Thousands of humpback whales starved to death after marine heatwave

The number of humpback whales in the North Pacific Ocean fell by 20 per cent between 2012 and 2021, according to a study that used artificial intelligence to identify individual whales from photos of their tails.

The decline coincided with a massive marine heatwave sometimes called the blob, which began in 2013 and lasted until 2016. The unprecedented intensity of the blob was almost certainly the result of global warming.

The findings suggest that around 7000 whales starved to death because of the marine heatwave, says Ted Cheeseman at Southern Cross University in Australia. The blob is known to have caused mass die-offs of many other animals, such as seabirds.

“This is unlikely to be a one-time event and if we do not rapidly curb the causes of climate change globally, more marine heatwaves decreasing ocean productivity worldwide will be our future,” says Cheeseman.

“This will hit humpback whales and other whale species, but we should recognise these whales are indicators of ocean health. We humans depend on that ocean health for many things,” he says.

Whale populations are usually estimated by methods such as ship surveys, where the number spotted in one area is extrapolated to get a rough idea of the overall population. This study is the first to exploit data from an international collaborative project called Happywhale, where anyone can submit photos via a website or app, along with the time and location of the sighting.

AI is used to identify individuals from the photos. In the case of humpback whales (Megaptera novaeangliae), this is based on the shape of their tail flukes and their pigment patterns.

Overall, nearly 800,000 photos have now been submitted to Happywhale, which was co-founded by Cheeseman. That has allowed more than 100,000 individual whales around the world to be identified.

“With collaborative work enabled by the AI-powered image-recognition technology, we can cost-effectively study and monitor species that have been historically too difficult to track in fine detail,” says Cheeseman.

A large international team analysed the data to provide the first detailed picture of how the North Pacific humpback population has changed over time. The researchers expected to find that the population was still slowly recovering, as it had been since whaling largely ended, or had stabilised. Instead, they found evidence of a major decline from around 33,500 in 2012 to 26,500 by 2021.

There are large uncertainties in the numbers, but the team thinks the decrease is real. “What were the exact numbers? We can’t know, but we are quite confident in a major decline, a major loss of life in the North Pacific humpback whale population,” says Cheeseman.

“If not for the breadth of this effort, I suspect we’d have never realised the extent of change caused by this one massive marine heatwave,” he says.