HONOLULU — Humpback whale sightings are becoming less common in the Hawaiian Islands.

Researchers have found that along with fewer sightings, fewer male Hawaiian humpback whales have been recorded singing, the Honolulu Star-Advertiser reported Saturday.

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Researchers have also observed a decline in the number of mother-calf pairs for the past three seasons.

Despite the downward trends, scientists are not too concerned just yet.

There are still plenty of whales out there, said Ed Lyman with the Hawaiian Islands Humpback Whale National Marine Sanctuary.

“They’re not all gone,” he said.

Researchers first noticed the decline in December 2015, Lyman said. They were alerted by whale-watching tour operators who noticed the whales were migrating to Hawaii from Alaska later than usual.

Lyman reached out to tour operators on other islands who reported similar findings.

Rachel Cartwright, a whale researcher on Maui with the Keiki Kohola Project, had observed a dramatic drop in mother-calf sighting over the past three years. She estimates that mother-calf sightings have dropped by about 80 percent over the past five years and the trend has continued into this migration season, she said.

At the same time, National Oceanic and Atmospheric Administration research coordinator Marc Lammers found that he wasn’t hearing whale singing as often as he used to.

“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.”

Researchers have yet to figure out what is causing these declines.

One idea is that the humpback whales have simply decided to go farther offshore or elsewhere than the usual spots where they are sighted. Another theory is that the number of whales has reached “carrying capacity,” meaning there are only so many the environment can support and it has reached that limit,” Lyman said.

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“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.

Whale researchers plan to meet in Hawaii at the National Oceanic and Atmospheric Administration Oahu headquarters in November to compare and correlate their findings.