| dc.description.abstract | Marine ecosystems have been under pressure for decades as a result of intensive
human activity. Cetaceans have also been negatively affected by our actions, so to
implement protection and conservation plans, it is crucial to know the current state
of the populations. In the last decades, passive acoustic monitoring (PAM) has
become an important tool to fulfil this purpose. This study, makes use of PAM to
investigate the spatial and temporal occurrence of cetaceans in the Norwegian
Skagerrak between March 2023 and February 2024. Two methods were used: i)
Monthly transect ship surveys from Norway to Denmark using a towed hydrophone
array; and ii) Moored Continuous Porpoise Detectors (CPODs, n = 6) deployed at the
Norwegian coast to monitor the presence and the vocal activity of harbour porpoises
(Phocoena phocoena). Towed hydrophone data were analysed using the PAM
software PAMGuard to study the cetacean occurrence along the year. The CPOD data
were used to investigate spatial, seasonal and diel patterns in porpoise presence and
vocal activity, as well as how sea temperature and currents affect these. Ship surveys
revealed the presence of harbour porpoises, northern bottlenose whales
(Hyperoodon ampullatus), minke whales (Balaenoptera acutorostrata) and
bottlenose dolphins (Tursiops truncatus) in the area. Generalized Additive Models,
fitted to the CPOD data, showed spatial, seasonal and diel variations on harbour
porpoise presence and vocal activity, while the effect of hydrographic conditions was
less clear. Significantly higher harbour porpoise presence and vocal activity were
registered 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 porpoise
calving 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 it
provided novel information about specially, the harbour porpoises in the Norwegian
Skagerrak. However, important knowledge gaps must still be filled to properly
manage them. | |
