Abstract

Harbour Porpoise (Phocoena phocoena) are a small marine mammal reaching a length of about 2.2 m and weight of about 75 kg when fully grown, making them the smallest cetacean in Canadian waters. The species exhibits sexual dimorphism, with females of this species generally being larger than males. Harbour Porpoise are often difficult to observe in the wild, in part due to the grey-brown counter-colouration on the dorsal surface with lighter lateral undersides. A
distinctive lateral grey-brown stripe(s) extending from the corner of the mouth, to the pectoral flipper on both sides of the animal can sometimes be observed. This is a shy species that seldom rides bow waves of vessels and rarely, if ever leaps out of water. Further complicating the observation of wild Harbour Porpoise is that the 15-20 cm high dorsal fin rarely makes an exit or entry splash.

In Canada, the species is found in the relatively shallow waters of the continental shelves and coastal waters on the east and west coasts of Canada. Pacific Harbour Porpoise are found throughout British Columbian waters, but certain areas appear to be seasonally favoured. More dense aggregations and increased seasonal densities have been reported from the southern Strait of Georgia and in Juan de Fuca Strait, near Victoria.

Little information is available on the abundance and population trends of the Pacific Harbour Porpoise in B.C. However, research on contaminant loading and genetic structure of the population suggests that Harbour Porpoise in B.C. may exist in stratified population sub-units, with little regional dispersal. There are reports that the Harbour Porpoise population of southern B.C. and northwestern Washington declined since the 1940’s. This decline is inferred from qualitative observations and as such, assessment of trends in relative abundance over the last half century is difficult. The potential for further reduction or displacement exists, as both the human population and use of coastal waters increases.

Given that Harbour Porpoise inhabit coastal areas and appear to be particularly sensitive to environmental disturbance, there are several identified threats to this species. The most significant of threats are: entanglement in fishing gear, habitat degradation, toxic spills, acoustic disturbance, and contamination by persistent bioaccumulative toxic chemicals. Despite uncertainties on dietary needs, competition with fisheries is also of moderate concern for long-term impacts to population health. Further research will assist in clarifying this threat.

For populations, such as Pacific Harbour Porpoise, which may occur over small ranges or exist in restricted habitats, the cumulative effect of any combination of threats may result in more deleterious consequences than any single threat alone. Reducing the risk of entanglement and coastal habitat degradation is essential for effective management of this population.

Uncertainties remain regarding Harbour Porpoise abundance and diet in B.C., and actions and objectives will address these and other issues. A multi-species approach to research efforts will allow for effective use of available resources.