Over the years, our research led to new findings concerning Risso’s dolphins social structure, differences in residency patterns between gender, calving seasons, critical habitat, and negative influences of local tourism. These results have motivated the Regional Government (March 2014) to adjust the local law in order to reduce anthropogenic pressure on Risso’s dolphins.
Fig. 1: Main study area (540 km2) and ocean based surveys conducted during 2004-2007.
Results of our study in a nutshell:
Risso’s dolphins studied in the Azores display a new and unique social structure amongst cetaceans, formed by a stratified community of highly associated social units, grouped by age and sex classes. Stable associations between frequently sighted individuals (male and female) occurred and seemed to be organised in separate clusters. Association patterns varied between age-classes, being most stable in adult males and most fluid in sub-adults. This model differs from that of bottlenose dolphins and pilot whale societies.
Our work has yielded the largest identified population of Risso’s dolphin known to date, with 1,250 individuals recorded within the processed data set obtained over a 4 year sequential period. There is a resident population of around 250 individuals living off Pico Island, together with a large number of non-resident animals. The resident animals showed high site fidelity and were encountered in relatively restricted areas. Different patterns between sexes appear and support the species complex social ecology, particularly the mating system.
Calving season & critical habitat:
The south coast of Pico Island is a critical habitat for Risso’s dolphins, that are using the study area for breeding and nursing. Calving takes place mainly from June to August, concentrated therefore in a summer calving season. Calving and nursing females can be encountered closer to the coast than other social units. This spatial segregation of calving females indicates the particular needs and vulnerability of Risso’s dolphins in this stage of their life cycle.
A new method to link scarification to age:
Using skin scarification patterns observed on mark-recapture photos of individuals resighted throughout the project period, we developed a new non-invasive and easy to apply method to estimate age for this species. The method was tested amongst a group of 28 rankers and has proven to work. Six age classes were developed and linked with an estimation of age using Bayesian statistics. These skin classes were linked with previously described maturation stages. It was discovered that Risso’s may reach over 50 years of age, which is older than the previously recorded 38 years , using the GLGs technique.
Influences of touristic activities:
Since 1992, whale watch and swim with dolphin activities have been growing steadily in the research area. We studied the influence of the presence of whale watching vessels during low and high season. We found that Risso’s dolphins shift their resting patterns in high season towards 1300hrs, when most vessels are in the harbour, compared with a peak in their resting rate at around 1100hrs and 1500hrs during low season. Their resting rate showed a significant negative relationship with vessel abundance, where the probability of observing resting behaviour decreased when more than five whale watching vessels were present in the research area.