Performance of theodolites versus drones in land-based studies of marine mammals.

Theodolites and drones are key instruments for observing small whales in coastal areas. This study compared their performance while observing the harbour porpoise (Phocoena phocoena) in the western Baltic Sea. The methods were used simultaneously providing information on location, behaviour and group size during a field campaign in 2022. Theodolite observers were able to detect surfacing positions during 80.5% of porpoise sightings while a drone collected data during 50.7% of total sightings detected by plain eye. The drone footage quality was poor during 47.3% of these sightings. An in-depth analysis of 75:36 h of good quality footage resulted in 16:55 h (22.4%) of cetacean appearance. The determination of group size was significantly more precise using drone footage while the theodolite was more accurate in determining the start/end of a sighting. The accuracy of locations was modelled using the distance (Dt-d) between recorded theodolite and drone coordinates of the same surfacing porpoise. Dt-d varied significantly based on the point quality. Sea state and porpoise to theodolite observer distance did not influence Dt-d. Both methods complement each other and should ideally be used simultaneously to obtain both accurate and detailed information on harbour porpoises and other marine mammals during land-based observation studies.