Aerial survey perspectives on humpback whale resiliency in Maui Nui, Hawai’i, in the face of an unprecedented North Pacific marine warming event

Abstract

After decades of population growth, the central stock of the North Pacific population of humpback whales, known as the Hawai'i Distinct Population Segment (HDPS), was delisted from its endangered status in 2016. At that time, however, an unprecedented heating event, the "Pacific Marine Heatwave" (PMH) was already underway. The PMH coincided with reports of major declines of sightings of humpback whales, including calves of the year, on both the Hawaiian wintering grounds and the feeding grounds of Southeast Alaska. To examine the resiliency of the Hawai'i Distinct Population Segment, we conducted aerial surveys of the high- density Maui Nui region immediately following the PMH event in 2019 and 2020, using distance sampling methods identical to those used in an earlier series (1993 to 2003). Results showed whale densities at or above those seen earlier, with mean density for 2020 highest overall. Crude birth rates (% groups containing a calf) were similarly comparable to those recorded in the earlier series, with an increase from 2019 to 2020. Overall, results suggest the central North Pacific humpback whale population stock to be resilient in the face of this major climatic event.

Bellwethers of change: population modelling of North Pacific humpback whales from 2002 through 2021 reveals shift from recovery to climate response

“…[77–79]) was reported after the 2014–2016 PMH, using boat-based transect surveys [38] and passive acoustic monitoring [65], as well as off the Kohala coast of Hawai‘i Island using shore-based systematic scans [64]. The relative abundance trend measured agrees with data from density estimates by fixed-wing aerial surveys of the entire Maui Nui region for 1993–2003 and 2019–2020 [80], though these surveys did not occur during the period of apparent steep increase then decrease, over the 2005–2016 period. Our results support the conclusions of Frankel et al .…”

“…[64] that environmental variables likely caused Hawaiian humpback populations to decline due to the PMH. Fluctuating but resilient humpback whale crude birth rates in Hawai‘i [38,80] suggest the measured −4.3% yr −1 decline from 2014 to 2021 may be temporary, stabilizing if ocean conditions in feeding areas return to the long-term mean. These trends point to an apparent ecological shift from a half century of recovery response following near-extirpation by industrial whaling to carrying capacity-limited abundance.…”