Assessments of the impact of offshore energy developments are constrained because it is not known whether fine-scale behavioral responses to noise lead to broader-scale displacement of protected small cetaceans. We used passive acoustic monitoring and digital aerial surveys to study changes in the occurrence of harbour porpoises across a 2000 km2 study area during a commercial 2-D seismic survey in the North Sea. Acoustic and visual data provided evidence of group responses to air-gun noise from the 470 cu inch array over ranges of 5-10 km, at received peak-to-peak sound pressure levels of 165-172 dB re 1 µPa and sound exposure levels of 145-151 dB re. 1µPa2 s. However, animals were typically detected again at affected sites within a few hours, and the level of response declined through the 10 day survey. Overall, acoustic detections decreased significantly during the survey period in the impact area compared to a control area, but this effect was small in relation to natural variation. These results demonstrate that prolonged seismic survey noise did not lead to broader-scale displacement into sub-optimal or higher-risk habitats, and suggest that impact assessments should focus on sub-lethal effects resulting from changes in foraging performance of animals within affected sites.