We showed that the offshore bottlenose dolphins (Tursiops truncatus truncatus) are, in general, transient in Rio de Janeiro waters (low degree of residence), but with a small subset of individuals with medium or high degree of residence in specific areas. These bottlenose dolphins also occur in coastal areas and continental shelf waters, up to the slope, likely influenced by environmental conditions and human activities. We found that the most suitable areas for these dolphins occur in high primary productivity sites, along the continental shelf, and in more gentle slopes, from shallow water, less than 50m deep. Shallow waters tend to be more productive, presenting a greater abundance of fishes (Fiperj 2020), which are typical prey of these dolphins. Indeed, the predicted suitable areas for bottlenose dolphins, including CF and RJ subareas. These are surrounded by fishing landing ports that land more than 90% of the local fishing resources caught in the study area (Fiperj 2020) reinforcing that these sites likely have high prey availability.
Primary productivity was an important predictor to explain suitable areas for bottlenose dolphins. However, bottlenose dolphins were not usually sighted in regions with the highest values for primary productivity, such as Ilha Grande, Sepetiba and Guanabara bays. These values were equivalent to those usually found in eutrophic waters (e.g. Marins et al. 2010; Aguiar et al. 2011; Castelo et al. 2021). The absence of bottlenose dolphins in those areas could be associated with the co-occurrence of other dolphins’ species. Indeed, these bays are also inhabited by resident populations of Guiana dolphins, Sotalia guianensis (e.g. Ribeiro-Campos et al. 2021). An aggressive interaction between bottlenose and Guiana dolphins were reported in Baía Norte, Southern Brazil (Wedekin et al. 2004), an area where both species overlap their niches by sharing consumptions of demersal mullet species; therefore, it is suggested that potential interspecific competition between both dolphin populations may be a limiting factor for the occurrence of bottlenose dolphins (Teixeira et al. 2021) which avoid these areas even if the habitat is suitable for their occurrence as indicated by our results. On the other hand, bottlenose dolphins are usually sighted outside the three bays, such as CF and RJ subareas (e.g. Tardin et al. 2013, 2019; Laporta et al. 2017). The frequent occurrence of groups in these two subareas, most of them including calves (87.5%), suggests that both subareas may be important for feeding and breeding bottlenose dolphins.
Considering CF subarea, bottlenose dolphins are already known to occur primarily in shallow and productive areas (Tardin et al. 2019). On a larger scale, on the shelf break of South and Southeast regions, the frequency of sightings is greater in areas close to the 500m isobath than in deeper waters (di Tullio et al. 2016), but worldwide bottlenose dolphins show plasticity in habitats used. This species tends to use shallow water with higher primary productivity either in sheltered or open waters with gentle slopes occurring in Australia, Namibia, Spain, and the United States (Cañadas et al. 2002; Zanardo et al. 2017; McBride-Kebert et al. 2019; de Rock et al. 2019). However, this species is also found in steeper slopes with high primary productivity caused by upwelling in shelf break or as barriers during feeding tactics in shallow habitats (Cañadas et al. 2002; McBride-Kebert et al. 2019).
Residence patterns in a specific area also vary among populations worldwide, from high (e.g. Simões-Lopes and Fabian 1999; Laporta et al. 2017; Carmen et al. 2021; Bennington et al. 2021) to low (e.g. Zolman 2002; Balmer et al. 2008; Akkaya Baş et al. 2019; Pace et al. 2021). Low residence patterns can indicate that the species use larger habitats than the studied area (Zanardo et al. 2016; Cobarrubia-Russo et al. 2019), while a high residence pattern to specific locations may suggest critical habitats for vital activities (Simões-Lopes and Fabian 1999; Ingram and Rogan 2002). Indeed, large range movements (from 700 to ca. 1.700 km) were observed for bottlenose dolphins tagged with satellite tags or photo-identified in Brazil (Cremer et al. 2018). Thus, it is likely that the individuals analyzed in the present study belong to a large population and groups remain in a certain area for short-term periods or regularly visit it to feed or breed. An individual variance in terms of residence may suggest complex habitat, social or population structures (Zolman 2002; Blasi and Boitani 2014). Residence in a specific area may be linked to the high availability of food resources and low predation risks (Knip et al. 2012; Habel et al. 2016). On the other hand, resident individuals tend to be more exposed to local threats (Warkentin and Hernández 1996; Atkins et al. 2016). Even those dolphins with low residence patterns might be exposed to local threats in our study area. The most suitable areas for the species, for example, are areas close to port complexes and shipping routes. These areas are surrounded by four port complexes (located on Campos municipality, Guanabara, Sepetiba and Ilha Grande bays), and are affected by nearby ports along the coast (located on Macaé, Búzios, Cabo Frio municipalities and Guaíba Island) (ANTAQ 2021). There is also high vessel traffic associated with oil and gas exploration occurring in the Campos and Santos basins (ANP 2021). The effects of port complexes and related activities, such as vessel traffic, on dolphins’ populations are well-reported worldwide (e.g. Halpern et al. 2015; Walker et al. 2019). Collisions of dolphins with vessels or their propellers, for example, may cause mutilation and even the death of individuals (van Waerebeek et al. 2007; Schoeman et al. 2020). Noisy areas, such as those near port complexes and shipping routes, may also change dolphin behavior and acoustic repertoire, cause acoustic masking, and lead to temporary or permanent habitat abandonment (Guerra et al. 2014; Marley et al. 2017; Erbe et al. 2019). Additionally, as aforementioned, we found that the highest suitable areas for bottlenose dolphins overlap with important fisheries activities (Fiperj 2020) increasing bycatch risks.
Besides, these suitable areas also have high levels of contaminants (Vidal et al. 2020), which may cause various adverse effects on cetaceans, such as contamination of calves crossing the placenta or through lactation, and immunosuppression of both calves and adults, which may result in skin diseases and even death (Moura et al. 2009; Bossart 2011; Vidal et al. 2020). Therefore, individuals occurring in this area may be under all these risks.
Understanding residence patterns together with the predictions of suitable habitats can contribute to safeguarding critical areas for these dolphins. Our study identified the most suitable habitats for bottlenose dolphins in an area with multiple human activities that may expose them to several different impacts. We also found that dolphins vary in how they use the area, suggesting complex social or populational structures. By mapping these critical areas and characterizing how dolphins use them, our findings may support additional and more effective conservation actions. For instance, a creation of MPAs to better manage local human activities, and then protect critical habitats for this important top predator since the MPAs network along Rio de Janeiro State does not encompass most of the highest suitable sites for bottlenose dolphins. Moreover, effective management of fisheries focusing on the protection of the ecosystem and reducing the bycatch of this species and any other marine species is urgently needed.