Presenter: Robert Dziak (NOAA-PMEL)

Description:
Human-made noise in the ocean has been increasing since the 1950s, likely due to increased ship traffic worldwide. However, economic indicators suggest a significant drop in northeast Pacific commercial ship traffic, as well as individual ship counts, during the COVID-19 pandemic. This decrease in ship activity likely resulted in reduced ship noise from 2019 to summer of 2020. To investigate whether decreased ship traffic resulted in reduced ocean sound levels, we reviewed several years of acoustic data (<2 kHz) from the NOAA-NPS ocean noise reference network (ONRS) hydrophones at Ocean Station Papa and Axial Seamount. As potential control sites, we also included sound records from the ONRS Arctic-Beaufort Sea station, and as well as a 4-day hydrophone deployment at Challenger Deep, Mariana Trench. We estimated the median weekly sound levels at 100 Hz (calculated from 1-minute averages) as well as the third-octave level at 63 Hz. Both 100 Hz and 63 Hz third-octave bands are thought to be the main frequency bands for ship noise. Sound levels in these bands at both Papa and Axial decreased by 1-2 dB from April to August of 2020 as compared to sound levels in non-pandemic years. Papa and Axial also exhibited a several dB seasonal variation in sound levels due to weather and biological sources (blue/fin whales). Additionally, Axial Seamount levels were dominated by a nearby research airgun survey during July-August 2019. In contrast, sound levels in the Beaufort Sea show typical seasonal fluctuations associated with sea-ice, while ship noise levels in the Mariana Trench were equal and/or exceeded levels observed during a 2015 acoustic survey. The northeast Pacific sound levels presented here are consistent with low-frequency noise trends observed at the Neptune Canada cabled hydrophones off Vancouver Island and in the Puget Sound (Thompson and Barclay, 2020), as well as an NSF cabled hydrophone located ~87 km west of the Oregon coast (Dahl et al., 2021).