Offshore windfarm developments are expanding, requiring assessment and mitigation of impacts on protected species. Typically, assessments of impacts on marine mammals have focussed on pile-driving, as intense impulsive noise elicits adverse behavioural responses. However, other construction activities such as jacket and turbine installation also change acoustic habitats through increased vessel activity. To date, the contribution of construction-related vessel activity in shaping marine mammal behavioural responses at windfarm construction sites has been overlooked and no guidelines or mitigation measures have been implemented.

We compared broad-scale spatio-temporal variation in harbour porpoise occurrence and foraging activity between baseline periods and different construction phases at two Scottish offshore windfarms. Following a Before-After Control-Impact design, arrays of echolocation click detectors (CPODs) were deployed in 25 km by 25 km impact and reference blocks throughout the 2017-2019 construction. Echolocation clicks and buzzes were used to investigate porpoise occurrence and foraging activity respectively. In parallel, we characterised broadband noise levels using calibrated noise recorders (SoundTraps and SM2Ms) and vessel activities using AIS data integrated with engineering records. Following an impact gradient design, we then quantified the magnitude of porpoise responses in relation to changes in the acoustic environment and vessel activity.

Compared to baseline, an 8-17% decline in porpoise occurrence was observed in the impact block during pile-driving and other construction activities. The probability of detecting porpoises and buzzing activity was positively related to the distance from vessel and construction activities, and negatively related to levels of vessel intensity and background noise. Porpoise displacement was observed at up to 12 km from pile-driving activities and up to 4 km from construction vessels. This evidence of broad-scale behavioural responses of harbour porpoises to these different construction activities highlights the importance of assessing and managing all vessel activities at offshore windfarm sites to minimise potential impacts of anthropogenic noise.