Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 6

Distribution

Global range

Harbour porpoises are widely distributed over the continental shelves of the temperate Northern Hemisphere (Gaskin 1984; IWC 1996). The species is found from the Barents Sea to Senegal in the eastern Atlantic; Upernavik, Greenland to Cape Hatteras (with occasional strandings in northern Florida) in the western Atlantic; the Mackenzie Delta to Monterey Bay, California in the eastern Pacific; and from Siberia to Wakayama, Japan in the western Pacific (Read 1999). An isolated sub-species, P. p. relicta, occurs in the Black Sea. Over the past few decades, harbour porpoises have largely disappeared from the English Channel and much of the Baltic Sea (IWC 1996), although the reasons for this disappearance are unknown.

Analysis of control region (d-loop) sequences of mitochondrial DNA indicates that harbour porpoises in the northwestern Atlantic are effectively isolated from those in the northeastern Atlantic (Rosel et al. 1999b; Tolley 2001) . Significant differences in DNA haplotype composition are maintained by the low level of dispersal, which is estimated to be 2.7 females per generation (Rosel et al. 1999b). Significant differences in mitochondrial haplotype frequencies and molecular diversity suggest a hiatus between Iceland and Norway, likely due to isolation caused by Pleistocene glaciation (Tolley 2001; Tolley et al. 2001).


Canadian range

In eastern Canada, the harbour porpoise occurs from the Bay of Fundy north to Cape Aston, Baffin Island, at approximately 70° N (Gaskin 1992). The southern range of the species extends into U.S. waters. Information on the distribution of this species is restricted largely to the summer months, when it is possible to conduct visual surveys for these small, cryptic animals (e.g. Palka 1995a). Additional information on distribution has been obtained from observations of bycatches and strandings and, in the Bay of Fundy, from the movements of individual porpoises equipped with satellite-linked radio transmitters (Read and Westgate 1997).

One mature female porpoise was tagged in the Bay of Fundy during early summer and was tracked as it moved to the Gulf of St. Lawrence (see below). This is the only porpoise (of 25 tracked) that left the range of the Bay of Fundy-Gulf of Maine population.


Figure 2. Distribution of harbour porpoises in eastern Canada

Figure 2. Distribution of harbour porpoises in eastern Canada.

Map courtesy of Dave Johnston, Duke University. Dashed lines indicate approximate delineations of the three subpopulations.


Information on the distribution of the species in Newfoundland and Labrador is sparse, particularly compared to knowledge of the species in more southern waters. Bycatches in groundfish gillnets (Lien et al. 1994; Lawson et al. 2004) show that porpoises occur around the entire island of Newfoundland (especially along the south coast, west coast and in Notre Dame Bay) as well as in southern Labrador. Bycatches were particularly common in parts of southeastern Newfoundland, such as St. Mary’s Bay, during the early summer in the 1980s (e.g. Lien 1989). Stenson and Reddin (1990) reported bycatches in experimental salmon drift nets across the entire Grand Banks as well as along the continental shelf as far north as Nain. They also reported a number of catches in the Labrador Sea between Newfoundland and Greenland. With the exception of the Strait of Belle Isle and western coast of Newfoundland, no surveys have been conducted for this species in Newfoundland or Labrador.

Surveys (focusing on bottlenose whales) along the 1,000 m contour on the Canadian side of Davis Strait to 61º15'N (mouth of Hudson Strait) in 2003 resulted in 13 sightings of harbour porpoises (group sizes ranging from one to five) between 6-13 August (H. Whitehead, pers. comm.). All of these sightings occurred north of 58ºN; no porpoise sightings were made south of that latitude during the deepwater survey in 2003 despite many sightings of other cetacean species. Bycatch records and opportunistic sightings data suggest that porpoises occur all the way up the Labrador shelf (G. Stenson, pers. comm.). The 2003 deepwater observations, together with the bycatch data mentioned in the preceding paragraph, raise the possibility that harbour porpoises can move across deep basin waters between Canada and Greenland.

During summer harbour porpoises are found throughout the Gulf of St. Lawrence, reaching upstream as far as the mouth of the Saguenay River. Porpoises are common along the north shore of the Gulf of St. Lawrence, along the Gaspé coast and in the Baie des Chaleurs (Fontaine et al. 1994; Kingsley and Reeves 1998). Densities of porpoises are lower in the southern Gulf of St. Lawrence. There is reason to believe that porpoises in the Gulf are migratory and that most of them move out of the Gulf in winter to avoid ice entrapment.

In the Bay of Fundy and northern Gulf of Maine, the summer distribution of harbour porpoises is concentrated in waters less than 150 m deep, along the coasts of Maine and New Brunswick and extending to the southwestern tip of Nova Scotia (Waring et al. 2001). Porpoises equipped with satellite transmitters move frequently into and out of U.S. waters during the summer (Read and Westgate 1997). Densities are quite low in the upper reaches of the Bay of Fundy and along the southern shore of Nova Scotia (Gaskin 1992). There is considerable inter-annual variation in the summer distribution of porpoises in this part of their range (Palka 1995b).

In winter, many porpoises from the Bay of Fundy disperse into the Gulf of Maine and along the U.S. east coast as far south as North Carolina, where they may mix with individuals from more northern areas (Rosel et al. 1999a). Some porpoises may over-winter in the Bay of Fundy (Gaskin 1992; Westgate and Read, unpublished data). Very little is known of the winter distribution of the porpoises from Labrador, Newfoundland, and the Gulf of St. Lawrence, although much of the Gulf of St. Lawrence is covered by ice during winter, so most porpoises must leave that area for open water.

No information exists on historical changes in the area of occupancy of this species in eastern Canada.

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