They generally inhabit coastal waters with a depth of less than 150 meters, and their common name is derived from their regular appearance in bays and harbours. Many populations of harbour porpoises are migratory and their elusive nature has limited our ability to determine where they spend much of their time. Natural History Harbour porpoises are one of the world's smallest cetaceans, growing to an average length of 1.55 meters and a mass of 55 kilograms.
Female porpoises are usually larger than males.
It is believed that porpoises can live as long as 20 years. Harbour porpoises become sexually mature between 3 and 5 years of age. After that time, female porpoises usually produce one calf every year. Porpoises also eat squid. Recently weaned porpoises eat euphausiid shrimp.