Acoustic deterrent devices (ADDs) are used globally by the aquaculture industry to attempt to reduce pinniped depredation. However, noise from ADDs sited in areas used by cetaceans may lead to changes in cetacean behaviour and displacement from habitats impacting conservation status. Here, we investigate whether ADD noise from aquaculture within a habitat reduces acoustic detections of harbour porpoises, indicating either displacement or changes in echolocation activity. Nine years (2011–2019) of boat-based acoustic line-transect data for harbour porpoises were collected across the west coast of Scotland together with ADD presence and perceived loudness recorded by volunteers at regular listening stations along each transect. The most intense ADD noise coincided with significantly fewer harbour porpoise detections than in areas with no ADD noise. This corresponded with an estimated displacement of 34 % of the total undisturbed population, assuming that there was no change in echolocation behaviour. Furthermore, significant responses occurred at estimated received sound levels from ADDs >108 dB re 1 μPa RMS (frequency weighted for harbour porpoises). These findings suggest porpoises are either displaced from habitats degraded by intense ADD noise and/or echolocate less. In Scotland, ADD use in aquaculture has required a license since 2021. Limiting licensing, and monitoring ADD use by aquaculture should be considered by conservation managers globally, particularly in areas used by acoustically sensitive protected species. Non-acoustic (and non-lethal) measures to mitigate pinniped depredation on aquaculture sites are available and should be encouraged.