Bottlenose dolphin (Tursiops truncatus) social structure in the Shannon Estuary, Ireland, is distinguished by age‐ and area‐related associations

Abstract

Social and network analyses that incorporate information on individuals within a population enhance our knowledge of complex species. In this study, the social structure of wild bottlenose dolphins in the Shannon Estuary, Ireland, was analyzed by examining the dynamics of the whole population and then of individuals classed by sex, age, and area. One hundred and twenty‐one dolphins were identified during 522 sightings between 2012 and 2015. The mean half‐weight association index (HWI) of the population was 0.07 ± 0.02. The highest HWIs for individuals of known sex were for female‐male pairs. Mean within‐class HWI was significantly higher than mean between‐class HWI for both age and area classes. Ordinations and sociograms were used to visualize social networks. Permutation tests revealed nonrandom associations for the population overall and both between and within classes. Temporal analyses showed associations persisting for >1,000 d. The whole population's best fit model was for two levels of casual acquaintances. Movement analyses demonstrated the use of the inner estuary by only 25% of the population revealing a potential community division by area. The difference between mean HWI when socializing (0.09 ± 0.03) compared to foraging (0.06 ± 0.03) was significant. These results highlight the importance of localized research, reflecting the complexity found in bottlenose dolphin societies globally.

Foraging Behavior of Bottlenose Dolphins in the Shannon Estuary, Ireland as Determined through Static Acoustic Monitoring

“…Furthermore, they are listed under Annex II of the Directive, which requires member states to protect them and maintain them at a favorable conservation status requiring the designation of Special Areas of Conservation (SAC) for this species [3,4]. The Shannon Estuary (Lower River Shannon, Site Code 002165) is one of only two designated SACs for bottlenose dolphins in Ireland [4][5][6].…”

Social Structure, Habitat Use and Injuries of Indo-Pacific Bottlenose Dolphins (Tursiops aduncus) Reveal Isolated, Coastal, and Threatened Communities in the South Pacific

“…Individuals observed in the same group were considered associated (Whitehead, 2009). The half-weighted association index (HWI, Cairns and Schwager, 1987) was used, as it accounts for incomplete photo-identification of individuals within groups (Parra et al, 2011;Smith et al, 2016;Baker et al, 2018;Genov et al, 2019). A social network was built from an association matrix using NetDraw 2.158 (Borgatti, 2002) to assess social communities.…”

“…Even if these spatial movements occurred both across and within years (e.g., 2009, 2018), this potential social connectivity is expected to be quite low as it represents only 2.1 and 7.8% of all individuals encountered in the south lagoon and the southwest lagoon, respectively. The social structure highlighted in our study during the winter months was established over all dolphins sighted at least twice, although most studies conducted on social structure of dolphins used individuals resighted at least five times to reduce data skew from individual rarely resighted (e.g., Chilvers and Corkeron, 2001;Moreno and Acevedo-Gutiérrez, 2016;Baker et al, 2018). Given the small sample sizes collected in the southwest lagoon and the Isle of Pines, we considered this choice more suitable to our dataset.…”

Survival Rates and Capture Heterogeneity of Bottlenose Dolphins (Tursiops truncatus) in the Shannon Estuary, Ireland

“…Between 1993 and 2005, analogue cameras of the models Canon EOS 50 SLR and Canon EOS RT were used for photo-ID work, with images being printed on film and slides (Berrow et al, 1996;Berrow, 2009). In 2005, the transition to digital photography was made and since then SLR Canon cameras including EOS 20D, 50D, 7Dii, 5D have been used for photo-ID (Baker et al, 2018b). Lenses varied between 200 and 500 mm auto-focus telephoto (Berrow, 2009), 70-200 mm f2.8USM and f3.0 300 mm (Berrow et al, 2012), 70-300 mm auto-focus and 200-500 mm lenses (Berrow et al, 1996;Baker et al, 2018b).…”

“…This indicates that mark severity is not the only factor causing individual heterogeneity in capture probabilities. Another explanation is habitat partitioning with some dolphins using less frequently surveyed areas and are thus rarely sighted (Ingram and Rogan, 2002;Baker et al, 2018b). Variation in site fidelity between individuals has been observed in bottlenose dolphins (Speakman et al, 2010;Zanardo et al, 2016), with Only covariates with a sum of ≥0.001 are included in this summary, the remaining covariates are listed in Supplementary Table 9.…”