A Quieter Ocean: Experimentally Derived Differences in Attentive Responses of Tursiops truncatus to Anthropogenic Noise Playbacks before and during the COVID-19-Related Anthropause

SIMPLE SUMMARY: The COVID-19 pandemic caused a decrease in human activity in many areas across the globe due to stay-at-home orders and the cessation of non-essential work activities. This dramatic decrease in human activity became known as an anthropause which created an opportunity for scientists to study the effects of decreased anthropogenic (human-made) noise. Bermuda did not host cruise ships or allow non-essential activities from early in the pandemic through to 20 May 2021. We presented three anthropogenic sound types (cruise ship, personal watercraft, and Navy low-frequency active sonar) to dolphins housed at Dolphin Quest Bermuda in 2018 and again during the COVID-19-related anthropause in 2021. We played each dolphin three anthropogenic sound types via an underwater speaker. The dolphins’ behavioral and acoustic responses were recorded. We found that decreased anthropogenic noise levels around Bermuda were associated with altered attention in the dolphins. The dolphins increased look durations and changed vocal response behaviors to playbacks during the anthropause and the dolphins responded to cruise ship playbacks up to fourfold more in 2021. These changes in response to this sound type may have implications on how dolphins habituate and dishabituate to sounds ever-present in their environment. ABSTRACT: The effects of anthropogenic noise continue to threaten marine fauna, yet the impacts of human-produced sound on the broad aspects of cognition in marine mammals remain relatively understudied. The shutdown of non-essential activities due to the COVID-19-related anthropause created an opportunity to determine if reducing levels of oceanic anthropogenic noise on cetaceans affected processes of sensitization and habituation for common human-made sounds in an experimental setting. Dolphins at Dolphin Quest Bermuda were presented with three noises related to human activities (cruise ship, personal watercraft, and Navy low-frequency active sonar) both in 2018 and again during the anthropause in 2021 via an underwater speaker. We found that decreased anthropogenic noise levels altered dolphin responses to noise playbacks. The dolphins spent significantly more time looking towards the playback source, but less time producing burst pulse and echolocation bouts in 2021. The dolphins looked towards the cruise ship sound source significantly more in 2021 than 2018. These data highlight that different sounds may incur different habituation and sensitization profiles and suggest that pauses in anthropogenic noise production may affect future responses to noise stimuli as dolphins dishabituate to sounds over time.