Griffiths ET, Kyhn LA, Sveegaard S, Marcolin C, Teilmann J, & Tougaard, J. 2023. Acoustic detections of odontocetes in Skagerrak. Investigation of clicks and whistles from delphinids at Gule Rev and Store Rev. Aarhus University, DCE – Danish Centre for Environment and Energy, 24 pp. Scientific Report No. 539 http://dce2.au.dk/pub/SR539.pdf
A pilot study was conducted in 2020-2021 to test the feasibility of performing passive acoustic monitoring for odontocetes other than harbour porpoises in the Danish North Sea and Skagerrak. Acoustic monitoring methods for porpoises are already established, but no attempts have previously been made to detect other cetaceans in the Danish North Sea. Four sound recorders (SoundTrap ST500 or ST600), capable of wideband recordings sufficient to capture both whistles and echolocation clicks, were deployed, two on each on the deep stone reefs Store Rev (32-58m) and Gule Rev (29-48m) in the Skagerrak.
Analysis of recordings by means of automated detectors in the software package PAMGuard, assisted by manual auditing, resulted in identification of numerous acoustic events clearly distinct for odonotocetes other than porpoises. Most of these vocalizations matched known echolocation clicks of white-beaked dolphins, assumed from surveys and sightings to be the second most common odontocete in the area after harbour porpoises. Other signals that did not match white-beaked dolphins were also found in the recordings, with lower peak frequency of the echolocation clicks and low first harmonic of whistles, consistent with signals from either killer whales or pilot whales, two species also observed in the area.
Since little is known about the annual occurrence of harbour porpoises in this area, dedicated harbour porpoise detectors (C-PODs) were deployed together with the wideband acoustic recorders. Porpoises were detected at the four stations on 92-100% of days during the deployment period, with most detections from February to May. The recordings covered the full year at Store Rev and from May to October at Gule Rev.
In conclusion, the pilot study has documented the feasibility of passive acoustic monitoring for not only harbour porpoises, but also white-beaked dolphins and other odontocetes in the Skagerrak and North Sea.
