Hawaii's pygmy killer whales do not roam

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 behaviour, including fishing and Navy sonar, said a paper published 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, Washington-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 Oahu, Lanai and Niihau.

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

The study showed that 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 behaviours 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 per cent, of 889 whale and dolphin sightings the researchers made off Hawaii between 2000 and 2007.

- AP