Fadia Al Abbar

Wageningen University & Research | WUR · Wildlife conservation and Ecology

Short-beaked common dolphins (Delphinus delphis) are the most sighted species in the Azores archipelago. Yet, little is known about their spatial and temporal distribution, seasonal variation and residency patterns in this region. In order to better understand the common dolphins sighted off São Miguel Island, Azores, a 2-year sightings dataset (2018-2019), collected by opportunistic observations on board whale-watching vessels, has been used to analyse the temporal and spatial distribution, and seasonal variation. As common dolphins occur in large numbers in the study area, and photo-identification can be extremely time consuming in terms of data collection and processing, no public catalogue has been created for the Azores archipelago yet. However, several studies in New Zealand, Australia and Greece have shown that photo-identification of common dolphins, using pigmentation and markings on dorsal fins, is reliable. In this study, a selection of “highly identifiable individuals” (HIIs) has been used to build a catalogue and determine the re-sighting rate of marked common dolphins. HIIs have been defined as dolphins with highly distinctive nicks/notches and/or extensive pigmentation of the left side of the dorsal fin. The research focus on HIIs aims to facilitate the data collection from a platform of opportunity, such as whale-watching vessels, as the time spent with the species is limited. Pictures have been collected between 2018 and 2019.The initial results show the presence of common dolphins year-round, with larger groups in summer months. Several HIIs have been re-sighted in the same season, in different seasons, and different years. These findings may indicate the presence of certain individuals within the same season around São Miguel. Further comparisons between seasons and the two years of the study will allow to determine if some individuals are resident off the coast of São Miguel Island.

It is of great importance to the conservation of cetaceans to improve and understand the methods used to study cetaceans at sea; it increases our understanding of them and improves conservation chances of entire marine ecosystems. The harbour porpoise (Phocoena phocoena) has been fluctuating in population and presence in the Dutch waters which is why their return opens opportunities to study them again. However, studying them is challenging since they are small, evasive, surface infrequently, and require almost ideal weather conditions to observe them visually. This is why acoustic methods such as CPODs are favourable devices to study them. Challenges with the use of CPODs are that they only detect presence and absence of harbour porpoises, with highly directional clicks of harbour porpoises the clicks are not always detected. So, to improve density estimates, this study aims to improve the knowledge on the detectibility of echolocation clicks in CPODs.