Harbour porpoise (Northwest Atlantic population) COSEWIC assessment and status report: chapter 7

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Subpopulation structure

The U.S. National Marine Fisheries Service (2005) continues to cite multiple lines of evidence in support of Gaskin’s (1984, 1992) original concept of four separate subpopulations of harbour porpoises in the western North Atlantic: (1) Gulf of Maine/ Bay of Fundy, (2) Gulf of St. Lawrence, (3) Newfoundland/Labrador and (4) West Greenland. These include analyses involving mitochondrial DNA (mtDNA) (Wang et al. 1996; Rosel et al. 1999a, 1999b), organochlorine contaminants (Westgate et al. 1997; Westgate and Tolley 1999), heavy metals (Johnston 1995), and life history parameters (Read and Hohn 1995). Although individual studies are not as definitive as one would wish, the balance of evidence indicates that there are multiple subpopulations of harbour porpoises in eastern Canadian waters.

Differences shown in genetic and organochlorine contaminants studies of harbour porpoises in Canadian waters are shown in Table 1. Unfortunately, many of the same animals were used as specimens in all of the studies. However, three uncorrelated measurements were made on mtDNA, microsatellites, and contaminants. Significant variation in sequence data from the control region of mtDNA indicated three subpopulations in eastern Canada – Newfoundland-Labrador, Gulf of St. Lawrence, and Bay of Fundy-Gulf of Maine – and a fourth subpopulation in West Greenland (Wang et al. 1996; Rosel et al. 1999a; Table 1). The Bay of Fundy/Gulf of Maine and Newfoundland-Labrador subpopulations both showed significant differentiation from the other two subpopulations. Porpoises from the Gulf of St. Lawrence and West Greenland were not genetically differentiated. Tolley et al. (2001) suggested that the weak differentiation may reflect recent colonization in northern areas following Pleistocene glaciation, and that insufficient time may have elapsed to allow significant differentiation in mitochondrial haplotype frequencies.

In contrast to analyses of mitochondrial DNA, microsatellite markers exhibited little differentiation among the four putative subpopulations (Rosel et al. 1999a). However, the pattern of genetic distances among them was the same as that demonstrated for mtDNA haplotypes (Rosel et al. 1999a). This remained the case even after doubling the number of nuclear markers used, considerably augmenting the sample sizes from all four areas, and incorporating specimens from additional parts of Newfoundland (P. Rosel, pers. comm., December 2005). The Newfoundland specimens used in the 1999 published analysis had all come from the south coast (G. Stenson, pers. comm.). It therefore seems likely that male-mediated gene flow is sufficient to maintain homogeneity among nuclear markers, while female philopatry maintains significant differentiation in the mtDNA (Wang et al. 1996; Rosel et al. 1999a).

Some mixing of porpoises from the various subpopulations occurs outside the late spring/early summer breeding season. Mitochondrial haplotype frequencies suggest that individuals from all four subpopulations in the northwestern Atlantic strand during winter along the eastern coast of the United States (Rosel et al. 1999a). Haplotypes unique to the Gulf of St. Lawrence and West Greenland appeared in a sample of stranded animals and eight of the 28 haplotypes present were unique to the winter sample, suggesting that source populations have not been sufficiently sampled to detect all of their diversity (Rosel et al. 1999a).

Harbour porpoises from the three Canadian subpopulations had significantly different levels of organochlorines in their tissues (Westgate and Tolley 1999; Table 1). This indicates that the three subpopulations, overall, feed in different areas at some times of the year. The animals from the Newfoundland-Labrador subpopulation had notably lower organochlorine concentrations than animals from the Gulf of St. Lawrence and Bay of Fundy/Gulf of Maine subpopulations.