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 northwestern Pacific and P. p. vomerina in the northeastern Pacific.

In parts of Atlantic Canada it is known colloquially as the puffing pig, and in Norway ‘nise’, derived from an Old Norse word for sneeze; both of which refer to the sound made when porpoises surface to breathe.

[5][6][7] Although conjoined twins are rarely seen in wild mammals, the first known case of a two-headed harbour porpoise was documented in May 2017 when Dutch fishermen in the North Sea caught them by chance.

[8] A study published by the online journal of the Natural History Museum Rotterdam points out that conjoined twins in whales and dolphins are extremely rare.

[11] In the Atlantic, harbour porpoises may be present in a curved band of water running from the coast of West Africa to the coasts of Portugal, Spain, France, the United Kingdom, Ireland, Scandinavia, Iceland, Greenland, Nova Scotia and Newfoundland and the eastern seaboard of the United States.

There is another band in the Pacific Ocean running from the Sea of Japan, Vladivostok, the Bering Strait, Alaska, British Columbia, and California.

It concluded that the harbour porpoise population structure is more complex than previously thought, with at least three genetically distinct subspecies in the North Atlantic.

[14] The highest densities are in the southwestern North Sea and oceans of mainland Denmark;[14] the latter region alone is home to about 107,000-300,000 harbour porpoises.

[12] They feed mostly on small pelagic schooling fish, particularly herring, pollack, hake, sardine, cod,[20] capelin, and sprat.

[21][22] A study (2024) shown that prey availability is an important driver of seasonal and diel dynamics of harbour porpoise acoustic activity in the Black Sea.

Researchers at the University of Aberdeen in Scotland have also discovered that the local bottlenose dolphins attack and kill harbour porpoises without eating them due to competition for a decreasing food supply.

[1][34] In prehistoric times, harbour porpoises were also hunted in many areas, for example by the Alby People of the east coast of Öland, Sweden.

Bycatch in bottom-set gill nets is considered the main anthropogenic mortality factor for harbour porpoises worldwide.

[37][38] Porpoise-scaring devices, so-called pingers, have been developed to keep porpoises out of nets and numerous studies have demonstrated they are very effective at reducing entanglement.

[39][40] However, concern has been raised over the noise pollution created by the pingers and whether their efficiency will diminish over time due to porpoises habituating to the sounds.

Noise from ship traffic and oil platforms is thought to affect the distribution of toothed whales, like the harbour porpoise, that use echolocation for communication and prey detection.

Noise from shipping traffic, particularly busy sea lanes, appears to instigate evasive behavior, with predominantly lateral movements during the day and deeper dives during the night.

[46][47] Marine top predators like porpoises and seals accumulate pollutants such as heavy metals, PCBs and pesticides in their fat tissue.

Porpoises may not experience any toxic effects until they draw on their fat reserves, such as in periods of food shortage, migration or reproduction.

Reduced stocks of sand eel along the east coast of Scotland, a pattern linked to climate change, appears to be the main reason for the increase in malnutrition in porpoises in the area.

[51][52][53] In addition, the harbour porpoise is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS) and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).