Bottlenose dolphins have a worldwide distribution, occupying different types of habitats, and so, for that reason, they acquire a considerable morphological variation. Although our taxonomic knowledge of these variations and the connectivity between them is still limited, the existence of a coastal (in-shore) and an oceanic (off-shore) ecotype has been widely recognized globally.
In the southwestern Atlantic, these ecotypes have been described based on phenology and genetics, and significant differences have also been discovered between various coastal and oceanic populations in the northeastern Pacific and northwestern Atlantic.
Oceanic dolphins have a more falcate dorsal fin and have a darker coloration. Coastal dolphins have 2 wider bands in the throat region and a longer rostrum. These two ecotypes also show differences in habitat preference. While the coastal ecotype prefers shallower waters up to 18 meters deep and at a maximum distance of more or less 7.5 km from the coast, the oceanic ecotype has a wider distribution and greater flexibility in habitat preference. These are found in both coastal and deeper waters, at depths that typically range from 34m to 750m and more than 34km from the coast.
As the Azores have deep waters (>200m) very close to the coast, the bathymetric limits that are normally attributed to coastal and oceanic populations in the North Atlantic cannot be applied in this case.
In the western North Atlantic, mitochondrial and DNA analyses proved the existence of genetic differences between these two ecotypes as well as between coastal populations.
However, contrary to what is observed in dolphins belonging to the coastal ecotype, oceanic dolphins can maintain high levels of gene flow.
After obtaining the genetic sequences from samples of bottlenose dolphins from the Azores and later comparing them with the sequences of populations from the western North Atlantic, it was possible to verify that there are no significant differences between the Azorean ones and the oceanic ecotype. These studies conclude that all bottlenose dolphins that occur in pelagic areas of the North Atlantic belong to a large oceanic population.
Another way to distinguish these two populations is through residence patterns, with coastal populations tending to be more resident while oceanic ones are usually more passengers.
Although bottlenose dolphins are present all year round in the Azores, photo identification analyses show that some individuals are seen repeatedly in the same area at different seasons. These animals could be residents. On the other hand, some individuals are seen very rarely and are probably just transient visitors to the islands. For this reason, we could assume that there are two populations present in the Azores, one coastal/resident, and one oceanic/transient.
However, a study revealed that there is only one bottlenose dolphin population in the archipelago and that this population is not significantly different from the oceanic population of the northwestern North Atlantic.
This lack of genetic differentiation between the supposed coastal bottlenose dolphins in the Azores from the other oceanic ones was not at all expected considering:
- that these two populations are recognized both in the Atlantic Ocean and in the Pacific Ocean;
- the patterns of residence observed in this region.
The bottlenose dolphins present in the Azores, therefore, belong to a single oceanic population (off-shore), despite being found in coastal areas. This contradictory information will most likely be a consequence of the topography of the Azores, where there is no continental shelf and where we have zones of great depth starting very close to the coast.
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Oudejans, M. G., Visser, F., Englund, A., Rogan, E., & Ingram, S. N. (2015). Evidence for distinct coastal and offshore communities of bottlenose dolphins in the North-East Atlantic. PLoS One, 10(4), e0122668.
Quérouil, S., Silva, M. A., Magalhães, S., Prieto, R., Matos, J. A., Mendonça, D., & Santos, R. S. BOTTLENOSE DOLPHINS OF THE AZOREAN ARCHIPELAGO WOULD BELONG TO A SINGLE POPULATION OF THE” OFFSHORE” TYPE.
Simões-Lopes, P. C., Daura-Jorge, F. G., Lodi, L., Bezamat, C., Costa, A. P., & Wedekin, L. L. (2019). Bottlenose dolphin ecotypes of the western South Atlantic: the puzzle of habitats, coloration patterns and dorsal fin shapes. Aquatic Biology, 28, 101-111.
Torres, L. G., Rosel, P. E., D’Agrosa, C., & Read, A. J. (2003). Improving management of overlapping bottlenose dolphin ecotypes through spatial analysis and genetics. Marine Mammal Science, 19(3), 502-514.
