Carcass non-recovery rate of franciscana dolphin (Pontoporia blainvillei), calibrated with a drift mark-recapture study at FMA Ia, Brazil

The franciscana dolphin (Pontoporia blainvillei) is one of the smallest dolphins globally and the most threatened in the Southwest Atlantic Ocean. Beach monitoring helps to investigate cetacean strandings within their distributions and assess impacts that cause mortality. Using drifters in mark-recapture studies helps to estimate recovery rates when carcasses are unavailable. The study aims to estimate the non-recovery rate of franciscana carcasses from FMA Ia by comparing the spatial distribution between carcasses and drifters along the coast; correlating the influence of cold fronts with the recovery rate of drifters; estimating the non-recovery rate of carcasses according to the drifters’ results and the meteorological profile in the pre-stranding period; characterizing the death diagnostic with temporal distribution of franciscanas by considering the stranding index and the carcass non-recovery rates. We repeated the release of 54 drifters in two campaigns close to the coast within the range of franciscana dolphins, where beaches are monitored daily, in the north region of Espírito Santo state, Brazil. The carcass stranding hotspots (21%) and drifters (18%) were 10 km apart. Cold fronts significantly increase the number of strandings. Considering the incidence of cold fronts in the pre-stranding period and linear regression from drifters, the median carcass recovery rate is 0.26, 95% IC [0.22 - 0.29], which means that for each stranding, the carcass non-recovery rate varies from 0.78 to 0.71. The range between 265 to 350 estimates the total of carcasses from 77 strandings observed from 2003 to 2021. The record year of strandings was 2014 (n = 14). About 52% of records occurred in summer, and January is the month with the highest occurrence of strandings. Of the conclusive diagnoses (n = 43), around 77% (n = 33) were attributed to incidental capture in gillnets. Estimating the number of carcasses based on stranding records is essential for population viability analyses and conservation purposes, especially considering small and isolated populations as in the present study. To prevent local extinction, a solution to avoid incidental capture, especially along summers, must be addressed quickly