Harbour porpoise (Pacific Ocean population) COSEWIC assessment and status report: chapter 3

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General Biology

Reproduction

Information on the reproductive biology of harbour porpoises has been collected primarily from stranded animals. Reproduction is seasonal, with births in British Columbia occurring from May through September (Baird and Guenther 1995). Mean age at sexual maturity of females from the North Atlantic has been estimated to be three or four years of age, although this is known to vary between populations (Read 1999), and has not been determined for the British Columbia population. In the North Atlantic, females may give birth each year (Read and Hohn 1995), while off California females give birth only every second year (Read 1999). Average longevity has not been determined. Using a modelling approach, Trites and Pauly (1998) estimate longevity (defined as the 99^th percentile of the age distribution) at 13 years, although maximum longevity based on aging of teeth has been reported as 24 years (Lockyer 1995; Read 1999). The oldest animal recovered from British Columbia (of 62 specimens aged) was 10 years of age (K. Robertson, SWFSC, NMFS, personal communication), and animals over 12 years of age are rare in other populations (e.g., Read and Hohn 1995).

Movements

Besides the relatively restricted habitat use noted above, several lines of evidence suggest limited movements by harbour porpoises off western North America. These include regional differences in pollutant ratios (Calambokidis and Barlow 1991; Osmek et al. 1995), cranial morphology (Yurick and Gaskin 1987) and genetic patterns. Chivers et al. (2002) provide evidence of genetic differentiation between harbour porpoise from inshore areas (such as the Strait of Georgia) versus offshore areas (such as the west coast of Vancouver Island), thus there appear to be at least two discrete stocks in the province. There has been very little genetic sampling of harbour porpoises in British Columbia north of southern Vancouver Island, thus it is not possible to state whether more population differentiation is occurring within the province. Radio-tracking data from two harbour porpoises tagged in Washington State (Hanson et al. 1999) also suggest limited movements by individuals. Flaherty and Stark (1982) photo-identified 29 harbour porpoise around San Juan Island, and resighted three individuals, all relatively close (8-30 kilometres) to where they were originally photographed, up to seven months after they were first sighted. The United States (U.S.) National Marine Fisheries Service has used this combined evidence to classify (and treat as separate management units) a number of stocks along the U.S. Pacific coast. These are (from south to north): Moro Bay (California); Monterey Bay (California); San Francisco-Russian River (California); northern California/southern Oregon; Oregon/Washington coast; and Washington inland waters stock (Carretta et al. 2002). Work on stock discrimination in U.S. waters is ongoing.

Nutrition and Interspecific Interactions

Harbour porpoises typically feed on small schooling fish. Prey taken may vary by season, reproductive status and age (see Read 1999). Stomach contents from 26 stranded or incidentally caught harbour porpoises from southern British Columbia indicate that they have a diverse diet of small fish and squid (Walker et al. 1998). Based on the calculated percentage of prey mass each type represented from the sample, market squid (Loligo opalescens) appears to be the most important prey item in this area. Other relatively common prey species (both of which were found in more than a quarter of the stomachs and, combined, represented about one quarter of the mass of prey) include Pacific herring (Clupea harengus pallasi) and Pacific hake (Merluccius productus). Prey items taken ranged in size from 80 to 371 millimetres in length, and from 4 to 317 grams in weight (Walker et al. 1998). Diet of harbour porpoise overlapped strongly with that of Dall’s porpoise recovered from the same area (Walker et al. 1998). Squid seem to form a larger proportion of the diet of harbour porpoise in British Columbia than has been reported elsewhere (cf. Smith and Gaskin 1974; Recchia and Read 1989; Fontaine et al. 1994; Gannon et al. 1998).

Harbour porpoises are typically found in small groups, although lone animals are frequently seen (Read 1999). A study in the northern San Juan Islands, near southern Vancouver Island, found group sizes ranging from 1-8, with a mean group size of 1.87 (Raum-Suryan and Harvey 1998). Associations with other species of cetaceans are uncommon, with some interactions being agonistic (Ross and Wilson 1996; Baird 1998; Morton 1999).