Natural markings and population parameters of Blainville’s beaked whales in Madeira Island

Population parameters have been studied for only five out of 24 beaked whale species worldwide and in limited locations. This study provides novel knowledge on the natural markings and population parameters of Blainville’s beaked whales ( Mesoplodon densirostris ). Using photographic-identification data from adults and subadults of both sexes in Madeira Island (Eastern North Atlantic, 2015-2020), we estimated abundance and survival using capture-recapture models, the Robust Design (RD) and the POPAN. We calculated a proportion of marked individuals of 0.88, with new marks appearing every 0.4 years (range = 0.1-0.7). Annual abundance estimates from both models were similar and showed no trend. The POPAN model estimated a corrected super-population of 112 individuals (95% CI = 99-134) between 2015 and 2020. The corrected annual abundance estimates from the POPAN best-fitting model ranged from 42 whales (95% CI = 39-46 in 2016, and 95% CI = 39-48 in 2019) to 61 (95% CI = 51-72 in 2015). Survival rates of 0.98 (95% CI = 0.83-1.0) and 0.99 (95% CI = 0.97-1.00) were estimated (excluding transients) from the RD and POPAN models, respectively, which are similar to other regions. Despite potential caveats discussed in our analysis, these plausible estimates for a relatively small population should be considered in conservation and management strategies, highlighting the need to monitor the potential impact of local human activities. • First photo-ID-based abundance estimations of Blainville’s whales • Results suggest population stability over time • Two open population capture-recapture models gave consistent estimates