Abundance Estimates of Guiana Dolphins (Sotalia guianensis; Van Bénéden, 1864) Inhabiting an Estuarine System in Southeastern Brazil

Introduction Baseline demographic information is essential The Guiana dolphin (Sotalia guianensis; Van for effective conservation and management stratBénéden, 1864) is a small cetacean (Delphinidae) egies for most living species. The abundance of that inhabits estuarine and shallow coastal waters Guiana dolphins (Sotalia guianensis) is poorly (up to 30 m) along the western coast of the Atlantic known, yet species conservation is considered a Ocean in Central and South America (Borobia high priority in areas where human activities may et al., 1991; Da Silva et al., 2010). Although induce population declines. This study estimated common in most of its range, little is known abundance for Guiana dolphins in the Cananeia about population parameters for S. guianensis; estuary (25° 03' S, 47° 55' W) in southeastern the species is listed as “Data Deficient” by the Brazil using mark-recapture data and Pollock’s International Union for Conservation of Nature Robust Design Model. Systematic boat-based (Secchi, 2012). Methods used for most abundance photo-identification surveys were based on data estimates presented in the past 30 years violated collected in the summer and winter of 2015 and important analytical assumptions, generating in the summer of 2016. A total of 55 capture high levels of uncertainty for many of the gathevents allowed identification of 133 different ered results (Santos et al., 2010c). As the species individuals. The best model indicated a populais susceptible to multiple human-related threats tion with random temporary emigration, a timethroughout its whole distribution (Crespo et al., constant survival rate, and heterogeneous time2010), robust abundance estimates are needed for varying capture probabilities among primary effective conservation strategies for locally surperiods. The temporary emigration rate (γ’’= γ’) veyed populations (Wilson et al., 1999). was 0.05 (± 0.03). Estimated population sizes Individual identification of wild cetaceans were 430 (95% CI: 410 to 451) individuals in the based on their natural markings has provided relsummer of 2015, 384 (95% CI: 366 to 403) indievant information in the field of marine mammal viduals in the winter of 2015, and 414 (95% CI: science, especially for population dynamics 392 to 438) individuals in the summer of 2016, (Hammond, 1990; Würsig & Jefferson, 1990). In indicating that environmental variables among general, natural markings on cetaceans comprise seasons may have a mild effect on the estimated fluke morphology and color patterns (e.g., Katona size of this surveyed population. These estimates et al., 1979), callosity patterns (e.g., Payne et al., should stand as an important baseline for future 1983), and nicks and notches along the dorsal fin comparisons. Systematic, long-term monitor(e.g., Würsig & Würsig, 1977). In photo-identiing of this population is recommended, and is fication studies, these marks are photographed required to accurately assess population trends. and then used for non-invasive re-identification of recognized individuals for spatial and temporal