A humpback in the Pacific Ocean off the coast of Maui in 2005. The waters there are usually breeding grounds, but whale sightings have dropped. (Reed Saxon/AP)

Researchers are meeting this week to compare clues about a significant decline in the number of sightings of North Pacific humpback whales in their traditional breeding grounds off Hawaii.

Fewer sightings doesn’t necessarily mean the iconic giants are dying off, or that they’re not still migrating to the islands. But the disappearance of many whales from this historically predictable location is causing concern. Some researchers believe there’s a link between warmer ocean temperatures in Alaska and the effect that has on the whales’ food chain. The drop in sightings is estimated at 50 percent to 80 percent over the past four years. Most humpbacks were taken off the endangered species list in 2016 but are still protected by the U.S. government.

Although scientists say it’s too early to draw any conclusions, the decline has sparked enough interest that a group of whale experts are meeting in Hono­lulu, Hawaii, to compare data and attempt to better understand what’s happening and what to do about it.

The humpbacks traditionally migrate each autumn from Alaska to Hawaii, where they mate and give birth during the winter. In the spring, they head to Alaska, where they feed during the summer months.

It is estimated that half of all North Pacific humpbacks make the journey to Hawaii each year, putting the total number of whales making the 6,000-mile round trip each year at about 11,000.

Researchers use a variety of monitoring methods to count the whales, including visual observations while aboard ships that follow specific coordinates. Another method is monitoring whale songs from fixed underwater locations. There is also a less scientific approach, where residents onshore report their sightings each year.

Officials with the National Oceanic and Atmospheric Administration hope this week’s meetings will help them figure out what to do to help ensure the species’ success.

Marc Lammers, research coordinator for the agency’s Hawaiian Islands Humpback Whale National Marine Sanctuary, said research groups have collected various data sets that all seem to point toward decreased whale sightings.

“People started to report fewer sightings, and there was concern about what might be going on,” Lammers said in a telephone interview.

“We just know that we’re not seeing them in the same places that we’re expecting to see them,” he said.

Many theories are being considered, including the possibility that warmer ocean temperatures are reducing food supplies in the north, said Rachel Cartwright, lead researcher at the Keiki Kohola Project. If the female whales are not getting enough food, they would be unable to reproduce, she said.

Cartwright said she doesn’t believe the whales are in danger of becoming massively depleted again but thinks it’s important to understand the connection between environmental and climate changes in Alaska and the whales’ behavior.