HONOLULU (AP) -- 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 don't swim out to the open ocean. There are very 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'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 didn't 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.
Baird said Hawaii's pygmy killer whales, like Cuvier's beaked whales and almost 10 other whale and dolphin species living in island waters, don't venture far because there isn't much food for them just outside Hawaiian waters.
The islands are their most reliable source of food, so they stay nearby.
Hawaii'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 Hawaii between 2000 and 2007.
The pygmy killer whale's small numbers stand in contrast to the humpback whale. It is an endangered species yet as many as 10,000 individuals migrate to Hawaii's waters from Alaska to breed and calve each winter.
Baird said the small number of pygmy killer whales made it difficult to monitor for harmful 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 the whales are so rare and because they generally spend their time miles offshore.
Environmentalists argue the Navy's mid-frequency active sonar can disrupt whale feeding patterns, and in the most extreme cases can kill whales by causing them to beach themselves.
The Navy acknowledges its sonar, used to hunt enemy submarines, may harm some marine mammals. But it says it takes steps to protect whales, including having ships power down their sonar when whales are nearby and posting marine mammal lookouts on deck.
Fishing is the another potential human source of harm to pygmy killer whales.
The study said there has been no report of a pygmy killer whale dying as a result of Hawaii's long-line tuna and swordfish fishery. But the mouth of a pygmy killer whale that stranded on Oahu in 2006 had hook and line marks, indicating fishing lines affect the animals.
Marine Mammal Science is published by the Society for Marine Mammalogy.