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Cause of Porpoise Deaths Remains Uncertain, Panel Says

By Kat Kerlin on February 9, 2004 in

A preliminary scientific report released today on the deaths last May of 11 harbor porpoises in the Puget Sound region finds no definitive evidence that the deaths were linked to a U.S. Navy mid-frequency sonar exercise.

UC Davis wildlife veterinarian Joe Gaydos, based at a UC Davis research lab in Puget Sound, is one author of the report.

Gaydos said the investigation was difficult because some of the carcasses were rotting when they were collected. "Additionally, we need to know more about the effects of mid-range sonar on marine mammals' hearing systems." The panel did find signs of illness or injury in some of the porpoises' ears, he said. "But we aren't able to distinguish damage caused by sonar from damage caused by other agents, such as decomposition."

Gaydos and 13 other scientists were asked to investigate after 14 porpoises stranded and died just before and after a U.S. Navy guided missile destroyer, the USS Shoup, conducted sonar exercises in Haro Strait.

The scientists included veterinarians, pathologists, biologists and an expert in porpoise ear anatomy. Joe Gaydos is staff veterinarian for the SeaDoc Society, a marine ecosystem health program of the UC Davis Wildlife Health Center.

For the past several years, with Richard Osborne, research director of the Whale Museum in Friday Harbor on San Juan Island, Gaydos has investigated the death of every marine mammal found dead on the shores of San Juan County. The area has more than a dozen different species of marine mammals, including orcas, harbor porpoises and harbor seals.

The porpoise carcasses in this case underwent a variety of studies, including high-resolution computer tomography (CT scanning) and tests for chemical toxins, diseases and parasites. The scientific team then analyzed that data to establish a possible cause of death for each animal.

This task was difficult, the report says, because of the advanced degree of decomposition of most of the carcasses.

The team declared a cause of death for only five of the porpoises. Two had died of "blunt-force trauma," which could include ship strikes or natural injury from coming ashore or being struck by another animal. The other three likely died of peritonitis, a bacterial infection (salmonellosis) and pneumonia.

The team said it could not find evidence of acoustic trauma in any of the animals but cautioned that lesions "consistent with acoustic trauma" can be difficult to interpret in decomposed animals. It said the possibility of acoustic trauma exacerbating or compounding the conditions that it found "cannot be excluded" in any of the animals.

The 60-page preliminary report was released today for scientific review. A final report is expected in April.

NOAA Fisheries' news release and copies of the preliminary report are available at

The Wildlife Health Center is a research program of the UC Davis School of Veterinary Medicine. The wildlife center's SeaDoc Society, with offices and laboratories on Orcas Island, Wash., focuses on the marine wildlife and ecosystem of the Pacific Northwest.

The SeaDoc Society conducts its own scientific studies and distributes grants for others; fosters stakeholder discussion; and conducts public education on marine issues. For more information see:

NOAA Fisheries is an agency of the U.S. Commerce Department's National Oceanic and Atmospheric Administration.

Media contact(s)

Kat Kerlin, Research news (emphasis on environmental sciences), 530-752-7704, 530-750-9195,

Joe Gaydos, Wildlife Health Center, 360-914-1083,