New Study identifies Offshore Population of Bottlenose Dolphins

3rd Feb 2014

A new study confirms that bottlenose dolphins inhabiting offshore waters in Ireland are a separate population to those occurring inshore. The study just published in the journal Molecular Ecology by French researcher Marie Louis and collaborators analysed samples from 381 bottlenose dolphins either stranded or biopsy sampled from Scotland to the south of Portugal and from the Azores. The IWDG supplied biopsy samples (small skin samples collected with a dart) from north Co. Mayo and one obtained offshore during the Marine Institute sponsored Cetaceans on the Frontier 3 survey, which is run jointly by the IWDG and the Galway-Mayo Institute of Technology (GMIT). Skin samples were also contributed from stranded dolphins collected by the IWDG Cetacean Stranding network and stored in the National Museum of Ireland (Natural History). The offshore sample was critical to this study for Ireland as this is the only offshore biopsy sample from any species taken in Ireland to date. It was obtained 150km offshore in February 2011 from the IWDG RIB Muc Mhara, which was launched from the RV Celtic Explorer during a brief weather window. Opportunities to sample cetaceans offshore are few and far between but the scientific value of these samples is immense.

The existence of an offshore population had been speculated by Luca Mirimin and colleagues in 2011 who showed that the Shannon Estuary population is genetically distinct from those bottlenose dolphins occurring all around the Irish coast (Coastal population). However some stranded animals did not fit into either population so he speculated they washed in from offshore waters.

The work was carried out by Marie Louis as part of a PhD in the Centre d’Etudes Biologies de Chizé at the University of La Rochelle, France. She is investigating the fine-scale population structure of a bottlenose dolphin population in the English Channel (Normandy coast and Channel Islands) with other populations in European waters using social structure, genetic and stable isotope analyses. Commenting on this new work Marine Louis said during a research visit to GMIT“This work enables us to assess bottlenose dolphin population structure across an unprecedently large geographical area, and get important results both in terms of ecology, evolution and management. Very little is known about pelagic bottlenose dolphins, and the Cetaceans on the Frontier surveys are a unique opportunity to gather data on these animals”.

Marie Louis also showed that coastal bottlenose dolphins from Ireland are more closely related to UK dolphins than French and southern European dolphins. She also used genetics to estimate effective population size which is a measure of how big the breeding population is. Estimates for coastal populations were very low <100 individuals while offshore populations were estimated to be in their thousands. These estimates, while not as precise as sighting surveys or mark-recapture studies, are scaled and indicate that bottlenose dolphins are much more abundant offshore than inshore.

This new study has important management implications. Bottlenose dolphins are entitled to protection under the EU Habitats Directive which includes designations of Special Areas of Conservation to protect important habitats. As a discrete population, legally offshore bottlenose dolphins are now also entitled to the designation of SACs to protect important habitats. We know too little to be able to recommend potential sites suitable for designation, and this population is highly mobile and thus unlikely to remain in a single site for extended periods. However it does require consideration of these populations separately when implementing management actions. With increased interest in oil and gas exploration in offshore Ireland this study requires offshore populations to have mitigation and monitoring specific to their requirements.

IWDG Executive Officer, Dr Simon Berrow said ”this excellent and exciting research, which is only possible through international collaboration, certainly increases the pressure for us in Ireland to understand more about the offshore environment in Ireland and not just implement management actions developed inshore. This certainly adds more complexity to our understanding of bottlenose dolphins in Ireland and reflects the international dimension required for compliance with EU Directives”.



Louis, M., Viricel, A., Lucas, T., Peltier, H., Alfonsi, E., Berrow, S., Brownlow, A., Covelo, P., Dabin, W., Deaville, R., de Stephanis, R., Gally, F., Gauffier, P., Penrose, R., Silva, M.A., Guinet, C. and Benoit S-B. (2014) Habitat-driven population structure of bottlenose dolphins, Tursiops truncatus, in the North-East Atlantic. Molecular Ecology. 23, 857-874.

Mirimin, L., Miller, R., Dillane, E., Berrow, S.D., Ingram, S., Cross, T.F. and Rogan, E. (2011) Fine-scale population genetic structuring of bottlenose dolphins using Irish coastal waters. Animal Conservation. 14(4), 342-353.