The Harbour porpoise is the porpoise that is most commonly seen, and it is also the most widely distributed cetacean in northern Europe, easily recognizable from its short triangular dorsal fin and lack of a beak. It is smaller than other porpoises and has a plump body and a dark gray to bluish back, pale belly with a rounded head. When born, the young have a dull color and usually have birth lines that look like folds in their skin, which last for a few hours after birth.
Harbour porpoises live in coastal regions in the North Atlantic, North Pacific and the Arctic Oceans, as well as the Black Sea and the Mediterranean regions. They inhabit estuaries, bays, and river mouths, and sometimes go further upriver. They prefer shallow, cold coastal waters.
The Harbour porpoise is a social species that travels in groups of two to five members, but larger groups can form during migration. Some populations migrate, but on return to their usual areas they are territorial, and patrol certain areas. These porpoises usually swim near the water's surface, rising up to the surface about every 25 seconds to breathe, and their blow is not easy to see at sea. They do not present an especially playful attitude, taking no notice of boats and hardly ever leaping above the water. They can often be detected by their loud puffing sound as they breathe at the surface.
The Harbour porpoise eats mainly smooth, non-spiny fish, such as herring, pollack, hake, cod and sardines. Other sea creatures including cephalopods and shrimp are also eaten.
Harbour porpoises are polygynandrous, two or more males mating with two or more female porpoises. Mating mainly takes place from June to September, with births occurring between May and August. Females give birth to a single calf every year or every second year, following a gestation of 10-11 months. The mothers usually take their newborns to secluded coves for nursing. Lactation lasts approximately 8-12 months, though calves start to eat solid food at around 5 months of age. Young porpoises stay with their mothers after weaning for up to a further 9 months. They reach sexual maturity at the age of 3-4.
The main threats are considered to be lack of food, entanglement in fishing nets, noise and chemical pollution, hunting, and boat traffic.
According to IUCN's Red List, the global number for this species is no fewer than 700,000 individuals. Harbour porpoises are classified currently as least concern (LC) on the list of threatened species.