Assessing Mooring Designs for Improved Acoustic Monitoring of Harbour Porpoise in the German Borkum Riffgrund, North Sea

This study evaluates two contrasting mooring methods for Passive Acoustic Monitoring (PAM) of harbor porpoise (Phocoena phocoena) in the German Borkum Riffgrund Special Area of Conservation (North Sea). The first method utilized a low-profile subsurface acoustic-release buoy, while the second employed a large surface-chained buoy. Porpoises were monitored using T- and C-PODs, which are autonomous echolocation click detectors. The experimental subsurface acoustic-release buoy yielded significantly higher-quality data compared to the surface buoy, likely due to reduced hydrodynamic drag and self-generated noise. Porpoise detection positive minutes per day were significantly higher at the experimental site, indicating superior performance of the subsurface mooring. The study highlights the importance of mooring design in PAM studies and suggests that subsurface acoustic-release buoys are more effective for the long-term monitoring of harbor porpoises in high-noise environments. These findings have implications for the design of future PAM studies and the mitigation of underwater noise impacts on marine mammals.