Multidisciplinary Investigation of Stranded Harbor Porpoises (Phocoena phocoena) in Washington State with an Assessment of Acoustic Trauma as a Contributory Factor
Stephanie A. Norman1; Brent Norberg1; Lynne Barre1; Stephen Raverty2; Joseph K. Gaydos3; Darlene Ketten4; Michelle Fleetwood5; William A. McLellan6; Tara Cox7; Brad Hanson8; Steve Jeffries9
1NOAA/National Marine Fisheries Service, Protected Resources Division, Seattle, WA, USA; 2Ministry of Agriculture Food and Fisheries, Abbotsford, BC, Canada; 3SeaDoc Society, UC Davis Wildlife Health Center, Eastsound, WA, USA; 4Biology Department, Woods Hole Oceanographic Institution, Woods Hole, MA, USA; 5Armed Forces Institute of Pathology, Washington, DC, USA; 6University of North Carolina, Wilmington, Wilmington, NC, USA; 7Marine Mammal Commission, Bethesda, MD, USA; 8NOAA/National Marine Fisheries Service, Northwest Fisheries Science Center, Seattle, WA, USA; 9Washington Department of Fish and Wildlife, Marine Mammal Program, Tacoma, WA, USA
Observations of altered behavior of marine mammals in the area of mid-range sonar use by the naval vessel USS Shoup in the eastern Strait of Juan de Fuca and Haro Strait on 5 May 2003 prompted the National Marine Fisheries Service (NMFS) to conduct an extensive investigation into the causes of coincidental harbor porpoise strandings. Fifteen harbor porpoises were reported stranded from 2 May-2 June 2004, which represented an abnormally high number when compared to the average stranding rate of six per year recorded over the past decade. Eleven of the stranded harbor porpoises were collected for this investigation.
NMFS assembled a multidisciplinary team to conduct extensive necropsy examinations on the specimens, complemented by high-resolution computerized tomography (CT) scans, followed by laboratory diagnostic and histological analyses. Samples were collected for a variety of analyses including disease screening, parasitology, chemical contaminant and lipid analyses, aging studies, prey identification and domoic acid analysis.
Over 70 percent of the specimens were in moderate to advanced states of decomposition, which made interpretation of the cause of death difficult to impossible. Nevertheless, for five of the eleven porpoises examined, a cause of death was apparent despite relatively poor preservation. Of these five animals, two were found to have clear evidence of blunt force trauma which may have been due to ship strike or some other collision. Evidence of serious illness (peritonitis, salmonellosis, pneumonia) was found in the remaining three cases. No clear cause of death could be determined for the remaining six animals. In none of the porpoises was there any differential diagnostic evidence of acoustic trauma. Lesions consistent with acoustic trauma can be difficult to interpret or obscured in animals in advanced postmortem decomposition. Since many of the carcasses investigated were in moderate to poor condition, the possibility of acoustic trauma from exposure to mid-range sonar as a contributory factor in the mortality of any of the porpoises could neither be confirmed nor excluded.