Friday, March 11, 2011
March 11, 2011 : Harbour Porpoise
The Harbour Porpoise (Phocoena phocoena) is one of six species of porpoise. It is one of the smallest marine mammals. As its name implies, it stays close to coastal areas or river estuaries and as such is the most familiar porpoise to whale watchers. This porpoise often ventures up rivers and has been seen hundreds of miles from the sea. The Harbour porpoise may be polytypic, with geographically distinct populations representing distinct races: P. p. phocoena in the North Atlantic and West Africa, P. p. relicta in the Black Sea and Sea of Azov, an unnamed population in the Northwest Pacific and P. p. vomerina in the Northeast Pacific.
The Harbour Porpoise is a little smaller than the other porpoises. It is about 67–85 cm (26–33 in) long at birth, weighing 6.4-10 kg. Both sexes grow up to be 1.4 m to 1.9 m (4.6-6.2 ft). The females are heavier, with a maximum weight of around 76 kg (167 pounds) compared with the males' 61 kg (134 pounds). The body is robust and the animal is at its maximum girth just in front of its triangular dorsal fin. The beak is poorly demarcated. The flippers, dorsal fin, tail fin and back are a dark grey. The sides are a slightly speckled lighter grey. The underside is much whiter, though there are usually grey stripes running along the throat from the underside of their body.
Harbour Porpoises are limited to northern or and subarctic waters. They are often found in fjords, bays, estuaries and harbors, hence the name. Harbour Porpoises feed mostly on small pelagic schooling fish, particularly herring, capelin, and sprat. They will, however, also eat squid and crustaceans in some areas. The porpoises mostly forage near the sea bottom in waters less than 200m deep. They will also forage near the surface when feeding on sprat. When in deeper waters, porpoises may forage for mid-water fish like pearlsides. Harbour porpoises tend to be independent foragers, but groups have been observed to collaborate to keep schools of fish closely together and herd them to the surface. Young porpoises need to consume about 7% to 8% of their body weight each day in order to survive, which is approximately 14 pounds or 7 kilograms of fish a day. Significant predators of harbour porpoises include white sharks and orcas. Researchers at the University of Aberdeen in Scotland have also discovered that the local bottlenose dolphins attack and kill harbour porpoises without eating them due to competition for a decreasing food supply.