Stranding Pattern of the Critically Endangered Franciscana Dolphin (Subspecies Pontoporia blainvillei pukusi) in Southeastern Brazil: Effect of Temporal, Spatial, and Environmental Factors

Using data from the Campos Basin–Espírito Santo Beach Monitoring Program (PMP-BC/ES) (2018-2025), this study analyzed spatiotemporal distribution of franciscana dolphin (subspecies Pontoporia blainvillei pukusi) strandings in Franciscana Management Area Ib (FMA Ib; northern Rio de Janeiro, southeastern Brazil) and the influence of sea surface temperature, Chlorophyll-a, wind speed, and surface buoyancy scale in these patterns. Total, sex-specific, and age class-specific mortality were characterized. Spatial patterns (hot spots) were identified via kernel density maps. Generalized Additive Models (GAMs) were used to assess temporal variations in strandings (seasonal and interannual) and their relationship with the environmental variables. As a result, 227 stranded franciscana dolphins were recovered (mean = 28.4 individuals/year) over approximately 767,000 km of survey effort. Between 2018 and 2023, the sex ratio remained approximately 1:1, whereas a male-biased sex ratio was observed in 2024 and 2025. The GAM’s result indicated a nonlinear seasonal and interannual variation in strandings with a significant increase in mortality over the years. The surface buoyancy scale exhibits a significant inverse linear relationship with strandings, indicating a possible relationship between stranding occurrence and periods or areas of high productivity and water mixture. The highest mortality rates are expected between October and February, while the lowest rates are predicted to occur between April and July. January had the highest overall stranding rates for females, males, and calves, marking it as the peak mortality month in the region. Strandings predominantly occurred south of the Paraíba do Sul River mouth (21° 37' S), extending to Saquarema (22° 56' S), and decreasing in frequency with increasing latitude. Strikingly, 12 strandings were observed south of Armação dos Búzios (22° 46' S), possibly indicating a recent southward extension of the species’ distribution beyond its previously documented range limit in the region.