, 2012b) – in order to exclude persistent but variable low-level noise from the fabrication yard at Nigg ( Fig. 1) which was not associated to vessel movements. A narrower

frequency range (0.1–1 kHz, not 0.01–1 kHz) was also used to calculate the broadband noise level, since the spectrum below 100 Hz was contaminated by flow noise (see Section 3). AIS analysis was only conducted for The Sutors, which had high (>80%) temporal coverage. Coverage at Chanonry was more sporadic, such that only a few illustrative examples could be produced. By comparing AIS vessel movements to the acoustic data, peaks in noise levels were classed as due to: (i) closest points of approach (CPAs) of vessel passages; (ii) due to other AIS vessel movements; and (iii) unidentified. To compute the sound exposure attributable to each event, noise levels exceeding find more the adaptive threshold on either side

of each peak were considered to form part of the same event. Ambient noise levels differed significantly between the two sites (Fig. 3). Compared to The Sutors (Fig. 3b), noise levels at Chanonry were relatively low, with only occasional vessel passages (Fig. 3a). Variability in ambient noise levels at Chanonry was largely attributable to weather and tidal processes, as example data in Fig. 4 illustrate. Higher wind TGF-beta inhibitor speeds were associated to broadband noise concentrated in the range Interleukin-2 receptor 0.1–10 kHz (Fig. 4a and b), while a Spearman ranked correlation analysis (Fig. 4d) shows a broad peak with maximal correlation to wind speed at ∼500 Hz, consistent with the spectral profile of wind noise source levels (Wenz, 1962 and Kewley, 1990). The influence of rain noise was less apparent, perhaps because of low rainfall levels during the deployment, though the peaks in rainfall rate appear to correspond to weak noise peaks at ∼20 kHz, which would agree with previous measurements (e.g. Ma and Nystuen, 2005). Tide

speed was correlated to noise levels at low and high frequencies (Fig. 4d). The high (20–100 kHz) frequency component was attributable to sediment transport, which can generate broadband noise with peak frequencies dependent on grain size (Thorne, 1986 and Bassett et al., 2013). Sublittoral surveys of the area show a seabed of medium sand, silt, shell and gravel in the vicinity of the deployment (Bailey and Thompson, 2010), which approximately corresponds to laboratory measurements of ambient noise induced by this grain size (Thorne, 1986). The low frequency component was caused by turbulence around the hydrophone in the tidal flow (Strasberg, 1979) known as flow noise, which is pseudo-noise (i.e. due to the presence of the recording apparatus) and not a component of the acoustic environment. Comparison of the tide speed (Fig. 4c) with the periodic low-frequency noise peaks in Fig.