Programmed unmanned aerial vehicles show great potential for monitoring marine megafauna in specific areas of interest

Abstract Targeted conservation measures are contingent on robust knowledge of spatio‐temporal animal distribution in areas of interest. We explore unmanned aerial vehicle (UAV) transect monitoring as a novel method for standardized digital aerial surveys of marine megafauna by investigating the fine‐resolution spatio‐temporal distribution of harbour porpoises ( Phocoena phocoena ) in a Swedish nature reserve along with drivers of this distribution and potential biases. Biweekly UAV video data were collected along pre‐programmed strip transects over 17 weeks from June to September 2023, totalling a survey area of 3.37 km 2 , thereby providing porpoise monitoring data covering 89% of a special area of conservation for the species. All UAV video data were manually reviewed by a primary observer, and 25% of the UAV footage was also reviewed by a second, unexperienced observer to identify observer bias and learning effects. No significant observer bias or learning effect was found, but increased sea state affected porpoise density negatively. From the monitoring data, we were able to calculate relative density estimates, identify small‐scale spatio‐temporal differences and detect negative effects of recreational boat activity on porpoise presence. We further demonstrate that within this restricted area, porpoises are found in higher relative densities outside a designated conservation area, compared to within the conservation area, providing important knowledge to guide fine‐scale local conservation actions. We highlight advantages and areas of improvement of UAV transect monitoring as an accessible, versatile and adaptable method to survey marine megafauna in spatially restricted specific areas of interest. We conclude that this method constitutes a promising and valuable tool for wildlife monitoring, especially as it can be easily adapted and modified for specific contexts and species.