A sprawling international study of humpback whales in the northern Pacific has found their population has shrunk significantly since 2012 — despite the once-endangered species' remarkable comeback from the brink of extinction.

The new research, published in Royal Society Open Science journal on Wednesday, estimated a roughly 20 per cent drop in the cetacean species' numbers over a decade.

But despite that, one of the study's nearly 75 listed authors said the findings are not yet cause for alarm.

Thomas Doniol-Valcroze is head of the cetacean research program for Fisheries and Oceans Canada's Pacific Biological Station.

"It's not catastrophic news just yet," he told CBC's On the Island in an interview. "But it certainly it took us by surprise."

The research he contributed to comes amidst concerns about a large number of the whales found dead worldwide in recent years, which scientists believe may have been caused by marine heat waves.

The new study, titled Bellwethers of Change, involved 46 organizations and modelled the whales' likely population partly based on thousands of Northern Pacific humpback photographs — collected by nearly 4,300 "community science contributors."

Precise counts of the giant but elusive marine mammals are near impossible.

"It's not like we can count them all ... but we're pretty confident in those trends," Doniol-Valcroze said.

He explained that after being hunted to near extinction in previous centuries, governments' moves to legally protect the species — officially outlawing commercial hunting in 1976, when their population was estimated to be at most 1,600 — allowed them to make a remarkable rebound.

By 2012, Northern Pacific humpbacks' population had exploded to what scientists believe was more than 33,000.

"Once we stopped killing those animals … the population grew quite fast, actually faster than we said was possible," Doniol-Valcroze said. "Those populations can't grow forever, obviously, at some point they have to slow down.

"It's still a very healthy level for a population of whales."

According to University of British Columbia zoology professor Andrew Trites, the likely culprit for the change in humpbacks' fortunes is a matter of balancing their relatively rapid population recovery.

"They're going to overshoot initially," he told CBC News in an interview, "until they realize they've overeaten and there'll be a correction.

"What we have to watch for is if they go down, down, down again … Should we be worried and upset? It's too soon to tell."

The federal body overseeing species at risk in the country, the Committee on the Status of Endangered Wildlife in Canada, now lists humpbacks as of "special concern" — because even though the species is recovering, it's still "not secure."

That's because humpbacks face several ongoing threats, the committee noted — such as being hit by ships, toxins, and ocean noise pollution. But "the major threat," officials warned, comes from marine heat waves reducing their food supply.

"There were changes in the environment, and in particular a long marine heat wave," said Doniol-Valcroze. "They've been happening more and more right now in the context of climate change.

"It seems to have hit the humpback whales pretty hard. It's like the ceiling came down on them — and they reacted to this by this big drop in their population."

UBC's Trites agreed climate change is likely having an impact on humpback populations. But it's still unclear exactly how.

"We just have to watch to see whether or not in the long term climate change is going to reduce their food supply — or maybe it's going to improve things for them," the director of the university's Marine Mammal Research Unit told CBC News. "We just don't know at this point."

He noted that aside from climate change, there are the normal, periodic fluctuations in temperature to consider, such as the current El Niño cycle.

Trites praised the researchers' use of data from thousands of contributors.

"This is an amazing example of citizen science," he said. "And when you see so many authors on this … it's another sign of amazing leadership as well — to pull this off and have everybody put their data together for a common goal."