Humpback whales, the winter stars of Hawaii’s marine environment, have been arriving and leaving island waters earlier than normal in recent years, according to a trend documented by scientists.

Other recent research suggests there actually may be two subpopulations of humpbacks coming to the island chain, and it appears that the marine mammals take what some might described as an afternoon siesta each day.

The findings were detailed last week by scientists with the Hawaiian Islands Humpback Whale National Marine Sanctuary as they gear up for another winter of research.

It has been estimated that more than 10,000 humpback whales travel each winter from Alaska to the warm and relatively shallow waters of Hawaii. The islands are where they breed and give birth primarily between January and March, although some whales are known to come as early as October and depart as late as June.

The sanctuary was established by Congress 30 years ago to provide a level of protection for an endangered species in an area encompassing 1,370 square miles of coastal waters in the main Hawaiian Islands, extending from the shoreline to the 600-foot depth in waters off Kauai, Oahu, Molokai, Lanai, Maui and Hawaii island.

Jointly managed by the National Oceanic and Atmospheric Administration and the state Department of Land and Natural Resources, the sanctuary not only brings educational awareness about the whales, but has funded millions of dollars in research over the years.

While the humpbacks are still protected by federal and state regulations, they are no longer an endangered species in Hawaii waters, having been delisted in 2016.

Concern remains about North Pacific population of the humpback whale, however, because whale migrations have exhibited much variability in recent years.

Marc Lammers, the sanctuary’s research ecologist, said whale migration to Hawaii dropped precipitously in 2016 due to a significant marine heat wave in the northeastern Pacific Ocean.

“This marine heat wave really caused havoc in the marine ecosystem,” Lammers told an online audience during a sanctuary webinar. “The whales presumably had to spend more time looking for food in different places and prioritized that over mating.”

The downward trend continued through 2018.

Lammers said the 2021 season was strong in terms of whale numbers, but the 2022 season appeared to drop off somewhat.

Scientists measure whale abundance through visual surveys and by measuring the sound the animals make.

The year before the marine heat wave, scientists recorded “chorusing” levels that have not been surpassed since, Lammers said. And the 2015 season lasted later than all the seasons since.

The peak levels of chorusing during the season — indicating peak abundance — has fluctuated in recent years, from early February to early March.

The 2020 season, he said, had a late peak and lasted longer than all other recent seasons. That was at a time when the COVID-19 pandemic swept across the islands.

“Whether that has some direct relationship, it’s a little bit unlikely, because I tend to believe that humpback whale migration patterns are not set by human activities — more by the food resources that are available,” Lammers said. “But it is an interesting coincidence.”

The 2018 and 2022 seasons peaked early and ended early — and it’s a trend overall of late.

The results of an acoustic study conducted across the entire archipelago shows that whales are present in both the main islands and the Northwest Hawaiian Islands from January to March.

“Maui had the highest levels of chorusing, reaffirming the fact that it is the epicenter of the humpback whale population here in the main Hawaiian Islands,” Lammers said.

In the study, a remote- controlled marine drone that uses waves and the sun to help propel it made ocean recordings up and down the island chain and detected whale song at every bank, shoal and seamount in 2020.

Surprising was strong chorusing in the northern part of the chain, with a region of relatively little whale sound wedged between the area of strong chorusing in the southern part of the chain, including the main islands.

“The structure of the whale population is more complex than previously thought,” Lammers said, adding that it might point to two subpopulations, one tied to the southeastern part of the archipelago and main Hawaiian Islands and one clustering toward the northwestern end.

But to really confirm that, he said, scientists will have to go up there and investigate.

“These data are certainly interesting and motivating us to go out and try to get more data from that area,” he said.

In another area of research, scientists have been studying behavior using suction cup tag monitors that they attach to individual whales. The tags record sound, video, depth levels and movements.

A recent study examined 86 hours of video from 25 different tagged whales and scored activity levels during different times of the day. A distinctive pattern emerged in the afternoon: decreases in activity and surfacing, and more resting.

“It looks like whales have a real tendency to rest between 1 and 4 p.m. This is something that anecdotally people have noticed, but there’s never been any quantitative evidence,” Lammers said.

“This is really new and exciting stuff for us — that we’re able to show these pretty clear patterns of resting behavior in the afternoon hours,” he said. “It’s kind of reminiscent of another species that we have here, the spinner dolphin, which is very active during nighttime hours but then spends its daytime hours more in a resting state.”

Asked for a prediction for peak whale arrival in Hawaii this upcoming season, Lammers said scientists haven’t reached the point where they are able to to accurately forecast what will happen in future seasons.

However, based on the trend from the previous eight years, he said, the probability of an early arrival and departure than historically “normal” is higher than a late arrival.

“Coincidentally, the first (whale) sightings have been rather early this year, including some mother-and-calf pairs, which are more common later in the season.”

As far as the number of whales that will come this year, that is more difficult to project, he said.

“2022 probably had somewhat fewer whales overall, but it’s unclear whether it represents the start of another downward trend, like we saw between 2016-2018, or just normal annual variation,” he said.

Lammers added that a marine heat wave doesn’t seem to be brewing in the northeastern Pacific Ocean.

“So right now, at least, there is no clear reason to expect a decline.”