Massive citizen science effort uncovers big decline in humpback numbers

Researchers say a heatwave is probably responsible for a dramatic decline in the number of humpback whales over the past ten years in the northern Pacific Ocean.

Using data collected by 4,000 whale watchers – the largest individual identification dataset ever compiled for a whale species – a team of researchers led by Dr Ted Cheeseman from Southern Cross University showed the regional humpback whale population has suffered a decline from 2012 to 2021 after peaking a decade ago.

That’s a massive loss of humpbacks.

“Humpback whales in the full North Pacific increased from 16,875 in 2002 to a peak of 33,488 in 2012 before a decline to 26,662 individuals by 2021,” Cheeseman told Cosmos, saying it coincided with a severe marine heatwave 2014 – 2016 that reduced food availability. 

Cheeseman says it takes 5-7 years for a whale to become sexually mature and “birth rates have not returned to pre-heat wave levels”.

The humpback population was decimated by whaling over 200 years with one estimate suggesting that before a final moratorium on commercial whaling in 1985, all populations of humpback whales were greatly reduced, most by more than 95 percent. 

“Global humpback populations are generally divided based on breeding areas into 14 populations,” Cheeseman says. “There is a lot of complexity and nuance in this but in general, 4 of these are considered still endangered while all others are considered recovering or recovered.”

Can there be a return to humpback halcyon days?

Cheeseman says populations in the West Pacific, Central America, East Atlantic and Arabian Sea could be considered “very much at risk of a marine heatwave-induced shock”.

“I would speculate that the largest populations – the southern hemisphere group that feed in the ocean off Antarctica – are likely more robust to individual marine heatwave events because of the oceanography.

“The circumpolar current means that no marine heat wave could sit in one area like the big Pacific Marine Heatwave did.

“Australian humpback whales feed mostly off Antarctica and populations appear to be very healthy. No study of this scale has ever been done on other humpback populations.”

Cheeseman is the co-founder of ‘Happywhale’, which encourages citizen scientists to report whale sightings for research. The project has data through research collaborators and citizen scientists for 14,200 eastern Australian humpbacks and 3,100 western Australian humpbacks, but because very few of these are seen in their feeding areas it is much harder to document the population compared to those in the North Pacific. 

“It is likely that as long as krill harvesting is not expanded to Australian Antarctic waters that Australian humpback whale populations will remain healthy, unless climate change radically alters Antarctic marine ecosystems,” he says.

Cheeseman believes it is likely that humpback whales are less affected by marine heatwaves than other whale species, because they are more flexible in what they feed on.

According to the recent report on State of the World’s Migratory Species, humpback whales are considered one of the few at relatively low risk… but, says Cheeseman, “if the whole ocean declines in productivity and health, many species — and our societies and economies — suffer”.

Cheeseman’s report is published  in Royal Society Open Science.