The harbour porpoise species is widespread in cooler coastal waters of the North Atlantic, North Pacific and the Black Sea.
The harbour porpoise (Phocoena phocoena) is one of six species of porpoise. It is one of the smallest marine mammals. As its name implies, it stays close to coastal areas or river estuaries, and as such, is the most familiar porpoise to whale watchers. This porpoise often ventures up rivers, and has been seen hundreds of miles from the sea. The harbour porpoise may be polytypic, with geographically distinct populations representing distinct races: P. p. phocoena in the North Atlantic and West Africa, P. p. relicta in the Black Sea and Sea of Azov, an unnamed population in the northwest Pacific and P. p. vomerina in the northeast Pacific.
The body is robust, and the animal is at its maximum girth just in front of its triangular dorsal fin. The beak is poorly demarcated. The flippers, dorsal fin, tail fin and back are a dark grey. The sides are a slightly speckled, lighter grey. The underside is much whiter, though there are usually grey stripes running along the throat from the underside of the body. Many anomalously white coloured individuals have been confirmed, mostly in the North Atlantic, but also notably around Turkish & British coasts, and in the Wadden Sea and Bay of Fundy.
The harbour porpoise is a little smaller than the other porpoises, at about 67–85 cm (26–33 in) long at birth, weighing 6.4-10 kg. Adults of both sexes grow to 1.4 m to 1.9 m (4.6-6.2 ft). The females are heavier, with a maximum weight of around 76 kg (167 pounds) compared with the males' 61 kg (134 pounds).
They feed mostly on small pelagic schooling fish, particularly herring, capelin, and sprat. They will, however, eat squid and crustaceans in certain places. This species tends to feed close to the sea bottom, at least for waters less than 200 m deep. However, when hunting sprat, porpoise may stay closer to the surface.
Porpoises mate promiscuously. Males produce large amounts of sperm, perhaps for sperm competition. Females become sexually mature by their third or fourth year and can calve each year for several consecutive years, being pregnant and lactating at the same time. The gestation of the porpoise is typically 10–11 months. Most births occur in late spring and summer. Calves are weaned after 8–12 months.
Some studies suggest porpoises are relatively sedentary and usually don't leave a certain area for long. Nevertheless, they have been recorded to move from onshore to offshore waters along coast.
The social life of harbour porpoises is not well understood. They are generally seen as a solitary species. Most of the time, porpoises are either alone or in groups of no more than five animals.