A social species, the harbour porpoise travels in groups numbering between two to five individuals (2), but larger groups may form during migration (4). It feeds on a variety of fish, including herring, mackerel and anchovy (2), a variety of invertebrates are also taken (4). It is known that echolocation is used in the detection and capture of prey, but this is not yet fully understood. Sight and passive listening (for the sounds made by prey) are also important during hunting (7). Mating occurs in summer, and gestation (pregnancy) takes 11 months (4). The calf is suckled for up to eight months and sexual maturity is reached at about four years of age (4). The harbour porpoise has possibly the shortest life-span of any cetacean; they rarely live for more than 12 years (2).