Harbour porpoises (Phocoena phocoena) are among the most abundant small cetaceans in the northeastern Pacific and there is a distinct bimodal seasonal stranding trend. Increased numbers of animals present in late April to mid May with fewer individuals stranding late August through to mid September. The cause of this temporal pattern is unknown. In May, 2010, 9 harbour porpoises stranded in a 4 day span along the southeast coast of Vancouver Island, British Columbia. All animals were adults, well fleshed and in good post mortem condition. There was no indication of human interaction. Six animals presented with pulmonary edema, 2 featured multifocal subcutaneous and muscular hemorrhage along the torso and 2 had healed rib fractures. Due to the acute deaths and clustered stranding in a geographical area, MR and CT scans were undertaken on the heads of 5 animals. Three individuals had extensive stippled hypoattenuating foci consistent with gas (-340 to -955HU) evident along the hypodermis/muscle interface along the dorsal margin of the head, within the melon, the calvarial vault, the globes, the retrobulbar tissues, throughout the mandibular fat,
lateral to the hemimandibles, and within the soft tissues rostromedial and dorsal to the bullae. On gross dissection and reflection of the calverium, gas was evident within the meningeal vasculature and histopathology of the acoustic fat revealed focal distension of blood vessels with clear, uni to multiloculated clear intravascular foci (gas) with scattered acute perivascular hemorrhage. Follow up whole body CT scan of a code 3 harbor porpoise in Puget Sound, WA disclosed subcutaneous gas near the blowhole and to a much lesser extent along the torso. Similar findings of gas bubble disease have been observed in a number of geographic regions and this is believed to be the first case series in the northeastern Pacific. The strandings could not be linked to an anthropogenic cause.