, 2004, Bailey and Thompson, 2010 and Pirotta et al., in press). Other marine mammal species are also regularly sighted in the area: harbour seal (Phoca vitulina), harbour porpoise (Phocoena phocoena), grey seal (Halichoerus grypus), and, further offshore, minke whale (Balaenoptera acutorostrata) and other smaller delphinid species ( Reid et al., 2003). In addition to the bottlenose dolphin SAC, six rivers around the Firth are SACs for Atlantic salmon (Salmo salar), while the Dornoch Firth is an SAC for harbour seals ( Butler et al., 2008). Two locations were selected for underwater noise monitoring: The Sutors (57°41.15′N, 3°59.88′W), Selleck Etoposide at the entrance to

the Cromarty Firth, and Chanonry (57°35.12′N, 4°05.41′W), to the southwest (Fig. 1). Both locations are deep narrow

channels characterised by steep seabed gradients and strong tidal currents, heavily used by the dolphins for foraging (Hastie et al., 2004, Bailey and Thompson, 2010 and Pirotta et al., in press). The Sutors supports commercial PLX-4720 in vivo ship traffic transiting in and out of the Cromarty Firth, while Chanonry is on the route to and from Inverness and to the west coast of Scotland via the Caledonian Canal (Fig. 2). Water depths at the deployment sites were 45 m (The Sutors) and 19 m (Chanonry). Proposed development of fabrication yards for offshore renewable energy at Nigg, Invergordon and Ardersier yard (Fig. 1) are expected to increase levels of ship traffic in the SAC. Several consecutive deployments of single PAM devices (Wildlife Acoustics SM2M Ultrasonic) were made at the two sites during Cepharanthine summer 2012. The units were moored in the water column ~1.5 m above the seafloor. The periods covered by the deployments are shown in Table 1. Gaps in the time series at The Sutors were caused by equipment malfunctions. Noise was monitored on a duty cycle of 1 min every 10 min at a sampling rate of 384 kHz and 16 bits. This regime allowed for detection of ship passages with a similar time resolution to the AIS data (∼10 min;

see below) while also providing recordings of marine mammal sounds up to 192 kHz. Additionally, noise was recorded at 192 kHz, 16 bits during the remaining 9 min of the duty cycle. These data were only used for detailed analysis of illustrative events. The PAM units were independently calibrated using a pistonphone in the frequency range 25–315 Hz. This calibration agreed with the manufacturer’s declared sensitivity to within ±1 dB, and so the manufacturer’s data were used for the entire frequency range (25 Hz–192 kHz). Acoustic data were processed in MATLAB using custom-written scripts. The power spectral density was computed using a 1-s Hann window, and the spectra were then averaged to 60-s resolution using the standard Welch method (Welch, 1967), producing a single spectrum for each 1-min recording. These were then concatenated to form a master file for subsequent analysis.