NOAA Technical Memorandum NMFS-NWR-34 (2004)


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 in-depth investigation on the causes of harbor porpoise strandings. Fifteen stranded harbor porpoises were reported during the period of 2 May 2003 to 2 June 2003, an abnormally high number when compared to the average stranding rate of 6 per year recorded over the previous decade. Eleven of the stranded harbor porpoises were collected for this investigation.
NMFS assembled a multidisciplinary team to conduct extensive classical forensic necropsy examinations on the 11 specimens, followed by laboratory diagnostic and histological analyses and complemented by high resolution computerized tomography (CT) scans. Samples were taken for a variety of analyses including disease screening, parasitology, chemical contaminant and lipid analyses, aging studies, prey identification and domoic acid analysis. The gross and microscopic findings from the necropsy examinations, laboratory results, and the analysis of the CT image data for each specimen are provided. Information on the discovery and collection of the stranded porpoises, and a comparison of this with porpoise strandings over the previous ten years is also included in this report.
Over 70 percent of the specimens were in moderate to advanced states of decomposition which made interpretation of the cause of death difficult. The cause of death was determined for five of the 11 porpoises examined by the multidisciplinary team. Of these five animals, two were found to have suffered blunt force trauma, while illness (peritonitis, salmonellosis, pneumonia) was implicated in the remaining three cases. No cause of death could be determined for the remaining six animals. The examinations did not reveal definitive signs of acoustic trauma in any of the porpoises examined. The multidisciplinary team noted that lesions consistent with acoustic trauma can be difficult to interpret or obscured, especially in animals in advanced postmortem decomposition. Because 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 not be ruled out.