Mitochondrial DNA sequence variation and phylogeographic patterns in harbour porpoises (Phocoena phocoena) from the North Atlantic

The harbour porpoise (Phocoena phocoena) experiences high rates of incidental mortality in commercial fisheries, and in some areas these rates are sufficiently high to justify concern over population sustainability. Given the high incidental mortality, the resolution of population structure will be important to conservation and management, but in the North Atlantic the relationships among many of the putative populations remain unclear. A previous genetic study demonstrated substantial genetic differences between eastern and western North Atlantic populations, however the location of this break remained unresolved. In the present study, we addressed this issue by including new samples from Iceland. To investigate population structure, variation in the mitochondrial DNA of 370 porpoises was compared among six locations corresponding to several of the putative populations (Gulf of Maine, Gulf of St. Lawrence, Newfoundland, West Greenland, Iceland, Norway). The first 342 base pairs of the control region were sequenced and genetic variation investigated by analysis of molecular variance (F_ST and Φ_ST) and X² with permutation. Although some fine scale population structure was detected, porpoises from Iceland were found to be more similar to the western populations (W. Greenland, Gulf of St. Lawrence, Newfoundland, Gulf of Maine) than to Norway. Furthermore, porpoises from Norway were different from all other regions. These patterns suggest the existence of a discontinuity between Iceland and Norway, possibly the result of isolating events caused by repeated range contractions and expansions throughout Quaternary glaciation events within the North Atlantic. These results suggest that harbour porpoise populations within the North Atlantic are distinguishable, but patterns must be interpreted in light of their historical biogeography.