been collecting bottlenose dolphin photo-id data from Brandon and Tralee Bays which is just to the south and west of the Shannon Estuary. Levesque et al. (2016) showed 67 of the 70 individual dolphins photographed in these bays between 2008 and 2016 belonged to the Shannon dolphin population. McGuire (2022) updated this work using data obtained during 12 surveys carried out between 2017 and 2020 and Shannon dolphins accounted for 62-100% of all bottlenose dolphins identified. A discovery curve did not reach a definitive plateau indicating that not all the dolphins which occur in Tralee Bay and Brandon Bay were captured during this period.

Ludwig et al. (2021) explored mortality and survival rates of the Shannon dolphin population using a 26-year dataset collected between 1993 and 2019.  Survival rate using data only from the Shannon Estuary was 80%, which is very low, but when data from Brandon and Tralee Bays were included it rose to around 95%, which is consistent with similar populations elsewhere and suggesting this is a continuous population. Some of the dolphins, once recorded regularly in the estuary are now only recorded in Brandon, suggesting a permanent emigration out of the Shanon estuary into the Bay. This demonstrates the importance of this habitat to the Shannon dolphins. Charish et al. (2021) showed using static acoustic monitoring between May and November 2013 that Brandon Bay was an important foraging habitat for dolphins with detections on 92% of days monitored and foraging recorded during 20% of all monitored hours.

Boat-based surveys along the west coast of County Clare have increasingly recorded Shannon dolphins outside the Lower River Shannon SAC. Large groups of Shannon Dolphins are regularly recorded off Kilkee and Doonbeg to the north of the SAC, and more occasionally further north off Doolin and now as far north as Galway Bay. de Bonis (2023) provided new insights on potential limitations of current SAC boundaries as he showed individual dolphins biopsied in Brandon Bay (Co. Kerry) and stranded in Doolin and Lahinch (70 km from the mouth of the Shannon) belonged to the Shannon dolphin population, supporting the recent evidence of range expansion.

The evidence suggests these areas in which Shannon dolphins were not previously regularly recorded represent a recent range expansion. Dolphins are now regularly recorded suggesting this is not a rare event, but are consistently present in these habitats. The IMMA therefore includes all of these newly occupied habitats to highlight the importance of these habitats for a small, genetically discrete and vulnerable (see population viability analysis presented in Blázquez et al. 2020) population of bottlenose dolphins to support conservation action and management.

Criterion D: Special Attributes

Sub-criterion D1: Distinctiveness

The common bottlenose dolphins in the Shannon Estuary are considered genetically discrete compared to bottlenose dolphins elsewhere in Irish waters. Mirimin et al. (2011) used both biopsy samples and samples from stranded individuals to explore genetic structuring and showed that there are three discrete populations of bottlenose dolphins in Ireland; a population residing in the Shannon Estuary, a coastal population with its core habitat off NW Ireland and a putative offshore population which dominated the stranding records. Interestingly samples from a “semi-resident” group which were regularly recorded in the mouth of Cork Harbour from 2005 to 2013 were genetically assigned to the Shannon Estuary. Cork Harbour lies on the south coast of Ireland, at least 170 km from the mouth of the Shannon Estuary. Louis et al. (2014) confirmed the existence of a large, highly mobile offshore population which was also stranded in Ireland. This study was recently updated by de Bonis (2023), using more data primarily from stranded individuals (n=67) and confirmed strong population structuring in the Irish bottlenose dolphin community.