A new study of pygmy killer whales - one of the least understood marine mammal species - shows that those living off Hawaii tend to stay close to the islands and do not swim out to the open ocean.

There are few of the whales, probably less than 200 individuals, in this distinct pygmy killer whale population off the islands.

The population's limited number make it more vulnerable than other whale populations 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 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 Oahu, Lanai and Niihau.

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

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

The analysis also showed 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 paper 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 for scientists to study.

Baird said Hawaii's group was the only known case of a pygmy killer whale population that remained isolated in one area and did not venture out to the open ocean.

On average the researchers spotted pygmy killer whales about 3.7 miles from Hawaii shores. The furthest offshore sighting was at 9.3 miles.