Whale researchers are spotting a trend in the Hawaiian Islands — a decline in humpback whale sightings.

Not only are there fewer sightings, fewer male Hawaiian humpback whales have been recorded singing, and the number of mother-calf pairs has been diminishing for the past three seasons, according to the researchers. While the trend has been consistent over the past three years, the scientists refrained from sounding an alarm about the whales disappearing as a new season is around the corner.

“They’re not all gone,” said Ed Lyman, who is with the Hawaiian Islands Humpback Whale National Marine Sanctuary. “We are seeing indications of fewer whales near the islands where our effort is … There are still plenty of whales out there.”

The peak of the season is usually between January and March, when thousands of humpback whales migrate from Alaska to Hawaii to mate, calve and nurse their young, although they can be spotted in November or earlier, and stay as late as May.

Lyman, a researcher and the sanctuary’s whale entanglement response coordinator, said for him, the first signs of the decline were in late December 2015.

That’s when he got calls from whale-watching tour operators on Hawaii island, inquiring about their late arrival. Lyman reached out to tour operators on other islands, as well, and found them saying the same thing — the whales were late, and there were fewer sightings. He got the same reports from contacts in Mexico.

This correlates with the findings of Rachel Cartwright, a whale researcher on Maui, who found a dramatic drop in mother-calf sightings over the past three years as well.

Cartwright, who is with the Keiki Kohola Project, estimates mother-calf sightings have dropped about 80 percent over the past five years based on her transect surveys. The number of mother-calf pairs peaked in 2013-2014, she said, then dropped slightly. She did not think that was unusual, but in 2015, the year of El Nino, there were signs of a dramatic drop, and it happened again in 2016.

This last season, there were also fewer sightings of mothers and calves.

At the same time, National Oceanic and Atmospheric Administration research coordinator Marc Lammers, who specializes in whale acoustics, noticed the chorus of songs from male whales has quieted down.

“What we’ve effectively found,” he said, “is that the decibel levels of singing at locations we’ve been recording have been decreasing each year.”

These findings are in line with the less scientific annual Ocean Sanctuary count by volunteers conducted from January to March from island shores, which also found dramatically fewer whale sightings last season.

The cause of the downward trend remains unknown, although researchers believe there are several possibilities.

One is that the food source for the whales at their feeding grounds in Alaska has diminished, forcing the cetaceans to change their migration patterns or keeping them from migrating down to Hawaii at all. The whales feed on small fish and krill in Alaska during the summer, but those food sources may have shifted due to ocean warming.

Other possibilities are that the humpback whales have simply decided to go farther offshore or elsewhere than the usual spots where they are sighted — including, perhaps, the Northwestern Hawaiian isles.

Yet another hypothesis is that the number of whales has reached “carrying capacity,” said Lyman, meaning there are only so many the environment can support and it has reached that limit.

“The whales cover the entire ocean here in the North Pacific, and there are areas they may be exploring where they didn’t explore historically,” he said.

To compare and correlate their findings, whale researchers from the Pacific, including Alaska, are meeting for the first time at NOAA’s Oahu headquarters in late November to discuss the declining trend.

“The goal is to basically figure out what we know, but even more importantly, what we don’t know, where the gaps in knowledge currently lie, and hopefully settle on some research priorities that will help us solve those knowledge gaps,” Lammers said.

The researchers plan to publish a report summarizing their key findings.

For Lammers, the discernible shift began in 2016, when he noticed progressively lower decibel levels of whales chorusing each following year.

There also was a shift in when they sang. The decibel levels in 2014-2015 were normal during the peak of the season, which historically occurs between mid-February and mid-March, but that peak seems to have changed. Now, Lammers said, more whales are singing earlier in the season, and winding down their chorusing sooner. By April, they were pretty much gone.

Lammers said acoustics, which he measures using hydrophones (underwater microphones), is one way to monitor the humpback whales continuously in multiple locations for long periods of time.

Researchers believe the whales sing during mating season to court females, although it is unclear whether it is a display for other males or to attract females.

Cartwright said the males may have fewer reasons to sing, given that fewer female humpbacks may be in the islands.

She said she thinks the likely cause is a disruption of the whales’ food base in Alaskan waters due to three major ocean-warming occurrences in the past few years.

Without enough food, mother humpback whales would not have the reserves to make the journey from Alaska to Hawaii and back. The whales seek Hawaiian waters free from orcas, their predators, but do not feed here, she said.

Aerial surveys that are part of a study she is conducting offer early indications that humpback whale moms here are thinner than usual, she said. She noted fellow colleague Christine Gabriel’s observation of fewer as well as thinner whales at their traditional Glacier Bay feeding grounds in Alaska, and fewer calves, as well.

“We talk about these changes and trends, but at the same time we don’t want to give the impression that the whales have cleared out,” Lammers said. “There are still a lot of whales in this migration that come to Hawaii. We’re really looking more at the long-term trends.”

“It’s certainly very possible that these trends we’ve been observing will reverse themselves this coming season,” he said. “Right now it’s too early to say. It’s important that we keep an eye on it and understand it a little better.”

The impacts are being noticed by some boat tour operators.

Jason Thurber, owner and operator of Hawaii Oceanic Charter Boat Expedition in Kailua-Kona, said he had a very popular tour called “The Humpback.”

He stopped offering that tour two seasons ago because his company guarantees sightings, and there were so few that it was no longer feasible. He still offers daytime snorkeling and nighttime manta ray tours, among other options. Having grown up in the area, he said he had always seen humpback whales from offshore.

“Something changed, and it changed fast,” he said. “There are so many things going on with the ocean and the climate and various things that you see being in the industry that we’re in. It’s definitely worrying because it seems like so many (things) affect them, from the plankton to the sonar to climate change and food sources.”

Humpback whales were placed on the endangered species list in 1973, and the population that migrates to Hawaii was removed in 2016 after their numbers improved. Researchers say it is too soon to make any conclusions, much less seek a relisting.

Cartwright, however, said it may be a good time for humans to give the humpback whales a little more space.

“Maybe this is the year we have to be a little more protective of them,” she said. “Maybe we increase our appreciation of whales, especially moms and calves coming to Maui waters by ensuring they have quiet places to rest.”