The harbour porpoise has a small, rounded head which slopes down to the mouth, a flat forehead and no beak. It has black lips and chin, and a mouth that curves upwards as if smiling. The harbour porpoise has a robust dark body, with a white or pale grey belly. It has a triangular dorsal fin with blunt tips which is set just past the centre of the back, and small dark, slightly rounded flippers. Young harbour porpoises are dull in colour compared to their parents.
The English word ‘porpoise' is derived from the Latin word for pig – porcus. The harbour porpoise used to be referred to as the ‘puffing pig' because of the sneeze-like puffing sound it sometimes makes when it breathes. Harbour porpoises are the only member of the porpoise family found in European waters. Both sexes are reported to live up to some 23 years but few survive past 12 years of age. Four subspecies of harbour porpoise are recognised; (1) P. p. phocoena in the North Atlantic; (2) P. p. vomerina in the eastern North Pacific; (3) P. p. relicta in the Black Sea; and (4) an unnamed subspecies in the western North Pacific.
The harbour porpoise typically keeps a low profile in the water, but its small size, characteristic rolling swimming style and small triangular dorsal fin make it reasonably easy to recognise if the observer is able to get a good look at it. Fish, especially small schooling fish, and cephalopods, are the main prey items. They usually swim slowly and alone or in small groups and can dive for as long as six minutes. The most common social group is that of mother and calf.
Harbour porpoises are found in coastal waters of the sub-Arctic, and cool temperate waters of the North Atlantic and North Pacific. They frequently visit shallow bays, estuaries, and tidal channels less than 200m in depth, and have been known to swim up rivers; the majority of sightings occur within 10km of land. Due to seasonal prey movements, they usually move inshore in the summer and offshore in the winter. There is some evidence of north-south migrations. Numbers of harbour porpoises in some areas have declined in the past few decades due to human activities and the global population is unknown although estimates have been suggested. The IUCN regards the species overall as of Least Concern (2008) although the subspecies found in the Black Sea is listed as Endangered and the population in the Baltic Sea is considered Critically Endangered. Historically, harbour porpoises were hunted in large numbers but the biggest current threat to harbour porpoises throughout their range is incidental capture in fishing nets with thousands of casualties each year.