A new study of pygmy killer whales — one of the least-understood marine mammal species — finds that those living off Hawai'i tend to stay close to the islands rather than swim out to the open ocean.

There are very few of the whales, probably fewer than 200, in Hawai'i waters.

The limited number makes this pygmy killer whale population more vulnerable than others to potentially harmful human behavior, including fishing and Navy sonar, said the paper published Tuesday in the journal Marine Mammal Science.

"It's just much more likely that human activities could impact the population, hurt the population," said Robin Baird, a marine biologist with the Olympia, Wash.-based Cascadia Research Collective and one of the study's authors.

The study was based on an ongoing photo identification project launched in the mid-1980s by Daniel McSweeney of the Wild Whale Research Foundation in Holualoa on the Big Island.

The study's authors examined 3,431 photos of pygmy killer whales taken over 22 years.

Most of the whales were spotted off the Big Island, though a few were found off O'ahu, Lana'i and Ni'ihau.

The authors used the photos to distinguish the whales by their body scars, dorsal fin shapes and other distinctive characteristics.

The study found that researchers repeatedly came across the same whales, including one individual that was spotted over a 21-year period.

The analysis also concluded that these pygmy killer whales appear to be social animals, with many staying close to other individuals for at least 15 years.

Their stable, long-term relationships resemble the social behaviors of killer whales and pilot whales, the researchers said.

Pygmy killer whales are found in tropical and subtropical waters around the world. Yet they are among the least-understood toothed whales, in part because they generally live in the open ocean and so are harder to study.

Baird said Hawai'i's group is the only known case of a pygmy killer whale population that remained isolated in one area and didn't venture out to the open ocean.

On average, the researchers spotted pygmy killer whales about 3.7 miles from Hawai'i shores.

The farthest offshore sighting was at 9.3 miles.

Baird said Hawai'i's pygmy killer whales, like Cuvier's beaked whales and about 10 other whale and dolphin species living in Island waters, don't venture far because there isn't much food for them just outside Hawai'i waters.

The islands are their most reliable source of food, so they stay nearby.

Hawai'i's pygmy killer whales are so rare, however, that they accounted for only 11, or 1.2 percent, of 889 whale and dolphin sightings the researchers made off Hawai'i between 2000 and 2007.

In contrast, as many as 10,000 humpback whales — an endangered species — migrate to Hawai'i's waters each winter from Alaska to breed and calve.

Baird said the small number of pygmy killer whales made it difficult to monitor for effects of human activity.

"They're encountered so infrequently that any particular population of the species could be dramatically declining and we would never know it," Baird said.

"That's one of the problems with very rare species."

The study said there has been no documented case of a pygmy killer whale being hurt by sonar. But it also said there's low probability anyone would be able to document such harm, given that the whales are so rare and generally spend their time miles offshore.

Environmentalists argue that the Navy's mid-frequency active sonar can disrupt whale feeding patterns, and can sometimes kill whales by causing them to beach themselves.

The Navy acknowledges that its sonar, used to hunt enemy submarines, and to practice to hunt enemy submarines, may harm some marine mammals. But it says it takes steps to limit the impact on whales, including having ships power down their sonar when whales are nearby and posting lookouts on deck to spot marine mammals.

Fishing is the another potential risk to pygmy killer whales.

The study said there has been no report of a pygmy killer whale dying as a result of Hawai'i's long-line tuna and swordfish fishery. But the mouth of a pygmy killer whale that stranded on O'ahu in 2006 had hook and line marks, indicating that encounters with fishing lines may a problem.

Marine Mammal Science is published by the Society for Marine Mammalogy.