Cetacean strandings have typically been classified into two types: 1) single strandings (including a mother/offspring pair); and 2) mass strandings. Singly stranded cetaceans have been reported world-wide wherever populations of cetaceans exist, and appear to reflect natural mortality patterns in a population, with most individuals stranding due to serious pathological conditions. Mass strandings are more unusual, and seem to be largely limited to a few species of high social odontocetes and are extremely clustered geographically, both on a local and world-wide scale. Most research on causes of mortality in cetaceans have focused on singly or mass stranded individuals. In recent years, a third category of marine mammal mortality has been recognized; that of large-scale mortality events or die-offs. In many cases these events have been linked, albeit in some cases only circumstantially, to biotoxins, viral outbreaks or parasitic infections (Geraci 1989; Geraci et al. 1982, 1989; Harwood 1990; Keyes 1965).
In British Columbia, the Stranded Whale and Dolphin Program (SWDP), coordinated by the Marine Mammal Research Group, has been investigating the causes of mortality of cetaceans since 1987 (see Baird and Guenther in press; Guenther and Baird 1993; Guenther et al. 1993). Mass strandings in B.C. are extremely rare, with the last recorded one occurring in the 1940’s (Carl 1945). Single strandings occur fairly frequently, and the number of records of single stranded animals has been increasing each year, likely due to increased public awareness of the stranding program (Baird 1994).
Prior to 1993, only one record of a possible cetacean die-off had been recorded in the province. This involved a report of 12 neonatal porpoises, likely all Dall’s porpoise (Phocoenoides dalli), washing up on North Beach, Graham Island, over a three day period in 1992 (Guenther et al. 1993). The remoteness of the site and lack of funds precluded detailed investigation of these strandings, and the lack of sighting records of cetaceans in that area made it impossible to conclude whether the large number of strandings was due to an increase in Dall’s porpoise use of the area, or whether some agent or agents was responsible for the deaths of an increased number of neonatal porpoises. A similar occurrence around southern Vancouver Island, extending over a two month period in the spring of 1993 and involving both Dall’s and harbour (Phocoena phocoena) porpoises, was investigated in much greater detail. The purpose of this report is to review the history of porpoise strandings in the area of the 1993 event, to summarize information collected from stranded porpoises from this event, and to present other information necessary in the determination of possible causes of the event.