Passive acoustic monitoring of cetaceans in the Norwegian Skagerrak
Abstract
Marine ecosystems have been under pressure for decades as a result of intensivehuman activity. Cetaceans have also been negatively affected by our actions, so toimplement protection and conservation plans, it is crucial to know the current stateof the populations. In the last decades, passive acoustic monitoring (PAM) hasbecome an important tool to fulfil this purpose. This study, makes use of PAM toinvestigate the spatial and temporal occurrence of cetaceans in the NorwegianSkagerrak between March 2023 and February 2024. Two methods were used: i)Monthly transect ship surveys from Norway to Denmark using a towed hydrophonearray; and ii) Moored Continuous Porpoise Detectors (CPODs, n = 6) deployed at theNorwegian coast to monitor the presence and the vocal activity of harbour porpoises(Phocoena phocoena). Towed hydrophone data were analysed using the PAMsoftware PAMGuard to study the cetacean occurrence along the year. The CPOD datawere used to investigate spatial, seasonal and diel patterns in porpoise presence andvocal activity, as well as how sea temperature and currents affect these. Ship surveysrevealed the presence of harbour porpoises, northern bottlenose whales(Hyperoodon ampullatus), minke whales (Balaenoptera acutorostrata) andbottlenose dolphins (Tursiops truncatus) in the area. Generalized Additive Models,fitted to the CPOD data, showed spatial, seasonal and diel variations on harbourporpoise presence and vocal activity, while the effect of hydrographic conditions wasless clear. Significantly higher harbour porpoise presence and vocal activity wereregistered during spring, especially in the northernmost areas of Raet National Park.Moreover, porpoise presence was significantly higher during the night,independently of the time of the year. Changes in prey availability, as well as porpoisecalving and mating seasons, and prey migration may be behind these observations.This study was the first of its kind performed in the Norwegian Skagerrak, and itprovided novel information about specially, the harbour porpoises in the NorwegianSkagerrak. However, important knowledge gaps must still be filled to properlymanage them.