Abstract
Source level is one factor that determines the effectiveness of animal signal transmissions and their acoustic communication active space. Ocean-bottom seismometers (OBS) are platforms of opportunity to monitor marine species because they record data as pressure fluctuations in the water using a hydrophone and/or as particle velocity of the seabed using a seismometer. This study estimates source levels of 20 Hz fin whale notes recorded simultaneously in these two OBS channels and in two areas of the North Atlantic (Azores and southwest Portugal). It also discusses factors contributing to the variability of the estimates, namely geographical (deployment areas), instrumental (recording channels and sample size), and temporal factors (month of detected notes, inter-note interval, and diving duration). The average source level was 196.9 dB re 1 µPa m for the seismometer (derived from particle velocity measurements) and 186.7 dB re 1 µPa m for the hydrophone. Variability was associated with sample size, instrumental characteristics, acoustic propagation, and month of recordings. Source level estimates were very consistent throughout sequences, and there was no indication of geographical differences. Understanding what causes variation in animal sound source levels provides insights into the function of sounds and helps to assess the potential effects of increasing anthropogenic noise.
Highlights
Baleen whales produce low-frequency and high-intensity sounds that can be classified as calls or songs [1,2,3]
This study presents estimates of source levels of 20 Hz fin whale notes recorded concurrently in the two types of recording channels of the Ocean-bottom seismometers (OBS), and discusses several types of factors contributing to the variability of the estimates, namely, geographical, instrumental, and temporal factors
The remaining two OBS from the Azores showed relatively similar source level estimates compared with the southwest Portugal OBS
Summary
Baleen whales produce low-frequency and high-intensity sounds that can be classified as calls or songs [1,2,3]. Over the past few decades, ambient noise levels in the ocean, especially below 500 Hz, have increased in some regions, which seems to be linked with global economic growth [13,14] These sounds can potentially have a more extensive impact on animals because they can travel farther distances due to their lower attenuation [15]. Many seismic recordings were obtained throughout the years from several areas of the world’s oceans that have not yet been analysed in terms of their baleen whale acoustic data, and they could provide legacy data that could have crucial information to assess long-term temporal changes in source levels of fin whale notes. This study shows that the directional components of the seismometer of OBS can provide reliable source level estimates of fin whale notes
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