Genetically-distinct breeding populations of bottlenose dolphins in Irish waters

dolphin (c) E.RoganUnderstanding the population structure of a species is imperative for wildlife conservation, writes Dr. Emer Rogan, School of BEES.

For terrestrial mammals, mountain ranges, oceans and deserts may act as “barriers” to dispersal and over geological time can lead to genetically separate and reproductively-isolated populations.

However, when it comes to the marine environment and marine mammals in particular, we tend to think of species such as dolphins, which are highly mobile and live in an apparently homogeneous environment, as occurring in a single large population on an ocean basin scale.  Interestingly, our research is finding evidence to the contrary.

Bottlenose dolphins (Tursiops truncatus) are a familiar species to many in Ireland.  Fungi, the Dingle dolphin, is perhaps the best known bottlenose dolphin here, but other groups are commonly seen in the outer Shannon estuary, along the Connemara and Mayo coasts and indeed, a small group is regularly seen in Cork harbour.  So are these highly mobile dolphins all from the same population?

With funding from Science Foundation Ireland to Dr Emer Rogan, a team which also includes Prof. Tom Cross, Dr Simon Ingram and Dr Luca Mirimin from the Department of Zoology, Ecology and Plant Science, now part of the School of Biology, Earth and Environmental Science (BEES) have set out to answer this and some other questions about these dolphins.

Samples were obtained through a biopsy programme (which takes a skin sample from a live dolphin without harming it) and also from dead animals washed up on the coast (referred to as stranded animals).  Using a combination of molecular genetic markers (similar to those used in human forensics) we found that there is fine-scale structure over very short geographical distances along the Irish coast.

The results of this work have just been published in the journal Animal Conservation (Mirimin, L., Miller, R., Dillane, E., Berrow, S.D., Ingram, S., Cross, T.F. and Rogan, E.  Fine-scale population genetic structuring of bottlenose dolphins in Irish coastal waters).

The Shannon estuary bottlenose dolphins have been the focus of many research projects over the last 15 years.  We estimate that approximately 120-140 animals use the estuary over the summer, with a smaller number over-wintering.

Through a process known as photo-identification, we can recognise individual dolphins from nicks and notches on their dorsal fins. We also know that many dolphins come back to the Shannon year after year and we have never seen these dolphins anywhere else we have surveyed.

Dolphins (c) E.RoganIt turns out that dolphins using the Shannon are genetically distinct from animals off Connemara and Mayo.  Interestingly, the animals using Cork Harbour are most closely related to the Shannon dolphins.  This group of seven animals (a calf was born last year) have only been regularly seen in Cork Harbour since 2006.  Thus, it may be that these animals emigrated from the Shannon estuary to take up residency in Cork Harbour.

Curiously, apart from the Shannon-Cork, Mayo-Connemara populations, we found evidence of a third population, of unknown origin but possibly from an oceanic population, amongst the stranded animals.  All of this information is crucial for management and conservation of the species in Irish waters.  Bottlenose dolphins are listed in Annex 11 of the EU Habitat’s directive, requiring Special Areas of Conservation to be designated for them.  But the work also raises lots of questions, e.g.

  • Why are the Shannon dolphins genetically distinct from those off Connemara and Mayo?
  • How long ago did they separate?
  • What mechanisms drove the separation and continue to maintain the genetic separation of adjacent communities (in the apparent absence of geographical barriers)?
  • What is the third population and where do these animals come from?
  • Are there other populations in Irish waters?
  • Which of these populations was Fungi originally from?

Like many things in research each answer raises many interesting questions!

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