Harbour porpoise (Pacific Ocean population) COSEWIC assessment and status report: chapter 2

Habitat

In general, harbour porpoises have been reported to typically inhabit waters less than 200 metres (m) in depth. There is one record from British Columbia of a harbour porpoise occurring approximately 55 kilometres (km) up the Fraser River (Guenther et al. 1993), suggesting that movements into large rivers occur occasionally (see also Scheffer and Slipp 1948). Off the outer Washington, Oregon and southern British Columbia coasts, Laake et al. (1998) noted that the density of harbour porpoises between 100 and 200 metres of depth was only one-sixth the density of those found in depths less than 100 metres. Aerial surveys by Calambokidis et al. (1997) found a much lower density of harbour porpoises (0.04 animals per km², uncorrected density) in the relatively deep Strait of Georgia than in the shallower waters around the Canadian Gulf Islands (0.16 animals per km²) or on the Canadian side of Juan de Fuca Strait (0.24 animals per km²).  In British Columbia, based on a review of sighting and stranding records, Baird and Guenther (1991, 1994, 1995) noted that harbour porpoises appear to be generally restricted to shallow (<125 m) waters, and tend to avoid areas of high current flow (see also Watts and Gaskin 1985). As well, harbour porpoises tend to be absent from some areas of what appear to be suitable habitat based on depth and currents. These areas, however, are relatively small in size or otherwise separated from larger patches of suitable habitat (e.g., small areas in between islands, or small shelf areas adjacent to deep water habitats; Baird and Guenther 1991). In inshore waters, harbour porpoise distribution overlaps with that of Dall’s porpoise, but there are clear differences in the distribution of sightings, with harbour porpoises generally being found in shallower waters (see maps in Baird and Guenther 1994). Thus some inshore areas on the British Columbia coast (e.g., the mainland fjords, central Strait of Georgia) appear to be marginal or unsuitable habitat for this species. Exceptions do exist, indicating that local conditions may be important. Raum-Suryan and Harvey (1998) studied habitat use by harbour porpoise off the northern San Juan Islands, and found them distributed over a depth range of 20 to 235 m, with a mean depth of 142 m, and with a greater than expected use of deeper (> 125 m) waters. Also, in U.S. waters in the southern Strait of Georgia, Hanson et al. (1999) tracked a radio-tagged harbour porpoise over a 215-day period. It spent most of its time over a deep-water (approximately 200 metres maximum depth) basin.

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