Estimated Density and Abundance of Humpback Whales (Megaptera novaeangliae) off West Maui, Hawai‘i

Approximately half of the North Pacific humpback whale (Megaptera novaeangliae) population migrates from high-latitude feeding grounds to Hawai‘i each winter and spring to breed. Beginning in 2015, fluctuations in whale abundance were reported in Hawai‘i, and this trend has continued in subsequent years. These reports were consistent with passive acoustic monitoring data, which showed a decreasing whale presence off Maui during the same period using male humpback whale chorusing as a proxy for relative whale abundance.

To further quantify whale abundance in Maui and investigate fluctuations in the population, vessel-based surveys in a focal area off west Maui using distance sampling methods were initiated in 2018. In total, 26 survey days were completed over the 2018–2019, 2019–2020, and 2020–2021 breeding seasons; however, survey coverage in January and April differed among years. Estimates of pooled abundance, density (whales/km2), and coefficients of variation (CV) during the peak of the three seasons (February–March) were:

• 1. 2018–2019: estimated abundance = 2,207, estimated density =1.20, CV= 0.08
• 2. 2019–2020: estimated abundance = 2,826, estimated density =1.59, CV= 0.08
• 3. 2020–2021: estimated abundance = 2,207, estimated density =1.24, CV= 0.10

Results indicate that after the reported period of decline in whale presence, the number of humpback whales using the west Maui area has generally increased, but has fluctuated, over the three seasons surveyed. Continued surveys and further studies are warranted to better understand the fluctuations occurring in the recently delisted Hawai‘i distinct population segment of humpback whales.