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

Abstract

For the 40 years after the end of commercial whaling in 1976, humpback whale populations in the North Pacific Ocean exhibited a prolonged period of recovery. Using mark-recapture methods on the largest individual photo-identification dataset ever assembled for a cetacean, we estimated annual ocean-basin-wide abundance for the species from 2002 through 2021. Trends in annual estimates describe strong post-whaling era population recovery from 16 875 (± 5955) in 2002 to a peak abundance estimate of 33 488 (± 4455) in 2012. An apparent 20% decline from 2012 to 2021, 33 488 (± 4455) to 26 662 (± 4192), suggests the population abruptly reached carrying capacity due to loss of prey resources. This was particularly evident for humpback whales wintering in Hawai'i, where, by 2021, estimated abundance had declined by 34% from a peak in 2013, down to abundance levels previously seen in 2006, and contrasted to an absence of decline in Mainland Mexico breeding humpbacks. The strongest marine heatwave recorded globally to date during the 2014-2016 period appeared to have altered the course of species recovery, with enduring effects. Extending this time series will allow humpback whales to serve as an indicator species for the ecosystem in the face of a changing climate.

Keywords: abundance estimation; carrying capacity; climate change; environmental variables; marine heatwave; mark–recapture modelling.

Figure 1.

Locations of photo-identification samples from humpback whales in the North Pacific 2001–2021 colour coded by geographical strata. Summer feeding area strata north of 32°N, west to east are: Kamchatka Peninsula, Russia (black), west Bering Sea (cyan), north and western Bering Sea (green), Gulf of Alaska (yellow), southeast Alaska and northern British Columbia, Canada (blue and purple), southern British Columbia, Canada and Washington, United States (salmon), and California and Oregon, United States (orange). Wintering area strata south of 32°N, west to east are West Pacific (purple), Hawai‘i, United States (blue), Baja California, Mexico (black), Revillagigedo Islands, Mexico (cyan), Mainland Mexico (green), southern Mexico and Central America (yellow).

Figure 2.

Bias-corrected estimates of humpback whale abundance for the full North Pacific Ocean. The red line represents a three-year moving average of abundance estimates. Vertical lines represent confidence intervals based on ± twice the standard errors (s.e.). S.e. include both the uncertainty in the abundance for the SPLASH years (CV = 0.04, mid-sample year = 2005 [30]) and the uncertainty in the bias-correction process. Inset: North Pacific humpback whale abundance in the context of post-whaling population estimates [25,26] and 1993 estimate [31].

Figure 3.

Relative estimates of humpback whale abundance for the Hawai‘i (red) and Mainland Mexico (black) regions based on mark–recapture comparisons with northern British Columbia and southeast Alaska, and California and Oregon, respectively. The lines represent 3-year moving averages of abundance estimates. Vertical lines represent confidence intervals based on ± twice the standard error (s.e.).

Figure 4.

Proportion of Hawai‘i and Mainland Mexico relative abundance of humpback whales during study period of 3-year pooled samples 2002 through 2021, with a slope of −0.8% per year.