Following each AMAR deployment, data were downloaded and converted to .wav file format using proprietary software (Jasco Applied Sciences Ltd). Matlab (The Mathworks; Massachusetts) scripts were developed to process the acoustic data. Hydrophone response curves provided by the manufacturer's calibration process were interpolated linearly to provide a hydrophone sensitivity value per 1 Hz and used to calibrate the data with an acoustic gain of 0 dB.
A Fast Fourier Transform (FFT) function was applied to each 30-min acoustic recording to provide a power spectral density (PSD) in square pressure (pRMS 2 ).
The FFT used a 1-s Hann window, with a 50% overlap and a scaling factor of 0.5 to remove the effect of the Hann window on the resulting amplitude (Cerna and Harvey, 2000). A noise power bandwidth correction of 1.5 was also applied to give a 1-Hz frequency resolution (Cerna and Harvey, 2000; Merchant et al., 2013). The means of the square pressure values were calculated per minute per Hz to facilitate practical data
storage requirements and computation time while maintaining a fine time resolution. To calculate third-octave levels for each 30-min acoustic recording, the mean minute square pressure values were summed together, within the frequency range 57–71 Hz (63 Hz third-octave band) and 113–141 Hz (125 Hz third-octave band) to provide a thirdoctave level for each 1-min period. Their means were calculated for each file to provide a 30-min mean third-octave level. To facilitate comparison of our data with other studies, and for the identification of trends, 24-h running-mean third-octave levels were calculated by taking the centred-mean of 24 consecutive 30-min mean third-octave levels. Seasonal patterns in third-octave levels were investigated by grouping the 30- min mean third-octave levels according to season (Spring; March–May, Summer; June–August, Autumn; September–November, Winter; December–February). The resulting values were then converted to dB, with the standard reference pressure of 1 μPa, once all processing and averaging was completed. The mean square pressure (pRMS 2 ), or arithmetic mean, as
well as percentile levels from pRMS 2 have been used in line with the latest recommendations