Dall’s porpoise, Phocoenoides dalli, is the only recognized species of its genus within the family Phocoenidae, the true porpoises. A special characteristic of the species is its distinctive color pattern, a black body with a conspicuous white oval patch on each side. Two morphologically distinct subspecies are currently recognized within the species based on distinguishable color patterns: P. d. truei and P. d. dalli. The truei form is abundant only in waters around the Kuril Islands and off the Pacific coast of northern Japan, while the dalli-type ranges across the northern North Pacific, from northern Japan to the Bering Sea and into California (Jefferson 1988).
The genetic evidence from our research supports the view that both forms are part of a single species whose origin is placed on the western portion of the range, probably in the Okhotsk Sea, at least between 600,000 to a million years ago. Highly biased migration of males was detected for all population comparisons, suggesting the existence of sexual selection. Results also argue for nine distinct dalli-type and one truei-type management stocks that should be treated as separate units when building predictive models, monitoring trends, and maintaining viability. These stocks were recognized by the International Whaling Commission in 2001.
Escorza-Treviño, S., Lang, A. and Dizon, A. E. 2002. Genetic Differentiation and Intraspecific Structure of Eastern Tropical Pacific Spotted Dolphins, Stenella attenuata, Revealed by Mitochondrial and Microsatellite DNA Analyses. SWFSC Admin. Rep. LJ-02-38, 20 p.
Escorza-Treviño, S. 2002. North Pacific Marine Mammals. En Encyclopedia of Marine Mammals (Perrin, W.F., Würsig, B., and Thewissen, H.G.M., eds.). Academic Press.