Determining the spatial and temporal extent of the influence of pile driving sound on harbour porpoises

Piling driving sound is known to impact harbour porpoise (Phocoena phocoena) distribution, but to date detailed knowledge on the combined spatial and temporal components of this impact over longer time periods remains lacking. From May to September 2016, pile driving was taking place at the Nobelwind wind farm located on the Bligh Bank in Belgium. In this period, porpoise activity was recorded using passive acoustic monitoring (PAM) using Continuous Porpoise Detectors (C-PoDs), at various distances from the construction site (1 – > 55 km). In this study, we compared porpoise detections before, during and after pile driving. During piling, porpoise detections decreased at stations located up to 20 km from the location of the piling event. At larger distances (20-55 km), porpoise detections either remained the same or increased slightly during piling events, which may be due to displaced porpoises entering the area. Underwater sound levels were extrapolated for the different locations. Pile driving sound levels at the furthest distance where reductions in porpoise detections were observed were ~159 dB re 1μPa (Lsub>z-p), which is close to the threshold level for major disturbance for harbour porpoise proposed in literature