Harbour porpoises are found in cool temperate to sub-polar waters of the northern hemisphere, usually near shore and in shallow water (Jefferson et al, 2008).
In the North Pacific, they range from southern California and northern Honshu, Japan, to the southern Beaufort and Chukchi seas (Jefferson et al, 1993).
In the North Atlantic, they are found from the southeastern United States to southern Baffin Island in the west, and Senegal, West Africa to Novaya Zemlya in the east (Jefferson et al, 1993).
Harbour porpoises do not regularly occur throughout most of the Mediterranean except in the northern Aegean Sea, and represent the only cetacean species regularly found in the Baltic Sea (Jefferson et al, 2008). It is the smallest and most numerous small cetacean found in north-western European continental shelf waters (Reid et al, 2003). It is limited to the continental shelf by its demersal foraging behaviour and diving capacity (Gaskin et al, 1993) and by the presence of concentrated aggregations of prey.
Porpoises are extremely mobile, can cover large distances in relatively short periods of time and in some areas, may utilize home ranges that encompass tens of thousands of square kilometres (Read and Westergate, 1997).
In the North Atlantic Ocean (including the Black and Azov seas), fourteen populations are recognized. In the North Pacific, at least ten different stocks occur (Jefferson et al, 2008).
Current estimates suggest that the global abundance of the harbour porpoise is around 700,000 animals, with evidence of declining populations in the Baltic and Black Seas (IUCN, 2009).