Distribution and potential residency patterns of bottlenose dolphins in São Miguel island

Bottlenose dolphins (Tursiops truncatus) are one of the species of dolphins that can be sighted year-round in the Azores. They are among the most sighted dolphins in our tours, and therefore we have plenty of chances to collect useful data about them on our trips. Some bottlenose dolphins are really well known since a long time ago, because of some special features or marks they have, as you can see in this article written by our biologist Mariana Silva. However, in order to improve our knowledge about this species around São Miguel, a student from the University of Algarve, Rita Catalão, has recently joined our team and decided to study bottlenose dolphins for her Msc. Thesis.

The definition of population describes a group of organisms of the same species occupying a particular space at a particular time. However, individuals from the same population can have different ranging patterns and move differently, showing different site fidelity, and therefore, with different impacts on population distribution and abundance, habitat selection, interactions with other species, and social structure (Defran et al., 1999; Börger, 2016; Nathan et al., 2008). Whether they stay in the same area or leave, their movements are usually driven by the distribution of limiting resources like food, mates, nursery, resting and predator avoidance areas (Nathan et al., 2008; Silva et al., 2007). Species with high site fidelity and restricted ranging patterns to be more vulnerable to population declines due to local threats (Warkentin & Hernández, 1996; Atkins et al., 2016).Therefore, it is important to define key areas for each species so that appropriate conservation measures can be applied according to the needs of the populations (Herfindal et al., 2005).

Photo-identification is now a very commonly used technique in the study of species abundance, site fidelity, movement patterns and social structure (Würsig & Jefferson, 1990). Its first application on cetaceans occurred in the early 1970s, and since then the data taken from this technique has been continuously growing (Neumann et al., 2002), particularly with the implementation of digital photography. Natural marks that are considered distinguishable are used to identify individuals, instead of physically capture the animals, proving to be very effective and non-evasive (Evans & Hammond, 2004; Neumann et al., 2002).

In the Azores, several studies have been conducted on this species about ranging and residence patterns, occurrence and distribution, survival and abundance. However, although a lot of data has been collected throughout the years, only a few studies have been done focusing mainly on social structure, in the central group islands (Silva et al., 2008) and on habitat preference around São Miguel Island (Fernández et al., 2018; González García., 2018).

To analyse the distribution of bottlenose dolphins and create a photo-identification catalogue of the individuals sighted around São Miguel, a 10-year sightings dataset (2010-2019) and a 6-year photographic dataset (2014-2019) collected on board Futurismo’s whale watching vessels, are used.

For the catalogue we use dolphins with distinctive nicks/notches, marks/scars and/or extensive pigmentation on the dorsal fin.

This study aims to verify the occurrence, identify associations (i.e. groups) and possible residents and transient dolphins, and also to assess temporal and spatial distribution of bottlenose dolphins around São Miguel. Preliminary results show that there are dolphins who were encountered together on different dates, and also individuals that were re-sighted several times. These findings may indicate the presence of some individuals for some time in the area, some may leave and comeback and some may be residents. The understanding of their site fidelity and distribution around São Miguel is helpful to provide realistic information to implement effective management and conservation plans around the island.

Article by Rita Catalão

References

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González García L. (2018). Cetacean distribution in São Miguel (Azores): influence of environmental variables at different spatial and temporal scales. PhD Thesis. University of Vigo.

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