Humpback whales are slow to return to warmer waters of Hawaii this season as December usually marks the animals' migration from Alaska. Experts are currently investigating what the holdup is.

"This isn't a concern, but it's of interest. One theory was that something like this happened as whales increased. It's a product of their success," Ed Lyman, a Maui-based marine biologist and response coordinator for the Humpback Whale Marine Sanctuary, said in a statement. "What I'm seeing out there right now I would have expected a month ago. We've just seen a handful of whales."

The whales' slow return to Hawaii is of particular interest because these iconic animals are a famous part of winter on the islands and a major source of income for local tour operators. Researchers are unsure exactly how many whales have traveled to Hawaii so far, since population counts are not recorded until the last Saturdays of January, February and March.

Each year thousands of humpback whales migrate upwards of 3,000 miles from Alaska to Hawaiian breeding grounds, according to the National Oceanic and Atmospheric Administration (NOAA). In order to travel such great distances - the farthest migration of any mammal, in fact - humpbacks spend summer months building up fat stores by filter-feeding on tiny crustaceans.

Humpback whales are protected under the Endangered Species Act and federal law prohibits approaching within 100 yards of them by boat. Despite past population declines threatening the species, experts are not yet concerned about their slow arrival to Hawaii.

"They don't necessarily show up in the same place at the same time every year," former sanctuary co-manager Jeff Walters said.

The whales may also be spending more time up north because of El Niño patterns and warmer waters or because their population has increased., experts say. 

"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.

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