Male humpback whale chorusing and what it reveals about the species’ ecology in Hawaiʻi

Abstract

Approximately half the stock of North Pacific Humpback whales (Megaptera novaeangliae) visit the shallow waters of the Main Hawaiian Islands each winter. Between December and April, male song chorusing becomes the dominant source of low frequency energy in the marine soundscape. Seasonal increases and decreases in amplitude mirror the staggered whale migratory pattern, suggesting that chorusing can be reflective of relative whale abundance. Data from long-term passive acoustic monitoring (PAM) collected with bottom-moored recorders deployed at three sites off Maui, Hawaiʻi, during the humpback whale breeding seasons of 2016–2021 were combined with 74 visual surveys from a shore station and 26 vessel-based line-transect surveys. Daily average chorusing levels showed a strong positive second-order polynomial correlation with abundance/density (land: 0.72 ≤ R2 ≤ 0.76, vessel: 0.81 ≤ R2 ≤ 0.85), indicating that male chorusing accurately tracks overall humpback whale abundance off Maui, including non-singing whales, up to a threshold, after which levels plateau/decrease with further increasing whale density. The Hawaiʻi humpback whale distinct population segment (DPS) was delisted from the U.S. Endangered Species List in 2016 following decades of positive population growth. Using male chorusing as a proxy for relative abundance, levels decreased by over 50% during the peak of the breeding season in February and March at six PAM sites off Maui between 2015 and 2019. This change in chorusing levels co-occurred with a broad-scale oceanic heat wave in the northeast Pacific, a major El Niño event, and a warming period in the Pacific Decadal Oscillation cycle. The reported level of reduction is consistent with results from visual studies in Hawaiʻi during the same time period. Male humpback whale chorusing levels showed opposing diurnal trends off Maui. Levels at four shallow/nearshore PAM recorders decreased during the day, with a concurrent increase at a deep offshore recorder. Noise levels in the adjacent frequency band associated with non-humpback environmental sounds increased four-fold at the deep site around sunset, but not at the other sites. Visual land-based observations revealed adult pods not containing calves moved offshore during the day. Acoustic localization of singers using vector sensors showed a daytime increase in spacing among nearshore singers. Combining these PAM and visual data suggests that the observed diel chorusing trends are the result of opposing drivers. Singers attempting to reduce nearshore intraspecific acoustic interference and acoustic masking from environmental noise offshore creates a dynamic of diel movements of singers that is repeated daily throughout the breeding season. These studies provide valuable insights into spatio-temporal habitat use patterns of an acoustically active migratory whale species in Hawaiʻi and information on population fluctuations in light of increasing climate change concerns. They demonstrate that PAM is a relatively low-cost, robust complement and/or alternative for studying and monitoring humpback whales in their breeding grounds that is able to capture small-scale fluctuations during the season and can inform managers about population trends in a timely manner.