An adult harbour porpoise Phocoena phocoena grows to around 1.3 – 1.5 metres in length and weighs in around 50-60 kg. They are the smallest species of cetacean found in Scottish and European waters. The porpoise looks stocky and has a rotund shape, a small rounded head with no distinct beak.and a small triangular dorsal fin which shows briefly when they surface for air. They are difficult to spot in choppy water and is rare to see them perform the kind of arial activity more commonly associated with dolphins. Harbour porpoises do not usually bow ride or approach boats although some of the resident porpoises in the upper Clyde seem rather curious and have a habit of approaching vessels and often stay with them for some time.
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.
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.
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.
The species is exposed to a variety of human activities. It was formerly hunted in drive fisheries in the Baltic and off the coasts of Holland and the Faroe Islands. The major current threat appears to be fisheries conflicts. Incidental catches occur in a variety of fishing gear including bottom set gillnets for hake, cod, turbot and sole, fixed nets or traps for cod or salmon, herring weirs, trawls, drift nets, and purse seines for cod, herring or plaice. Recent independent observer schemes have revealed annual by-catches in English and French bottom set gillnet fisheries of 6.2% of the harbour porpoise population (2,237 out of an estimated population of 36,280) in the Celtic Sea west of Cornwall; and in the Danish bottom set gillnet fishery of 2.6% of the porpoise population in the central & southern North Sea (4,500 out of an estimated 175,000). Any by-catch above 1.7% of the population size is considered to be unsustainable.
Harbour porpoises from many areas (including the UK, Baltic, and Canada) have been found to have high pollutant burdens, including organohalogen compounds such as DDT and dieldrin, PCBs, dioxins, furans, and heavy metals. The effects are not clearly known, although links with causes of mortality have been made in UK stranded porpoises, and there is evidence that some pollutants (e.g. PCBs) may have an immunosuppressive function as well as cause reproductive impairment in cetaceans. Transplacental transfer of organochlorine compounds can also occur from females to their foetuses.
Porpoises often live in the vicinity of vessel traffic. A study in the Shetland Islands of reactions by porpoises to various types of vessel showed short-term negative effects from speedboats and large ferries, although reactions varied with group size, social status, and season.