Acoustic harassment devices (AHDs) are used by the Atlantic salmon (Salmo salar) industry to deter harbour seals (Phoca vitulina) from aquaculture cage sites. Two preliminary behavioural studies suggest that many harbour seals in the Bay of Fundy, Canada, have habituated to the sounds of AHDs. Although the sample size was low, no response or change in behaviour of seals in the water occurred when a nearby AHD was activated. Some seals came within 45 m of an active AHD. Seals also passed close by an active AHD when moving to a haulout site. In situ sound pressure level measures were made around the periphery of nine aquaculture cage sites in the Deer Island area of the Bay of Fundy. Sound pressure levels at 1, 5 and 10 m depths within the aquaculture cage sites were generally c162 dB re 1 Pa. The individual pulse lengths of the AHD pulse trains were typically 2 msec. Ambient noise levels at 9–11 kHz did not vary with time of day, but were influenced by weather. The highest sound level recorded at cage sites within 10 m of the surface was 168 dB re 1Pa which is about 80 dB above the seal's detection threshold for short pulses at these frequencies. On most days, AHD sounds would be clearly audible to harbour seals at ranges of 1.1 to a theoretical maximum of 20.2 km, depending on the ambient noise levels and sound transmission characteristics in the region adjacent to the cage site. Our results suggest that harbour seals in the study area are not frightened away by the sounds of AHDs and that the sound levels throughout most of the cage sites do not reach the likely pain threshold.