Humpback whales which abundantly arrive in the waters of Hawaii in wintertime have been slow to show up this year, puzzled scientists have reported.

The annual return of the iconic creatures is a highlight of the season and a considerable source of income for operators of whale-watching tours; however, the giant marine mammals have been reportedly difficult to spot so far.

It's unclear why so few have been observed, said an official at the Humpback Whale Marine Sanctuary.

"This isn't a concern, but it's of interest," said Maui-based Ed Lyman, a resource protection manager who acts as response coordinator for the sanctuary. "One theory was that something like this happened as whales increased. It's a product of their success."

The population may have gone up, which could have affected the timing of their annual winter journey from Alaskan waters to warmer seas of Hawaii.

Before setting off on their migrations, the whales build up fat reserves by feeding on krill and small fish.

"With more animals, they're competing against each other for that food resource, and it takes an energy of reserve to make that long migration over 2,000 miles," Lyman added.

He also suggested that these whales could also be spending more time in northern waters, possibly because of ocean changes linked to El Niño.

A study [PDF] by the World Wildlife Fund and the Whale and Dolphin Conservation Society published in 2007 suggested that rising ocean temperatures might cause some species of whale to "shift their distributions to remain within optimal habitat."

While there have been fewer sightings around Hawaii so far this winter, an accurate count will have to wait for the annual official whale counts conducted at the end of January, February and March, said Jeff Walters, former co-manager of the sanctuary.

"They don't necessarily show up in the same place at the same time every year," he said.

Every winter, about 10,000 humpbacks make the journey from Alaska to Hawaii, usually in small groups of three or four whales, to mate and give birth among larger groups, known as pods that congregate in Hawaiian waters.

The humpback whale winter season in the Hawaiian archipelago normally runs from November to May.

Humpback whales enjoy protection as an endangered species, and by federal law, cannot be approached closer than 100 yards by boats.

Adult humpbacks can grow to 40 or 50 feet long and weigh about 80,000 pounds.