A Tale of Two Soundscapes: Comparing the Acoustic Characteristics of Urban Versus Pristine Coastal Dolphin Habitats in Western Australia

Underwater noise environments are increasingly being considered in marine spatial planning and habitat quality assessments. Although the overall aim of regulation is to quieten anthropogenically noise-rich habitats whilst maintaining pristine habitats free of man-made noise, effective management plans require knowledge upon which to base decisions. This is particularly true for managers of acoustically specialised species. This study aimed to compare the acoustic environment of coastal dolphins in two locations within Western Australia by comparing a 'pristine' habitat (Roebuck Bay) with an 'urban' habitat (Fremantle Inner Harbour). Autonomous underwater acoustic recorders collected approximately 940 and 1080 h of data from these two sites, respectively. Additionally, in Roebuck Bay opportunistic in situ recordings with concurrent visual observations were collected in the presence of two dolphin species. Acoustic data were assessed via weekly spectrograms, power spectrum density percentile plots and probability densities, octave band levels, broadband noise levels, and generalised estimating equations. Results indicated that the two sites had highly contrasting acoustic environments. In Roebuck Bay, the local soundscape was dominated by biotic sounds, with only sporadic vessel noise. However, in Fremantle Inner Harbour, anthropogenic noise was prevalent. On average, Roebuck Bay was 20 dB quieter than the Fremantle Inner Harbour over the frequency band 10 Hz-11 kHz. Dolphin communications had a greater potential to be masked in Fremantle Inner Harbour than in Roebuck Bay based on elevated anthropogenic noise levels. If noise levels were to increase in Roebuck Bay, coastal dolphins may show behavioural and/or acoustical responses as observed at other locations.