Scientists with a 21-year photographic record of seldom-seen ocean mammals say that two species of beaked whales appear to be semi-permanent residents of Hawaiian waters.

The two are the Cuvier's beaked whale and the Blainville's beaked whale — part of the least-known and most numerous family of cetaceans.

"These beaked whale populations around the Hawaiian Islands are not just part of an open ocean population that's passing by," said Robin Baird, a biologist with Cascadia Research Collective, based in Olympia, Wash.

Scientists are now spending more time learning about them, in part because beaked whales have been seen as a family that may be particularly susceptible to Navy high-volume sonar. The National Marine Fisheries Service in recent years has funded efforts to get a handle on the Hawaiian populations of these animals.

The finding of a local population of the whales could have effects on military exercises in the Islands. Just last week, several conservation groups sued the Navy and the National Marine Fisheries Service to halt Navy anti-submarine exercises around Hawai'i using high-intensity, mid-frequency sonar.

Beaked whales are toothed whales, along with oceanic dolphins, river dolphins, sperm whales and others. There are 21 known species of beaked whales, and there may be more that haven't yet been identified.

Very little is known about them, for several good reasons, Baird said.

They tend to stay in deep water. They tend to be wary of boats. And they dive longer and deeper than any air-breathing mammal. So it's rare to come across them, Baird said.

"They are the least-known family of large mammals worldwide," he said.

"We see a beaked whale about every five days on the water," he said. They are so hard to see that researchers go out only when the sea is calm, and they cruise with multiple observers scanning the ocean in all directions.

Baird, and Big Island-based Wild Whale Research Foundation researchers Dan McSweeney and Sabre Mahaffy, have collected fewer than 100 sightings of the animals in more than two decades of collecting. But they got enough repeat sightings to conclude that some of these whales appear to hang around Hawai'i year-round, and for years at a time. Some have been spotted for as long as 15 years.

"I began studying beaked whales off Kona in 1982 along with the 'toothed' whale species found around the Hawaiian Islands," McSweeney said in an e-mail. Ultimately, that research has led to sightings around all the main Hawaiian Islands. The researchers have managed to place depth recorders on some whales, and tracked dives.

"By deploying suction-cup radio tags onto the backs of these animals, we are beginning to get a picture of what they are doing below the surface of the water. Both the Cuvier's and Blainville's beaked whale dive for incredibly long periods (up to more than an hour) and go very deep both during the day and at night," McSweeney said. In other studies, the animals have been found to dive more than a mile deep, and the longest recorded dives are nearly an hour and a half long.

Around Hawai'i, the Cuvier's whales seem to be found in the deeper waters more than a mile in depth, while Blainville's whales are found closer to shore on the deep slopes of the Islands.

In a paper published yesterday in the journal Marine Mammal Science, the researchers said these beaked whales may choose to stay around Hawai'i either for food or for protection.

Protection may be less of a factor, since animals of both species are freckled with white scars from the bites of cookie-cutter sharks. And at least one also was photographed with what appeared to be a large curved scar, perhaps the result of a bite by a large shark.

It is more likely, the paper says, that the whales frequent Hawai'i because of the relative abundance and predictability of food. "Unfortunately, virtually nothing is known regarding diet of either species within our study area," the paper says.

The scar patterns make it comparatively easy to identify individual whales, McSweeney said.

Researchers have taken skin samples from the animals, in hopes that DNA studies will show whether the Hawaiian populations are genetically distinct from open-ocean beaked whales. This kind of information "will in time allow us to determine if these whales are reproductively isolated from populations in the eastern tropical Pacific. Where the boundaries to these populations are is unknown. More research is needed in offshore areas around the islands," McSweeney said.

The animals have not been implicated in strandings or other responses to naval sonar use in the Hawaiian Islands, but they have in other areas.

"It's been documented in enough different areas at enough different times that there is no doubt that they're reacting to sonar," Baird said. In some cases, the whales have been found injured or dead, with internal injuries called gas bubble lesions. It could be the result of swimming toward the surface so quickly that they get the equivalent of the "bends," which human divers can get from surfacing too quickly from great depth.

McSweeney said that link between these whales and military exercises is leading to more research.

"I think that their tendency to strand subsequent to naval sonar exercises has brought the species into public's awareness," he said.

Reach Jan TenBruggencate at jant@honoluluadvertiser.com.