Abstract
Approximately half of the North Pacific humpback whaleMegaptera novaeangliaestock 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 µPa2during 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.
Highlights
Humpback whales Megaptera novaeangliae are baleen whales of the family Balaenopteridae that migrate annually between high-latitude summer feeding habitats and low-latitude winter breeding grounds
The Main Hawaiian Islands (MHI) — the largest islands of the Hawaiian archipelago consisting of Hawai‘i Island, O‘ahu, Kaua‘i, Ni’ihau, and the 4-island region of Maui, Moloka’i, Lāna’i, and Kaho’olawe called Maui Nui — are among the most important breeding grounds for North Pacific humpback whales, with more than half of the total North Pacific stock wintering in the islands (Herman & Antinoja 1977, Calambokidis et al 2008)
rootmean-square sound pressure level (RMS SPL) began to increase in November, peaked between February and March, before decreasing through April at all sites and during all monitored years (Figs. 3 & 4; Fig. S1)
Summary
Humpback whales Megaptera novaeangliae are baleen whales (mysticetes) of the family Balaenopteridae that migrate annually between high-latitude summer feeding habitats and low-latitude winter breeding grounds. These whales occur globally and have discrete populations in the different ocean. In the North Pacific, humpback whales spend the summers in subarctic and arctic regions such as British Colombia, Alaska, Kamchatka, and the Bering Sea and migrate to their tropical breeding grounds in Hawai‘i, Mexico, Central America, southern Japan, and the Philippines (Baker et al 1986, Calambokidis et al 2008, Clapham 2008, NOAA NMFS 2016). The observed trends have resulted in renewed concerns about the status of the Hawaiian DPS (NOAA 2019)
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