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ESR 43:421-434 (2020)  -  DOI:

Fluctuations in Hawaii’s humpback whale Megaptera novaeangliae population inferred from male song chorusing off Maui

Anke Kügler1,3,*, Marc O. Lammers2,3, Eden J. Zang2,3, Maxwell B. Kaplan4,5, T. Aran Mooney4

1Hawai‘i Institute of Marine Biology, University of Hawai‘i at Mānoa, Kāne‘ohe, HI 96744, USA
2Hawaiian Islands Humpback Whale National Marine Sanctuary, Kihei, HI 96753, USA
3Oceanwide Science Institute, Makawao, HI 96768, USA
4Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA
5Present address: Fisheries and Oceans Canada, Ottawa, ON K1A 0E6, Canada
*Corresponding author:

ABSTRACT: Approximately half of the North Pacific humpback whale Megaptera novaeangliae stock visits the shallow waters of the main Hawaiian Islands seasonally. Within this breeding area, mature males produce an elaborate acoustic display known as song, which becomes the dominant source of ambient underwater sound between December and April. Following reports of unusually low whale numbers that began in 2015/16, we examined song chorusing recorded through long-term passive acoustic monitoring at 6 sites off Maui as a proxy for relative whale abundance between 2014 and 2019. Daily root-mean-square sound pressure levels (RMS SPLs) were calculated to compare variations in low-frequency acoustic energy (0-1.5 kHz). After 2014/15, the overall RMS SPLs decreased between 5.6 and 9.7 dB re 1 µPa2 during the peak of whale season (February and March), reducing ambient acoustic energy from chorusing by over 50%. This change in song levels co-occurred with a broad-scale oceanic heat wave in the northeast Pacific termed the ‘Blob,’ a major El Niño event in the North Pacific, and a warming period in the Pacific Decadal Oscillation cycle. Although it remains unclear whether our observations reflect a decrease in population size, a change in migration patterns, a shift in distribution to other areas, a change in the behavior of males, or some combination of these, our results indicate that continued monitoring and further studies of humpback whales throughout the North Pacific are warranted to better understand the fluctuations occurring in this recently recovered population and other populations that continue to be endangered or threatened.

KEY WORDS: Humpback whale · Passive acoustic monitoring · Song chorusing · Population monitoring · Hawai‘i · North Pacific · El Niño · The Blob

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Cite this article as: Kügler A, Lammers MO, Zang EJ, Kaplan MB, Mooney TA (2020) Fluctuations in Hawaii’s humpback whale Megaptera novaeangliae population inferred from male song chorusing off Maui. Endang Species Res 43:421-434.

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