As many as 25 percent or more of the humpback whales that usually make the annual winter trek from Alaska may have canceled their Hawaii travel plans this year.

A season that started off with many wondering why the whales were arriving so late developed into a relatively lackluster season overall, with ocean tour companies, researchers and scientists reporting appreciably fewer sightings.

In addition, fewer humpbacks were seen from the shoreline during the season’s annual citizen whale counts. The count held on Maui in late February at the usual peak of the season recorded half as many whales as last year.

There was so much anecdotal evidence of fewer sightings that the Hawaiian Islands Humpback Whale National Marine Sanctuary has decided to push for an updated population study that will include a scientific evaluation of the numbers.

“It is so different out there,” acknowledged Ed Lyman, the sanctuary’s Maui-based large whale entanglement response coordinator.

But Lyman and other scientists say they aren’t too worried. They say fewer sightings could be linked to the warmer waters of El Nino and the disruption of feeding patterns, causing some whales to stay put this winter or to travel to other areas for the season.

Or it could even be a reflection of the success of the species’ recovery. A growing number of humpbacks may be competing for the same amount of food, Lyman said, causing some to linger in their summer feeding grounds to build up the energy reserves necessary for migration later.

There are indeed reports that more whales were seen off Alaska this winter, some researchers said.

It has been estimated that more than 10,000 humpbacks travel each year from Alaska to Hawaii, where they breed and give birth primarily between December and April, although some whales come as early as October and leave as late as June.

The annual migration helps sustain a whale-watching industry said to be worth more than $20 million a year to Hawaii’s economy.

Hawaii’s population of humpback whales, according to many scientists, has been growing steadily in recent years. The humpbacks have been doing so well, in fact, that the National Oceanic and Atmospheric Administration is moving to delist the North Pacific population from the Endangered Species List.

On the surface, that healthy population growth seems at odds with this year’s humpback season. There is plenty of circumstantial evidence pointing to the whale deficit.

For example, there were only five confirmed entanglements compared to 13 last year and 13 the year before, Lyman said, while there were only two reports of a whale colliding with a boat.

“A few years ago there were 12 or 13 incidents, and people were starting to get concerned,” he said.

The sanctuary’s three ocean counts in January, February and March — conducted with the help of citizen volunteers — recorded some of the lowest sighting numbers in five years, Lyman said.

And on March 27, Pacific Whale Foundation volunteers counted 732 humpback whales at 12 locations along the west coast of Maui during the nonprofit’s Great Whale Count. That number was less than half of last year’s total of 1,488.

“A lot of whale researchers were calling me directly and saying, ‘What do you think?’” Lyman said.

Many in the whale-watching industry were puzzled as well.

“There’s hardly any (whales) now compared to previous years,” said Maehkyne Mason, manager of Maui Adventure Cruises, which runs whale watching tours out of Lahaina.

Mason said that while her tours don’t have trouble encountering whales, they often have to travel farther to see them.

Talk on the street about fewer whales has led to the company taking a hit in the bottom line, she said.

“When tourists ask (hotel) concierges if there are any whales to see, they are saying it’s not great,” she said, which leads some to search for other activities.

Jim Coon, owner of Maui’s Trilogy Excursions, said that while revenue is right on target for his company, he did call it an unusual year.

“We had days where we had tremendous whale showings,” Coon said. “And then there were other days not so much.”

Veteran whale researcher Adam Pack, an associate professor at the University of Hawaii at Hilo, said that well into January and early February there were far fewer whales off Hilo than in previous years.

By mid-March there appeared to be far fewer whales in Maui waters, too, he said, and whale-watching boats that traditionally had no problem locating pods were struggling to find them.

“That trend appeared to continue perhaps indicating not only a delayed start to the season but overall a much shorter season than in previous years,” Pack said in an email. “Combined with the low numbers reported by other groups, this seemed like an unusual aberrant whale season.”

Why the change?

One possibility, he said, is that the extreme El Nino in the North Pacific led to lower productivity on the feeding grounds, resulting in the whales remaining longer in Alaska and other feeding grounds along the northern Pacific rim. It is also possible that this season more whales than usual have overwintered in the feeding grounds, he said.

Beth Goodwin, vice president of Hawaii operations for the Jupiter Research Foundation, said she noticed fewer whales in the waters off Puako, Hawaii island, where she puts hydrophones in the water to record the whale songs of the sexually mature males and then streams them online.

Judging from this year’s recordings, the singer males arrived a few weeks later in the season and left a few weeks early, prompting her to pull her hydrophones from the water early.

“I don’t think it means too much,” said Goodwin, a marine biologist with 40 years experience. “I’m not in any way, shape or form worried about it.”

Next year could be a great year again, she said.

UH-Hilo marine science professor Jason Turner said his Biology of Marine Mammals class conducted weekly surveys outside Hilo Bay from January to March and found numbers down from previous seasons, probably on the order of 10 percent to 25 percent.

“The most obvious difference this year is that it is an El Nino season,” Turner said in an email. “However it’s difficult to tell exactly what impact that might have had on these migrating humpbacks, or what effect it has had on them in past El Nino years. Of course this is just speculation.”

A Pacific Whale Foundation research vessel recorded about a 25 percent drop in whale numbers from January to March in the ocean around Maui, Molokai, Lanai and Kahoolawe, the epicenter of the whale migration in Hawaii.

But the foundation’s executive director and chief scientist, Greg Kaufman, said he isn’t ready to sound the alarm.

“It is important to remember that one year does not make a trend,” he said, noting that Pacific Whale Foundation’s data have been showing a steady 7 percent to 8 percent annual growth of Hawaii’s humpback whale population. “2016 may be a slightly down year for arrivals, but it does not mean … something dire has happened.”

Research has also shown that not all whales migrate each year and there is variability in numbers from year to year, Kaufman said, and data have indicated that it is not unusual to have off years as well as peak years that correspond with birthing cycles.

“The impacts of El Nino cannot be understated or underestimated, either,” he said.

Humpback whales are opportunistic feeders and will exploit a prey source if available, he said. In fact, there are new reports from British Columbia in Canada, Washington, Oregon and northern California that humpbacks are overwintering there and feeding on record anchovy schools.

Kaufman said researchers studying humpback whales off Mexico and Costa Rica have reported lower humpback numbers as well.

Kaufman, who founded the Pacific Whale Foundation 36 years ago, said he recently returned from southern Chile where an unusual number of Southern Hemisphere humpback whales are being reported off Patagonia. The whales appear to be exploiting “the fruits of El Nino” in the form of greater amounts of prey. Normally the whales would be found feeding in the Straits of Magellan or off Antarctica.

Despite the off year in Hawaii, Kaufman insisted it has not affected the quality of whale-watching. He said his crews have reported some of the best April encounters in years and that he anticipates good whale-watching through May.

Pack, the chairman of the sanctuary’s advisory council, said that given that NOAA has proposed to delist the Hawaii and Alaska stocks of humpback whales from their endangered status, it is important to continue to survey the humpbacks and examine data from satellites to explore productivity in different feeding areas.

The data should help to determine whether the observations this winter are simply an unusual “blip” or a trend, he said.