In a study replicating Navy sonar, the Hawai'i Institute of Marine Biology found that a captive bottlenose dolphin had to remain relatively close to a high-intensity sonar source for a prolonged period to experience even temporary hearing loss — a finding the Navy seized upon to say it "may have vastly overestimated impacts of mid-frequency active sonar on marine mammals."

"We are still reviewing the report, but the research indicates that higher decibel levels are required to cause a shift in hearing than in the Navy's more conservative models," said Mark Matsunaga, a spokesman for U.S. Pacific Fleet.

But the Natural Resources Defense Council, which has sued the Navy repeatedly over alleged harm by Navy sonar on marine mammals, said the Hawai'i Institute findings pertain to one bottlenose dolphin in captivity and don't speak to sonar effects on other marine mammal species.

The findings by the Coconut Island-based institute, a research arm of the University of Hawai'i, were published online yesterday in the journal Biology Letters.

"At bottom, this paper does not address the central concerns that the scientific and environmental communities have had about mid-frequency sonar," said Michael Jasny, senior policy analyst with the NRDC. "Those concerns involve types of impacts other than hearing loss — such as behavioral impacts and stress effects, and such impacts have been found to occur at vastly lower thresholds of sonar exposure."

Jasny said the Hawai'i Institute study adds a "data point" to the ongoing study of sonar impact on marine mammals.

The biggest concern, Jasny added, is over injuries to deep-diving whales from sonar that can cause the animals to stop vocalizing, abandon habitat and, in worst-case scenarios, suffer hemorrhages or emboli similar to the "bends" sometimes experienced by divers.

The Hawai'i Institute report, by T. Aran Mooney, Paul Nachtigall and Stephanie Vlachos, said there is increasing concern that human-produced ocean noise is adversely affecting marine mammals, with several mass strandings possibly caused by Navy mid-frequency sonar.

"However, it has yet to be empirically demonstrated how sonar could induce these strandings or cause physiological effects," the report states.

Experiments were conducted in open-water pens at Coconut Island from August to October 2007 using a captive-born and trained Atlantic bottlenose dolphin accustomed to noise exposure experiments, according to the study.

The report said the "fatiguing noise" was an actual mid-frequency Navy sonar signal recorded in Puget Sound, Wash., in the summer of 2005 before a marine mammal stranding event.

Nachtigall said non-invasive electrodes were placed on the dolphin to detect brain wave patterns in response to the sound. According to the Hawai'i Institute, to get a temporary hearing loss, a dolphin would have to remain for five minutes within about 120 feet of a sonar source to receive the threshold 214 decibels of sound.

Navy sonar typically operates at 235 decibels at its source.

Nachtigall, the director of the Marine Mammal Research Program, said the study is groundbreaking in that it is the first to examine the direct effect of sonar on dolphins. It also shows Navy sonar is no different than other ocean sounds, and is similar to oil drilling noise or the underwater sounds from a tanker, which can be 220 decibels in a lower frequency, he said.

"So there are a lot of loud sounds in the ocean — not just Navy sonar," Nachtigall said.

The study was funded by Congress through the Office of Naval Research.