Porpoise




dolphin




by Susan

Cetacea describes the order of mammals which consists of whales, porpoises and dolphins. Baleen whales, of which there are about 12 species, have filters called baleen set within the roof of their mouths allowing their filter feeding on krill. Toothed whales consist of dolphins, porpoises, white whales, sperm whales and beaked whales all belonging to the sub-order odontoceti. Cetaceans are the most specialised of all mammals. They are fish-shaped, have hairless bodies and flipper-like front limbs and what is called vestigial back limbs sited within the body wall. They are mammals, breathe air and suckle their young. Porpoises, my main study choice, are differentiated from dolphins by the lack of domed heads, but, as life continues to show us, exceptions are to be found.

The Finless porpoise, found in the Indian Ocean and the West Pacific, does have the domed head but lacks a dorsal fin. Its length is about 5 - 6 ft and weight around 160 lb. Its slightly beaked snout and lack of fin means it is a distinctive breed within the 6 species of porpoise. Unlike other porpoises it rarely leaps out of the water but can adjust its body to an almost vertical slant to view all around. It feeds mostly alone but sometimes with as many as 10 others. Its diet consists of small fish, molluscs and crustaceans nearest to the sea bed and it has between 13 and 22 pairs of teeth in each jaw.

The Harbour porpoise, due to its habitat being more shore-like, is suffering losses owing to fishing nets trapping it from resurfacing to replenish its air supply. Its predators are killer whales, bottlenose dolphins and sharks. It mostly forages alone diving 600 ft or so in search of shellfish but also groups with others to feed on shoaling fish. It weighs 110 - 200 lb and measures 4 - 6 ft. It has in excess of 45 pairs of teeth distributed in both jaws. The dorsal fin is shark-like and its colouring is black or chocolate brown with white underneath. It lives in the waters of the North Pacific, North Atlantic and the Baltic Sea.

Dall's porpoise is larger than the aforementioned two. It measures 7 - 7 ft, weighs about 380 - 440 lb and is known for its high speed. Its teeth number between 23 and 28 pairs in each jaw. Its head and flippers are small compared with its body size and it is mostly black with a portion of white flank. It inhabits the waters of the North Pacific in small groups which quite often join up to form thousands. It feeds on fish and squid and dives to middle depth but is mostly a surface eater.

The Vaquita, also called the Gulf of California porpoise owing to its habitat, is critically endangered. It is about 5 ft long and weighs about 105 lb. Living in shallow waters less than 130 ft deep make it very vulnerable. The vaquita has a grey body paling to white underneath and has a dark patch around its eyes. Although mostly solitary it can live in small groups. Its diet consists of small fish and squid. About 30 deaths a year are reported as a result of trapping by fishing nets. Also, deaths are being caused by pollution and by the disturbance of boat noise. Another threat to the vaquita's survival is the exploration of oil.

Native American tribes living near the oceans have many stories about porpoises and dolphins leading people to safety by steering them to the shore out of rough waters or from driving away menacing sharks. Porpoises are occasionally used as a clan crest or carved on totem poles in Northwestern tribes.

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