Two years ago, during Rim of the Pacific naval exercises, a pod of about 200 deep-water melon-headed whales appeared in the shallow waters of Hanalei Bay, Kaua'i.

The animals were herded out of the bay by beachgoers in canoes and kayaks, but marine mammal experts on the scene said the whales were behaving strangely. A small calf was later found dead on shore.

Public attention quickly turned to the large-scale Rim of the Pacific, or Rimpac, exercises and associated sonar use, adding to the growing worldwide body of concern about sonar and its impact on marine mammals.

Rimpac is coming back, and so is the ongoing sonar controversy that's about as broad and deep as the ocean blue.

In late June and through most of July, the U.S. Navy — with an aircraft carrier strike group as its centerpiece — is expected to conduct the biennial war games off Hawai'i with eight other nations.

This year, debate over Rimpac sonar, already under way behind the scenes here, is being affected by a Navy plan an ocean away in the Atlantic and a challenge in federal court.

InsideEPA.com reported last month that federal marine regulators are pressuring the Navy to re-examine its threshold for sonar harassment of marine mammals.

The National Oceanic and Atmospheric Administration is on record saying the Navy's proposed harassment threshold of 190 decibels for a planned East Coast underwater sonar training range is too high. But Rimpac, which will be completed before the range is in operation, will be the first test of the new threshold, and whatever decisions are made on the matter could set precedent for other Navy sonar use during training.

InsideEPA.com said legal pressure from environmental groups may be forcing the Navy for the first time in Rimpac's 35-year history to seek authorization from the National Marine Fisheries Service, which is part of NOAA, for "takes," or the incidental harassment of marine animals.

"That's been the indication to us, that they (the Navy) are going to be coming in with an application," said Donna Wieting, deputy director of the Office of Protected Resources for NOAA.

Asked whether the potential for the National Marine Fisheries to withhold permits for sonar harassment would alter plans for Rimpac, the Navy only would say that "environmental planning for Rimpac is currently ongoing."

"(The) Navy and (National Marine Fisheries) are pursuing an open, professional dialogue on the potential effects of the exercise on marine mammals, an issue that is both the subject of developing science and legislative clarification," Lt. Cmdr. Christy Hagen, a U.S. Pacific Fleet spokeswoman, said in a statement.

The Navy is facing lawsuits over that developing science, which has already affected its use of sonar — a key issue for Hawai'i with extensive anti-submarine warfare training conducted off the state.

MANDATORY TRAINING

With the proliferation of diesel submarines in the Pacific, Adm. Gary Roughead, who commands U.S. Pacific Fleet from Pearl Harbor, has made anti-submarine warfare the fleet's top maritime war-fighting priority.

Roughead mandated that all aircraft carriers and expeditionary strike groups deploying from the West Coast conduct several days of anti-submarine warfare, or ASW, training, near Hawai'i. Since January, three strike groups, each with a half dozen or more ships and submarines, have rotated through for the training, which relies heavily on sonar.

In 2003, after the Natural Resources Defense Council sued, the Navy agreed to limit training and use of low-frequency sonar to regions around the eastern seaboard of Asia, and not to use the system, known as SURTASS LFA, in waters off Hawai'i and other locations.

In October, the Defense Council, a national nonprofit advocacy group for environmental issues, again took the Navy to federal court, saying "there is no dispute that the Navy's use of mid-frequency active sonar can kill, injure and disturb many species, including marine mammals."

Among the Defense Council's complaints in its most recent lawsuit is that the Navy regularly fails to comply with federal environmental law in connection with mid-frequency sonar, including failure to obtain "small take permits" or "incidental harassment authorizations" from National Marine Fisheries, as required by the Marine Mammal Protection Act.

The Defense Council described active military sonar as acting like a floodlight, emitting sound waves, or pings, that sweep across tens or even hundreds of miles of ocean, and revealing objects in that path from the bounceback. Passive sonar listens for sound.

The environmental group said the Navy's low-frequency system can generate 215 decibels — sound as intense as a twin-engine jetfighter at takeoff.

Some mid-frequency systems produce 235 decibels, as loud as a Saturn V rocket at launch, it said.

Evidence of sonar-related harm first began to surface in March 2000, when 17 whales of four different species stranded themselves on beaches in the Bahamas after a United States battle group used active sonar, the Defense Council said.

A study published in 2003 in the journal Nature found that high-powered Navy sonar may give whales and other marine mammals a form of the bends, or decompression sickness, with bubbles forming in organ tissue.

But the Navy and some marine scientists maintain that the sonar debate is far from clear-cut.

"Sure, there's growing concern (with active sonar). Absolutely," said Paul Nachtigall, director of the Marine Mammal Research Program at the University of Hawai'i. There have been beaked whale strandings in places like the Bahamas and Canary Islands associated with mid-frequency sonar, Nachtigall said.

But there are 85 species of whales and dolphins. "And of the 85 species of whales and dolphins worldwide, we have tested the hearing of 12 species," he said.

Most of the smaller species, echo-locators like dolphins and small whales, "are high-frequency specialists," Nachtigall said. Dolphins probably hear best at frequencies between 20 kilohertz and 100 kilohertz.

Mid-frequency active sonar systems are conventionally defined as those that emit sound at frequencies of up to 10 kilohertz, which is a measure of the frequency of the oscillation of the sound wave, or its pitch.

In lower frequencies, where a lot of mid-frequency Navy sonars operate, "they don't hear as well. They still hear, but not as well as, say, the larger whales, presumably," he said. "But nobody has ever tested the hearing of a larger whale."

THRESHOLD LEVELS

While scientists pursue more precise definitions of "harassment," the Navy will likely remain stuck in the middle of the sonar debate.

"We will use the best available science, and we will always base our decisions on good science and good data," said Lt. William Marks, a Navy spokesman at the Pentagon.

In 2004, Congress defined "Level B" harassment of marine animals, which the Navy now has started to use for exercises like Rimpac, as "any act that disturbs or is likely to disturb" marine mammals in the wild.

For both the East Coast sonar training range proposal and Rimpac, the Navy set the threshold for "Level B" harassment — meaning behavioral disturbance without physiological effects — at 190 decibels.

The Navy tested four dolphins and two white whales in captivity to arrive at the figure. NOAA countered that was not representative of animal reaction in the wild and said the Navy should develop "a more conservative" acoustic threshold than 190 decibels.

Wieting of NOAA said the same conclusion wouldn't necessarily apply to Rimpac exercises because the proposal for the upcoming war games is "in a different part of the world with different marine mammals."

"I really can't comment until we see what the (Navy) application looks like," she said.

In an environmental assessment, the Navy said it expects 532 hours of sonar operation in 44 anti-submarine warfare exercises over 21 days during Rimpac.

No "Level A" harassment of marine animals, which equates to injury, is predicted by the Navy, but it does project 289 "Level B" harassment "exposures."

Humpback whales should not be present during the exercise, the Navy said. However, plans include having at least two lookouts on each ship, and active sonar can be shut down when marine mammals are encountered.

The Marine Mammal Research Program's Nachtigall, who also is involved with the Hawaiian Islands Stranding Response Group, said while some people blame the 2004 Kaua'i Hanalei Bay gathering on Rimpac, others don't.

"There are no smoking guns," he said.

The Navy's proposed 190 decibel harassment threshold level is "pretty loud," Nachtigall said, but a humpback whale also sings at about that level.

"What we have to say is we know that mid-frequency sonars have been associated with beaked-whale strandings in a variety of places in the last five years," Nachtigall said. "So, yes, we have to be concerned (about sonar). But we also know that we haven't had any beaked-whale strandings associated with sonars in the Hawaiian Islands."

So, while he thinks there are definitely areas of concern with strandings elsewhere around the world, the waters around Hawai'i do not appear to present a "high level of concern" when it comes to exercises such as Rimpac.

"There have been lots of (anti-submarine warfare) exercises going on in Hawai'i for a long time," Nachtigall said, "and we've never had that same circumstance here in Hawai'i."

Reach William Cole at wcole@honoluluadvertiser.com.