The harbour porpoise (Phocoena phocoena) is the smallest species of cetacean found in European waters, measuring around 1.3 - 1.5 metres in length and weighing 50 - 60 kg. It is often confused with dolphins, particularly the bottlenose dolphin. The porpoise is rotund in shape, with a small triangular dorsal fin which shows briefly above the surface - usually little of the animal is seen, as it rarely leaves the water entirely. It has a small rounded head with no distinct beak.
Harbour porpoises do not usually approach boats nor bow ride, although they can be observed at close quarters from a dinghy or small inflatable boat, and in late summer, may actually approach vessels.
The harbour porpoise eats a varied diet of fish, cephalopods and crustaceans, related to local availability of food; in European waters, herring, mackerel, sand-eel, gobies and a wide range of gadoid fish such as cod, saithe, pollack, and whiting are all known to form prey of porpoises. The apparent flexibility in diet helps the porpoise to avoid being adversely affected by local human over-exploitation of any single fish species. However, intense exploitation of fish stocks overall can put great pressure on marine mammals like porpoises that are dependent on them for food.
Harbour porpoises generally live in groups of two or three animals, or singly, but occasionally forming groups of 10 - 20 animals. Larger aggregations of up to several hundred porpoises have also been seen seasonally (Feb-March & Aug-Oct), either associated with food concentrations or long-distance movement.
The basic social unit appears to be the mother and calf, which may sometimes be accompanied by a yearling. Segregation by age and sex may also occur in larger groups. DNA studies indicate that females can form genetically distinct groups, while males are more likely to move away. During late summer, porpoises are more social, and sexual activity can be observed. In calm seas, animals frequently lie in a resting state just below the surface.
The main mating season is summer, and birth takes place 10-11 months later (usually between May and August with a peak in June). Calves are suckled for between four and eight months, and the mother usually reproduces every 1-2 years. Porpoises take three to four years to reach sexual maturity and have a relatively short life span usually of no more than 15 years, although animals have been recorded up to 24 years of age.
STATUS AND DISTRIBUTION
As the name suggests, the harbour porpoise is commonly seen in coastal areas, although it ranges over much of the European continental shelf. It is the commonest and most widely distributed
of all cetacean species in northern Europe, favouring comparatively shallow, cold waters.
There are seasonal concentrations of harbour porpoises off south-west and western Ireland, west Wales, the west coast of Scotland, Northern Isles, and eastern Scotland - porpoises may be permanent residents in these areas, with the greatest numbers usually between July and October. Like the bottlenose dolphin, the species was once a regular visitor to the south coast of England and the southern part of the North Sea during the summer months, but then became a rare sight in these areas.
Genetic studies have indicated that around the British Isles, there are separate populations in the Irish Sea and off the Welsh coast; in the northern North Sea; eastern (Denmark) and western (UK) North Sea; and southern North Sea (Netherlands). Further research may reveal other genetically distinct populations.
Despite the fact that the harbour porpoise is probably the commonest small cetacean in UK waters, it is thought to have undergone substantial declines in numbers over the last fifty years, with the species becoming rare in the southernmost North Sea and Channel. Although reasons for this status change are not known for certain, pollution, disturbance, lack of food and entanglement in fishing nets have all been implicated.