Source Level of Antarctic Blue and Fin Whale Sounds Recorded on Sonobuoys Deployed in the Deep-Ocean Off Antarctica

Susannah Calderan,Russell Leaper,Elanor Bell,Kathleen M. Stafford,Michael C. Double,Ana Širović,Elanor J. Miller,Brian S. Miller

doi:10.3389/fmars.2021.792651

Abstract

The source levels, SL, of Antarctic blue and fin whale calls were estimated using acoustic recordings collected from directional sonobuoys deployed during an Antarctic voyage in 2019. Antarctic blue whale call types included stereotyped song and downswept frequency-modulated calls, often, respectively, referred to as Z-calls (comprising song units-A, B, and C) and D-calls. Fin whale calls included 20 Hz pulses and 40 Hz downswept calls. Source levels were obtained by measuring received levels (RL) and modelling transmission losses (TL) for each detection. Estimates of SL were sensitive to the parameters used in TL models, particularly the seafloor geoacoustic properties and depth of the calling whale. For our best estimate of TL and whale-depth, mean SL in dB re 1 μPa ± 1 standard deviation ranged between 188–191 ± 6–8 dB for blue whale call types and 189–192 ± 6 dB for fin whale call types. These estimates of SL are the first from the Southern Hemisphere for D-calls and 40 Hz downsweeps, and the largest sample size to-date for Antarctic blue whale song. Knowledge of source levels is essential for estimating the detection range and communication space of these calls and will enable more accurate comparisons of detections of these sounds from sonobuoy surveys and across international long-term monitoring networks.

Highlights

Antarctic blue whales (Balaenoptera musculus intermedia; ABWs) nearly became extinct due to 20th century industrial whaling (Rocha et al, 2015), and today they remain Critically Endangered (Cooke, 2018a)
Though we have focused on the calls of Antarctic blue and fin whales on their Antarctic feeding grounds, our method for estimating SL from sonobuoy detections should be generally applicable to other call types and environments – provided that there is sufficient environmental information to model transmission losses (TL) with fidelity
We have demonstrated a viable method of estimating source levels of baleen whale calls from arrays of Directional Frequency Analysis and Recording (DIFAR) sonobuoys

Summary

Introduction

Antarctic blue whales (Balaenoptera musculus intermedia; ABWs) nearly became extinct due to 20th century industrial whaling (Rocha et al, 2015), and today they remain Critically Endangered (Cooke, 2018a). Due to the low number of animals, the large area of the Southern Ocean that they potentially inhabit, and the expense of conducting scientific operations in the Antarctic, field biologists have sought newer, more efficient means of studying large whales than traditional line-transect surveys. Both ABWs and fin whales produce high-intensity, low-frequency vocalizations (Širovicet al., 2004; Rankin et al, 2005) which can be detected over distances much larger than those over which whales can be seen from a ship (Širovicet al., 2007; Samaran et al, 2010a; Miller et al, 2015). Listening for these sounds via passive acoustic monitoring (PAM) offers an efficient means of detecting calling whales

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Conclusion