A marked increase in the frequency of harbor porpoises (Phocoena phocoena) stranded in North Carolina in 2005 was declared as an Unusual Mortality Event (UME). Strandings occurred in January through May when harbor porpoises are seasonally present. Increased stranding rates were measured relative to a threshold to determine that the UME was occurring. The threshold analysis also revealed elevated strandings during 1999, an undeclared UME year. Recovered carcasses during 1999 and 2005 accounted for 39% of 261 strandings during 1997–2009. During 2005, of 43 strandings, primary or secondary causes of mortality included fishery interactions, emaciation, and interspecific aggression. Apart from small but significant differences in timing and condition of strandings, composition of strandings during UME and non-UME years was similar, with most being young-of-the-year and occurring during March and April, north of Cape Hatteras. Porpoises had high levels of parasitic infestation typical for this species. However, no indication of infectious disease and no cause of the 2005 event were found from gross and histologic findings. Response to UMEs is challenging, particularly along the expanses of North Carolina beaches, requiring additional effort to obtain carcasses in sufficiently fresh condition to determine the cause of these events. 1. Introduction Of the six species in the odontocete family Phocoenidae, only one is found in the Atlantic Ocean, the harbor porpoise (Phocoena phocoena). Members of this family, including the harbor porpoise, generally occur at high latitude, while harbor porpoises are found only in the northern hemisphere [1]. Although it is primarily a cold-water temperate and boreal species, documented takes in gillnet fisheries in the western mid-Atlantic region in winter [2] support the mid-Atlantic coast of the United States of America (USA) being part of the normal winter range for the species. Further, harbor porpoises are one of the most commonly stranded species along the extensive beaches of North Carolina (NC) [3]. Historical data indicate that along the US Atlantic coast only Massachusetts has more documented strandings than NC [4]. Worldwide, harbor porpoise strandings have been associated with infectious and noninfectious diseases. Infectious diseases include morbillivirus (e.g., [5, 6]), herpesvirus [5], brucellosis [7, 8], bartonellosis [9, 10], and verminous pneumonia [6, 11, 12]. Papillomavirus has been reported to result in self-limiting cutaneous lesions rather than mortalities [13–15]. Noninfectious diseases found in harbor
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