In search of silent seas

Ocean noise degrades the “ancient ambient” acoustic conditions under which marine animals evolved. Critical habitats of endangered southern resident killer whales and North Atlantic right whales are sufficiently altered... [ view full abstract ]

Ocean noise degrades the “ancient ambient” acoustic conditions under which marine animals evolved. Critical habitats of endangered southern resident killer whales and North Atlantic right whales are sufficiently altered that whales lose more than 90% of opportunities to communicate over biologically meaningful ranges. Much of the world's ocean remains unsampled. In partnership with National Geographic’s Pristine Seas program, we used a rapid-assessment, “acoustic prospecting” approach to capture recordings with minimal anthropogenic contribution. Spot recordings from Palau, Rapa Nui, and Bali allowed us to generate ambient noise spectra and compare these to Wenz’s minimum noise levels. Across the full 0-48 kHz frequency range, we found that 95th percentile noise levels off Bali were approximately 10 dB higher, Palau 3-6 dB higher, and Rapa Nui 2-3 dB re 1 µPa lower than Wenz’s minimum curve. Across an “anthropogenic band” between 10 and 1,000 Hz, Palau was 108 dB//µPa, Rapa 119 dB//µPa, and Bali was 95 dB//µPa, the lower theoretical limit. Given increases in global shipping, we risk losing opportunities to study ecological effects of ocean noise due to lack of control sites and times. Spatially, we urge action to identify candidate sites for quieter MPAs, not only sites that need mitigation. Temporally, we recommend taking advantage of holidays, such as Bali’s Nyepi (Day of Silence), where we observed a 6 dB drop in the noisiest percentile of low-frequency levels. Now, our focus is shifting toward practical solutions to reduce ship noise through simple changes in shipping practices in important whale habitats.