The biggest study ever to count a whale species has found that the number of humpbacks in the North Pacific has been increasing significantly. There are now 18,000 to 20,000 humpbacks in the region. From Sydney, Phil Mercer reports.

The comprehensive international study of whales in the northern Pacific was undertaken by 400 scientists from 10 countries, including the United States, Canada and Australia.

It took them three years to complete.

A young adult humpback whale breaching into the air during a migratory frolic in Australia's Hervey Bay Marine Park off Fraser Island, north of Brisbane (File)

Their findings: the number of humpback whales in the region has surged in recent years, increasing at about five percent a year.

Researchers think that humpbacks are as common now as they were before commercial whaling began more than 150 years ago.

There are concerns, however, that whaling nations such as Japan will use the new evidence to argue for increasing its annual whale harvest. Although commercial whaling is banned, Japan hunts whales for what it calls scientific reasons, although the meat is later sold.

Other conservationists say that discussions about the health of whale populations should not be limited to looking at numbers or the effects of hunting.

Nick Gales, an expert in marine mammal ecology at the Australian Antarctic Division, says whales face other threats, including the effects of global warming.

"The arguments for and against whaling are about a lot of things beyond just numbers, and the whole recovery of populations is differential, as I said, there are some populations that have recovered really well and others haven't," said Gales. "This is showing good recovery and really important information for the humpbacks in that area, but there are plenty of other threatening processes that are going to be really important to address as well as whaling."

The research also pointed out that some isolated populations of humpbacks, especially in the Western Pacific, have not recovered at the same rate as others.

The study began in 2004 and identified 8,000 individual whales as part of a project to map migratory patterns and estimated populations.

Conservation group Greenpeace doubts the findings of this study. A spokesman in Australia said whale populations have not "even come close to recovering to historical levels."