Humpback Population In North Pacific Decline By 20% Due To Marine Heatwave

A new study revealed that the humpback whale population in the North Pacific decreased by 20% between 2013 and 2021 as warmer water upended the ecosystem.

Marine Heatwave

The marine heatwave significantly reduced ocean productivity, putting the humpback whale population at risk.

Humpback whales, which can weigh up to 40 tons and reach lengths of 17 meters, are well-known for their melodious underwater calls and spectacular breaching performances. However, centuries of killing nearly drove the animals to extinction.

Humpback whales in the North Pacific were estimated to be down to 1,200 to 1,600 animals in 1976. Humpback whales recovered dramatically after the International Whaling Commission outlawed commercial whaling in 1982.

The latest analysis predicts a high of roughly 33,500 humpbacks in the North Pacific in 2012, with an average population growth rate of 6% between 2002 and 2013.

Humpback whales were removed from the US Endangered Species Act in 2016 due to their excellent 40-year population growth pattern.

That same year, an intense marine heatwave continued to warm waters in the northeast Pacific. Maximum water temperatures between 2014 and 2016 were 3-6 degrees Celsius above average.

This resulted in fewer nutrients for phytoplankton, the plants at the bottom of the marine food chain. The effects spread throughout the ecosystem, leaving less food for sardines, seagulls, and sea lions.

According to a new study, around 7,000 humpback whales perished in the North Pacific between 2013 and 2021, most likely due to a lack of food.

"Humpback whales are flexible, and willing to switch from krill to herring or anchovies to salmon fry. But when the whole ecosystem decreases in productivity, it hurts them big time," said Ted Cheeseman, a biologist at Southern Cross University in Lismore, Australia who led the study.

Pregnancy Of Whales

Humpback whale research in Antarctica has demonstrated that warming water conditions mean less food for whales, resulting in reduced pregnancy rates.

Ari Friedlaender, an ecologist in the Ocean Sciences Department at the University of California, Santa Cruz, who led the Antarctic research, believes that the 2014-2016 marine heatwave "impacted the pregnancy rates in the population" and "led to the demise of a certain number of animals" in the North Pacific.

Long-term surveys of humpbacks in the Au'au Strait between Maui and Lanai yielded similar results. Mother-calf contact rates in this Hawaiian waterway dropped by approximately 77% between 2013 and 2018, indicating a dramatic decline in humpback reproduction.

Scientists noted that the scarcity of food supplies during the maritime heatwave led to reproductive failure in 2018.

They observed that only three humpback calves made it from Hawaii to Alaska, and before the end of the feeding season, all three had disappeared.

During the brief marine heatwave that hit the northeast Pacific in 2021, the 24 females with calves lost weight throughout the feeding season, despite the fact that these mothers generally gain roughly 16kg per day.

"These animals are truly sentinels of the ocean. Healthy oceans make healthy whales and vice versa," said Lars Bejder, the director of the Marine Mammal Research Program at the University of Hawaii at Manoa and a co-author on the recent study.