Multiyear ocean sound levels at the Northeast Pacific OOI Sites and Beaufort-Sea Arctic recorded using calibrated moored hydrophones

Low-frequency (<500 Hz) ocean sound levels are driven by both natural (e.g. wind, waves, marine animals) and anthropogenic (e.g. ship traffic, seismic exploration) sources. To assess the impact of human-made sound in the northeast Pacific and Alaskan Arctic oceans, we reviewed several years (2018-2020) of acoustic data from the NOAA-NPS Ocean Noise Reference Network hydrophones at Ocean Station Papa, Beaufort Sea and Axial Seamount. We estimated the weekly median sound levels in the 63 and 125 Hz one-third octave bands, thought to be the main frequency bands of commercial ship noise. Sound levels ranged from 80 to 103 dB re µPa2 at the three sites, with Papa exhibiting the highest overall levels annually (93-103 dB). The Papa and Axial levels also show winter highs and summer lows due to wind and sea-surface wave effects, as well as a 1-2 dB (~30%) decrease during Spring 2020 as compared to 2018-2019. This Spring 2020 decline in levels was likely due to reduced commercial shipping in the NE Pacific at the start of the Covid-19 pandemic. A seismic research airgun survey from the R/V Langseth temporarily dominated the levels at Axial Seamount during July-August 2019. The survey was at distances between 1.5-30 km from the hydrophone, generating hourly mean received levels of 75 - 112 dB re µPa2. These observed airgun received levels are consistent with the predicted 160 dB re 1µPa-rms levels (at distances of 0.5-6.7 km) considered the level B harassment zone for marine mammals [Dubuque et al., 2019]. The Beaufort Sea records show seasonal sound level fluctuations likely associated with sea-ice coverage, with low sound levels (~80 dB) during high sea-ice coverage in winter-spring, and high sound levels (~96 dB) during late summer-fall when sea-ice is lowest. The Beaufort 2018 levels were ~10 dB higher in late summer than 2019, probably due to annual differences in sea-ice conditions or human activities.