Although seal scarers are widely used both to reduce economic losses at fish farms caused by seal predation and to reduce risks posed to marine mammals by offshore pile driving activities, the spatial extent of their deterrent effect on harbour porpoises is still largely unclear. However, this information is crucial to understanding the effects these devices have on the marine environment and to judge their potential as a mitigation measure. A study was conducted in the German North Sea, using passive acoustic monitoring and to some extent simultaneous aerial surveying to specifically study the spatial extent of the deterrence effects of a seal scarer on harbour porpoises. In order to link porpoise detections at various distances to actual sound levels, sound measurements of the seal scarer signal were carried out at several distances from the source. C-POD recordings revealed a significant deterrence effect on harbour porpoises up to 7.5 km away (at about 113 dB re 1 Parms), much further than previously reported. During seal scarer operation the number of porpoise detections within 750 m of the C-PODs decreased by between 52 and 95% of the value before the seal scarer was activated. An aerial survey revealed a significant decrease in porpoise density from 2.4 porpoises km-2 before to 0.3 porpoises km-2 during seal scarer operation within the 990 km2 study area, showing that the decrease in porpoise detections by passive acoustic monitoring was probably indeed the result of a decrease in porpoise abundance. These results may raise serious concerns about unwanted disturbance effects on harbour porpoises in the context of seal scarer use at fish farms and also highlight the need for caution when applied as a mitigation measure during offshore construction.