Humpback whale abundance in Hawai'i: Temporal trends and response to climatic drivers

In 2001, we began long-term monitoring using scan sampling methodology to study the relative abundance and distribution of humpback whales (Megaptera novaeangliae) on their Hawaiian breeding grounds. Samples were collected annually (2001-2019) from a shore site overlooking Kawaihae Bay, HawaiModified Letter Turned Commai Island. Each sample included the number of whales, pod composition, and position. Whale numbers increased from 2001 to 2015, with increasing variability after 2010. Numbers then declined, including a precipitous 60% drop between 2015 and 2016. Crude birth rate fell from 6.5% in 2015 to 1.1% in 2016. We used generalized additive models, including predictor variables of year, date, observational conditions, and observer, to establish a base model. Climatic indices were individually added to base models, and their contributions were evaluated using Akaike information criteria. Models indicated that whale numbers and crude birth rate fell when climate indices reflected warmer water on high latitude feeding grounds in the North Pacific. Our results show that oceanographic conditions during prior feeding seasons correspond with the reproductive rate and the number of whales in HawaiModified Letter Turned Commai. Continued long-term monitoring of whale populations is essential to documenting and understanding marine ecosystem responses to global climate change.

Automated summary

The study found that the number and birth rate of humpback whales in Hawaii decreased when water temperatures in high latitude feeding grounds in the North Pacific rose. Long-term monitoring of whale populations is crucial for understanding how marine ecosystems respond to global climate change.