In 2020, the global COVID-19 pandemic presented a unique opportunity to understand the effects of vessel noise on the underwater acoustic environment in many places around the world. This and other descriptions of the pandemic-induced quiet, and the gradual return to increased activity, can help inform efforts to improve existing methods to mitigate vessel noise impacts and maintain the ecological integrity of marine protected areas. Our results suggest that all types of vessels had a role in the quieter underwater sound environment in 2020, with the combined acoustic footprint of tour vessels and cruise ships most evident in the decrease in the 95th percentile loudest sounds. A 3 dB decline in median sound level in the 125 Hz one-third octave band in 2020 reflects a change in medium and large vessel noise energy and/or harbor seal vocalizations. At the 95th percentile levels, morning and afternoon peak times in 2020 were 6.3–9.0 dB quieter than previous years.
Focusing on morning (06:00–09:00 LT) and afternoon (15:00–18:00 LT) time-blocks when tour vessels and cruise ships enter and exit Glacier Bay, median broadband sound levels were 3.3–5.1 dB lower in 2020 than prior years. Broadband (17.8–8,910 Hz) sound levels in the 2020 Visitor Season were 2.7 dB lower than 2018 and 2.5 dB lower than 2016. In all years, we found clear seasonal and diurnal patterns in vessel generated noise, focused from 06:00 to 20:00 local time (LT) in the summer months. Overall, the number of vessel entries in Glacier Bay was 44–49% lower in 2020 (2020: n = 1,831 2018: n = 3,599 2016: n = 3,212) affecting all vessel classes, including the complete absence of cruise ships and only three tour vessel trips. Park tourism occurs mainly in May–September. We used continuous, calibrated hydrophone recordings to examine 2020 ambient sound levels compared with previous years: 2018, the most recent year with data available, and 2016 for historical perspective. Glacier Bay National Park in southeastern Alaska has monitored underwater sound since 2000. Marine vessel traffic has long been known to affect the underwater acoustic environment with direct and indirect effects on marine ecological processes. The global COVID-19 pandemic caused a sharp decline in vessel traffic in many areas around the world, including vessel-based tourism throughout Alaska, USA in 2020. 3Department of Fisheries, Wildlife, and Conservation Science, Marine Mammal Institute, Oregon State University, Newport, OR, United States.2Center for Conservation Bioacoustics, Cornell Lab of Ornithology, Ithaca, NY, United States.1Glacier Bay National Park and Preserve, Gustavus, AK, United States.