There was a time when the ocean waters around the Northwest Hawaiian Islands were thought to be marginal as breeding grounds for humpback whales. A 1976 aerial and shipboard survey found few, if any, whales in the area.

However, a newly published National Oceanic and Atmospheric Administration acoustic study detected humpbacks up and down the remote island chain in what appears to be significant numbers.

The study, published in January in Frontiers in Marine Science, deployed a remote-controlled marine drone in the winter of 2020 to record whale songs on a 2,600-mile journey from Hawaii island to some of the farthest reaches of the island chain in Papahanaumokuakea Marine National Monument.

The scientists also placed moored recorders throughout the islands for the study that was part of NOAA and the Navy’s Sanctuary Soundscape Monitoring Project, also known as SanctSound.

“Based on recordings of whale song, nearly the entire Hawaiian archipelago is visited by humpback whales during the winter and early spring months,” said lead author Marc Lammers of the Hawaiian Islands Humpback Whale National Marine Sanctuary.

Not only that, but the study suggests the archipelago may be home to two separate populations of humpbacks: one that includes the whales that winter in the main islands and in the southern part of the northwest islands, and another one farther up the chain.

In the monument, whale songs were heard most at Middle Bank, just beyond Kauai, and gradually decreased farther to the northwest, reaching a minimum level at Gardner Pinnacles. Song occurrence increased again at Raita Bank and remained high between there and the Northampton Seamounts.

Among the locations monitored with moored recorders, the highest and most sustained seasonal chorusing levels were measured off Maui, followed by French Frigate Shoals, Hawaii island, Middle Bank, Oahu, Kauai, Gardner Pinnacles and Pearl and Hermes Reef, respectively.

THE MARINE drone, known as Wave Glider, was operated by Beth Goodwin of the Big Island’s Jupiter Research Foundation. The drone returned from its two-month-plus journey with 92,408 one-minute recordings. The researchers used artificial-intelligence computer software to help pinpoint where the songs were recorded.

There are 14 distinct humpback whale population segments worldwide, including four in the North Pacific. The group of whales that breeds in Hawaii waters is known as the Hawaii distinct population segment.

Another distinct group of whales, the West Pacific segment, feeds in the summer in the Bering Sea and in the Aleutians off Alaska. But scientists have not been able to track all the areas where that group breeds in the winter.

Lammers said he suspects the whales that were detected in the far reaches of Papahanaumokuakea Marine National Monument are part of the West Pacific population.

“That’s the mystery at this point,” he said. “There may be a bigger story to be told.”

Could these be main-­island whales just moving up the chain?

LAMMERS SAID he doesn’t think so. The results of satellite tagging by Oregon State University indicate that almost all of the humpbacks in the main Hawaiian Islands come and go from the main islands.

If these mystery whales are actually determined to be part of the endangered group, there may be management and conservation implications for the monument, he said. While the Hawaii humpback whale population was removed from the U.S. endangered species list in 2016, the West Pacific segment continues to be labeled endangered.

NOAA is now considering designating marine portions of Papahanaumo­kuakea Marine National Monument as a national marine sanctuary under the National Marine Sanctuaries Act, which would add a layer of protection to the monument’s waters.

Lammers said more research is needed to investigate the whale mystery, and he can’t wait to get out there.

“It’s got many of us excited,” he said.

But it could take a year or two because it will require fundraising and the assembly of a team of experts to visit the remote area in the winter to conduct fluke photo identification and biopsies and collect lots of other information.