Genetic structure of harbour porpoise Phocoena phocoena populations in thenorthwest Atlantic based on mitochondrial and nuclear markers

The harbour porpoise, Phocoena phocoena, experiences high levels of nonnatu ral mortality owing to interactions with commercial fisheries throughout it s range. To accurately evaluate the significance of this bycatch, informati on on population structure is required. We have examined the population str ucture of this species in the northwest Atlantic Ocean using mitochondrial DNA (mtDNA) sequence and nuclear microsatellite data. Samples from four pre viously proposed summer breeding populations-the Gulf of Maine, eastern New foundland, the Gulf of St Lawrence and West Greenland-were analysed. Contro l-region sequences revealed a significant partitioning of genetic variation among most of these summer populations, indicating that northwest Atlantic harbour porpoises should not be considered one panmictic population. Analy sis of females alone yielded the highest levels of population subdivision, suggesting that females are more philopatric than males. At least three man agement units may be defined for harbour porpoises in the northwest Atlanti c based on these data. Analysis of six microsatellite loci failed to detect significant population subdivision. Male-mediated gene flow may maintain h omogeneity among nuclear loci, while female philopatry is sufficient to pro duce a signal of population subdivision in the maternally inherited mtDNA g enome, mtDNA analyses also indicate that winter aggregations of harbour por poises along the US mid-Atlantic states comprise animals from more than one summer breeding population.