| Harbour porpoise[1] | |
|---|---|
| Size comparison against an average human | |
| Conservation status | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Cetacea |
| Suborder: | Odontoceti |
| Family: | Phocoenidae |
| Genus: | Phocoena |
| Species: | P. phocoena |
| Binomial name | |
| Phocoena phocoena (Linnaeus, 1758) |
|
| Harbour porpoise range | |
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.[3]
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The English word porpoise comes from the French pourpois (old French porpais, 12th cent.), which is from Medieval Latin porcopiscus, which is a compound of porcus (pig) and piscus (fish). The old word is probably a loan-translation of a Germanic word, cf. Danish marsvin and Dutch mereswijn (sea swine). Classical Latin had a similar name, porculus marinus (used by Pliny), and the notion behind the name likely is a fancied resemblance of the snout to that of a pig or the sound of a porpoise breathing resembling a pig snort. The species is sometimes known as the common porpoise in texts originating in the United Kingdom, though this usage appears to be dying out. It is also called a "puffer" or "puffer pig" by fishermen in New England and eastern Canada.[4] The species' taxonomic name, Phocaena phocaena, is the Latinized form of the Greek φώκαινα, phōkaina, "big seal", as described by Aristotle; this from φώκη, phōkē, "seal".
The harbour porpoise is a little smaller than the other porpoises, at about 67–85 cm (26–33 in) long at birth, weighing 6.4-10 kg. Adults of both sexes grow to 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 the body.
The harbour porpoise species is widespread in cooler coastal waters of the North Atlantic, North Pacific and the Black Sea.[5] The populations in these regions are not continuous[4] and are classified as separate subspecies with 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.[3][5] Recent genetic evidence suggests the harbour porpoise population structure may be more complex, and they should be reclassified.[6]
In the Atlantic, harbour porpoises may be present in a curved band of water running from the coast of West Africa to the coasts of Spain, France, the United Kingdom, Ireland, Scandinavia, Iceland, Greenland, Nova Scotia and Newfoundland and the eastern seaboard of the United States.[4][5] There is a similarly-shaped band in the Pacific Ocean running from Sea of Japan, Vladivostok, the Bering Strait, Alaska and California.[4][5] The harbour porpoise has a global population of 700,000.[5] Areas with sizable numbers include the North Sea, where porpoises number at about 350,000;[7] and well as the Gulf of Maine/Bay of Fundy area, the US west coast and Alaska.[8]
Harbour porpoises are limited to northern or and subarctic waters.[4] They are often found in fjords, bays, estuaries and harbours, hence the name.[4] They feed mostly on small pelagic schooling fish, particularly herring, capelin, and sprat.[5] They will, however, also eat squid and crustaceans in some areas.[5] The porpoises mostly forage near the sea bottom in waters less than 200m deep.[5] They will also forage near the surface when feeding on sprat.[5] When in deeper waters, porpoises may forage for mid-water fish, such as pearlsides.[5] 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.[5] Young porpoises need to consume about 7% to 8% of their body weight each day to survive, which is approximately 15 pounds or 7 kilograms of fish. Significant predators of harbour porpoises include white sharks and killer whales (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.[9]
Some studies suggest porpoises stay within an area for an extended period of time.[5] Nevertheless, they have been recorded to make onshore/offshore movements parallel to the coast.[5] Dives of 220m by harbour porpoises have been recorded.[5] Extended dives tend to last a minute, but five-minute dives have been recorded.[10]
Little is known about the social affiliation and interactions among harbour porpoises. They are generally seen as an asocial and solitary species.[4] They are mostly seen alone or in small groups of two to five individuals.[4] Porpoises have a promiscuous mating system, with each individual mating with several other individuals.[5] Males produce large amounts of sperm which suggests they are sperm competitors.[5] Females, after reaching sexual maturity at around three to four years, can produce young annually for several years.[4] With this they are over great nutritional stress due to being pregnant and lactating at the same time.[4] The gestation of the porpoise lasts 10 to 11 months.[4] Most births tend to take places from May to August.[5] Calves are nursed for eight to 12 months.[4]
Harbour porpoises were traditionally hunted for food, as well as for their blubber, which was used for lighting fuel. The drive hunt in the Little Belt strait, Denmark, is the best known example. Thousands of porpoises were caught there until the end of the 19th century and again in smaller scale during the world wars.[11] Currently, however, this species is not subject to commercial hunting, but it is hunted for food and sold locally in Greenland. In prehistoric times, this animal was hunted by the Alby People of the east coast of Oland, Sweden.
The Harbour porpoise populations of the North Sea, Baltic Sea, western North Atlantic, Black Sea and North West Africa are listed on Appendix II[12] of the Convention on the Conservation of Migratory Species of Wild Animals (CMS). This listing means that these populations have an unfavourable conservation status or would benefit significantly from international co-operation organised by tailored agreements.
In addition, the Harbour porpoise is covered by the Agreement on the Conservation of Small Cetaceans of the Baltic, North East Atlantic, Irish and North Seas (ASCOBANS), the Agreement on the Conservation of Cetaceans in the Black Sea, Mediterranean Sea and Contiguous Atlantic Area (ACCOBAMS) and the Memorandum of Understanding Concerning the Conservation of the Manatee and Small Cetaceans of Western Africa and Macaronesia (Western African Aquatic Mammals MoU).
Currently, the total population is in the hundreds of thousands and the harbour porpoise is not under threat of extinction. There are, however, a number of threats that impact population distribution and numbers.
The main threat to porpoises is static fishing techniques such as gill and tangle nets. Bycatch in bottom-set gill nets is considered the main anthropogenic mortality factor for harbour porpoises worldwide. Bycatch is reported from the Black Sea, the Baltic Sea, the North Sea and the east coast of the United States and Canada. Bottom-set gill nets are anchored to the sea floor and are up to 12.5 miles (20 km) in length. It is unknown why porpoises become entangled in gill nets, since several studies indicate they are able to detect these nets using their echolocation.[13][14] Porpoise-scaring devices, so-called pingers, have been developed to keep porpoises out of nets and numerous studies have demonstrated they are very effective at reducing entanglement.[15][16] However, concern has been raised over the noise pollution created by the pingers and whether their efficiency will diminish over time due to porpoises habituating to the sounds.[17]
Mortality resulting from trawling bycatch seems to be less of an issue, probably because porpoises are not inclined to feed inside trawls, as dolphins are known to do.
An increase in the temperature of the sea water is likely to affect the distribution of porpoises and their prey, but has not been shown to occur. Reduced stocks of sand eel along the east coast of Scotland seems to have been the main reason for the malnutrition in porpoises in the area.
Overfishing may reduce preferred prey availability for porpoises. Overfishing resulting in the collapse of herring in the North Sea caused porpoises to hunt for other prey species. Reduction of prey may result from climate change, or overfishing, or both.
Noise from ship traffic and oil platforms is thought to affect the distribution of toothed whales, like the harbour porpoise, that use echolocation for communication and prey detection. The construction of thousands of offshore wind turbines,planned in different areas of North Sea, is known to cause displacement of porpoises from the construction site,[18] particularly if steel monopile foundations are installed by percussive piling, where reactions can occur at distances of more than 20 km.[19] Noise levels from operating wind turbines are low and unlikely to affect porpoises, even at close range.[20][21]
Marine top predators like porpoises and seals accumulate pollutants such as heavy metals, PCBs and pesticides in their fat tissue. Porpoises have a coastal distribution that potentially brings them close to sources of pollution. Porpoises may not experience any toxic effects until they draw on their fat reserves, such as in periods of food shortage, migration or reproduction.