Humpback whales are one of Hawaii’s many tourist draws, but this year experts say fewer of them than usual have returned to the state’s warm waters for the winter.

According to the Associated Press, officials at the state’s Humpback Whale Marine Sanctuary say the giant mammals, who travel annually from Alaska to Hawaii to spend much of November through May around its warmer environs, have yet to populate the archipelago. An estimated 10,000 whales make the underwater trek each year to Hawaii, but this year the animals seem to be missing their usual numbers. “What I’m seeing out there right now I would’ve expected a month ago,” said Ed Lyman, a response coordinator for the sanctuary, in an interview with the AP.

Why have the whales steered clear of their traditional stomping ground? Their apparent absence may be due to El Nino, which leads to warmer temperatures in the Pacific Ocean and could be a factor in delaying the whales’ migration. An estimated 10,000 whales make the underwater trek each year to Hawaii. Another theory is that the whales’ disappearance actually means their population has gotten larger and it is therefore taking the animals longer to compete for enough food to make the long journey to Hawaii.

“One theory was that something like this happened as whales increased. It’s a product of their success,” said Lyman.

Read More: Humpback Whales May No Longer Be Endangered

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