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Cause of porpoise deaths remains uncertain

No definitive evidence linking the deaths of 11 harbor porpoises to a May 2003 U.S. Navy sonar exercise in Haro Straight has been found according to a preliminary scientific report. UC Davis wildlife veterinarian Joe Gaydos of Orcas Island was one of the authors of the report released Feb. 9, 2004. 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," he said. 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."

The porpoise carcasses 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.

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.

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.

Gaydos is staff veterinarian for U.C. Davis Wildlife Center's SeaDoc Society which has offices and laboratories on Orcas Island. SeaDoc Society focuses on the marine wildlife and ecosystem of the Pacific Northwest.

For the past several years Gaydos and Richard Osborne, research director of The Whale Museum in Friday Harbor, have 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 60-page preliminary report which is available online at the National Oceanic and Atmospheric Administration http://www.nwr.noaa.gov/mmammals/cetaceans/necropsypage.htm was released Feb. 9, 2004 for scientific review. A final report is expected in April, 2004.


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