The relationship between male humpback whale song chorusing and whale abundance on the Hawaiian breeding ground

Passive acoustic monitoring (PAM) with autonomous bottom-moored recorders is widely used to study cetacean occurrence, distribution, and behaviors, as it is less constrained by factors that often limit other traditional visual observation methods, such as weather and accessibility. During the breeding season, male humpback whales produce an elaborate acoustic display known as “song.” The typical asynchronous chorusing of numerous singing males at any one time can provide challenges for monitoring abundance using PAM. Chorusing becomes the dominant source of low frequency (0–1.5 kHz) noise in the marine soundscape in Hawai‘i and seasonal levels mirror the whales’ migratory patterns. However, the relationship between chorusing levels and overall whale numbers, including non-singing whales (e.g., mother-calf pairs and juveniles), has remained poorly defined. We combined long-term PAM conducted between 2014/15 and 2020/21 off West Maui with concurrent visual land- and vessel-based observations. We found that daily median root-mean-squared sound pressure levels (RMS SPLs) correlate strongly with whale numbers (land: 0.71 ≤ R2 ≤ 0.76, vessel: 0.81 ≤ R2 ≤ 0.85 for three different PAM locations). Applying these results, we were able to use PAM to document significant population fluctuations between 2015 and 2021, as well as study habitat use patterns off West Maui.