Researchers are tagging and tracking whales during international naval exercises off Hawai'i this month in a first-of-its-kind effort to monitor how sonar affects the animals.

Scientists have tagged whales before to study how sonar affects them, but this is the first time researchers have done so during a major U.S. Navy exercise.

The studies will last through the biennial "Rim of the Pacific" exercises involving navies from 10 countries including Japan, Australia and Chile. The drills are due to last through July 31.

The effect sonar has on whales has come under scrutiny because whales have recently stranded, and in some cases beached themselves, near naval exercises. But in many cases scientists don't know enough about whales — particularly how they react to underwater sound — to definitively determine what's causing the strandings.

One theory, not confirmed, is that sonar noise startles the whales, prompting them to surface unusually rapidly. This may give them injuries similar to the bends in human divers.

Researchers from Woods Hole Oceanographic Institution, the National Oceanic and Atmospheric Administration, and other organizations and universities aim to fill some of those knowledge gaps with this month's studies.

"The whole question of how do you look at impacts of sonar on animals is obviously something a lot of people have been interested in. The problem is it's a really, really difficult question to answer," said Robin Baird, a marine biologist with the Olympia, Wash.-based Cascadia Research Collective.

One reason is that no one knows at what distance the mammals react to underwater sound, said Baird, who is leading one of the research teams.

It's also unclear exactly what sound levels whales try to avoid, he said.

Baird's team aims to attach satellite tags to 30 to 35 whales in Hawaii waters. It will target two species of beaked whales, Cuvier's and Blainville's, both of which have shown some particular sensitivity to sonar in the past. False killer whales and short-finned pilot whales are also on the list.

The group has already successfully tagged five melon headed whales from three separate whale pods off Kauai and the Big Island. It has also tagged one pilot whale.

The satellite tags feed researchers the locations of the whales, including the depths to which they dive.

Acoustic tags, which another research team will use, will provide readings of ambient sound, including sonar. They'll show whether a tagged whale tries to move away when it hears sonar, and what levels of sound the whale responds, or doesn't respond, to.

Baird said researchers won't be able to assess the data they gathered until the exercise is over and they're able to compare the Navy's sonar use records with whale movements.

The Navy has adopted a set of 29 measures to protect whales when its sailors are practicing with sonar.

But environmentalists charge those measures fall short. The 9th U.S. Circuit Court of Appeals in San Francisco agreed, ordering the Navy earlier this year to adopt more stringent protective measures.

The U.S. Supreme Court agreed this month to consider the Navy's appeal of the ruling.

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On the Net:

Cascadia Research Collective: www.cascadiaresearch.org/

Rim of the Pacific Exercise 2008: www.c3f.navy.mil/RIMPAC—2008.html