The harbour porpoise is the smallest and most common cetacean found in the coastal waters of Northern Ireland. The population is believed to have decreased over the last 50 years – this may be due in part to thousands of porpoises getting caught and drowning in commercial fishing nets every year around the British and Irish coast.

In brief

• Found in the seas around Northern Ireland.
• Smallest and most commonly sighted cetacean in Irish waters.
• Can be seen all year round.
• Main threats to the population are accidental capture and drowning in commercial fishing equipment (by-catch) and pollution.

Species description: The body is short and rounded – adults are usually between 1.5 and 1.9 m long. The back is dark grey/black and the belly is pale/white. The pale markings extend up the sides above the flippers. A dark line runs from the mouth to the flippers. The head is small and rounded, there is no beak. There are numerous spade-shaped teeth present in the upper and lower jaw. The small, triangular dorsal fin is in the middle of the back.

The blow is short and usually not visible but can often be heard, giving rise to the name ‘puffing pig’.

Life cycle: A coastal species found only in the cool temperate and sub-polar waters of the Northern Hemisphere. Present all year round in many areas. Some populations, however, move inshore in summer and offshore in winter; these seasonal movements may be due to food availability and breeding. Some sightings data suggests that females may move offshore to give birth, before returning inshore with calves. Porpoises become sexually mature between two and four years of age. Pregnancy lasts about 10 to 11 months and a single calf may be produced every 1 to 2 years. Calves are born during the summer. The average life span is between 12 and 15 years. Porpoises are usually seen in small groups of two or three, but where feeding is good, 20 to 30 animals may be seen together. They feed mainly on small shoaling fish such as herring, mackerel and sand eels but will also take squid and octopus. Harbour porpoises are capable of diving to depths of 200m and can stay underwater for up to six minutes.

Similar species: Unlikely to be confused with any other species due to their small size. There is less surface activity than with dolphins – usually only the back and dorsal fin is seen and there is very little splashing. Porpoises are slow swimmers, tend to avoid boats and do not bow ride.

How to see this species: Sightings off the coast of Northern Ireland are common at all times of the year. The best places to see harbour porpoises are headlands and bays when the sea is calm. Feeding seabirds may help pinpoint their location.

Current status: Declining. Harbour porpoises are seen regularly in the seas around Northern Ireland. No evidence of porpoise breeding in Northern Irish waters, aside from some anecdotal accounts. A 1994 survey of small cetaceans in the North Sea, English Channel and Celtic sea (SCANS) estimated the population of harbour porpoise to be around 350,000. SCANS II carried in in 2005 estimated the North Sea block to be around 216,415 individuals, and the Irish Sea population was estimated to be around 15,000. Current population estimates are not known. European population listed as 'stable' on IUCN but no detailed information available for Britain and/or Ireland.

Why is this species a priority in Northern Ireland?

• Criterion 2: Decline based on available data and expert judgement.
• Criterion 4: NI stronghold or endemic – both stronghold range/population and designation as ‘endemic’ subject to expert judgement.

Threats:

• By-catch – the accidental killing associated with commercial fishing equipment – trawls, seines, cod traps and, in particular, bottom-set gill nets are the biggest threat to harbour porpoise populations.
• Other threats include disturbance and injury by speedboats and jet skis, climate change and ocean pollution (chemical and noise).
• Whales and dolphins communicate mainly by sound; they also use sound to navigate and find food. Many man-made sounds are introduced into the oceans, some of these, such as noise due to seismic exploration for oil and gas and disturbance from marine traffic, may pose a threat to whales and dolphin populations.
• Many species of fish eaten by porpoises are also fished commercially (herring, mackerel, sprat, pilchard, whiting, cod). Reduction in fish numbers due to commercial fishing may threaten porpoise populations.
• There have been documented attacks by bottlenose dolphins in Strangford Lough. They do not appear to eat the porpoise so the reasons behind this are currently poorly understood.

Action: Records of new sites for this species, with details of habitat and population are always valuable. Observations with a photograph can be recorded using iNaturalist, find the MBDP-NI iNaturalist project here.

Alternatively, records can be sent to CEDaR, National Museums Northern Ireland, 153 Bangor Road, Cultra, Holywood, County Down, BT18 0EU. Tel 028 9039 5257, e-mail cedar.records@nationalmuseumsni.org, or via the online portal.

Objectives:

• Maintain the current geographical range of the harbour porpoise.
• Maintain the current abundance of the harbour porpoise.
• Investigate ways to reduce bycatch as this seems to be a major threat to the species in this region.

Conservation:

• Post-mortem and tissue studies of stranded corpses are carried out to establish the cause of death and condition of the animals at the time of death.
• They are protected under Schedule 5 of the Wildlife (Northern Ireland) Order 1985.
• Protected under Bern Convention (Convention on the Conservation of European Wildlife and Natural Habitats) Appendix II.
• Listed in Appendix II of the Convention on the Conservation of Migratory Species (The Bonn Convention).
• Listed in Annex A of EU Council Regulation 338/97 and therefore treated by the EU as if they are on CITES, Appendix I.
• Covered by the terms of the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS).
• Listed in Annex II and IV (Animal and Plant Species of Community Interest in Need of Strict Protection) of the EC Habitats Directive.

Distribution Map from NBN: Phocoena phocoena at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Phocoena phocoena at iNaturalist World Species Observations database.

WoRMS: Phocoena phocoena at World Register of Marine Species. Accepted name: Phocoena phocoena (Linnaeus, 1758). AphiaID: 137117.

Classification: Biota; Animalia; Chordata; Vertebrata; Gnathostomata; Tetrapoda; Mammalia; Theria; Cetartiodactyla; Cetancodonta; Cetacea; Odontoceti; Phocoenidae; Phocoena

Adapted from text by Angela Ross (2026). Phocoena phocoena. (Linnaeus, 1758). [In] Priority species, Marine Biodiversity Portal NI. https://www2.habitas.org.uk/marbiop-ni/priorityaccounts.php?item=ZK850. Accessed on 2026-09-13