绪论 虎鲸类型和种群 (第5/7页)

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Transient or Bigg's: The diets of these orcas consist almost exclusively of marine mammals. They live in the same areas as residents， but the two avoid each other. Transients generally travel in small groups， usually of two to six animals， and have less persistent family bonds than residents. Transients vocalize in less variable and less plex dialects. Female transients are characterized by more triangular and pointed dorsal fins than those of residents. The grey or white area around the dorsal fin， known as the “saddle patch“， often contains some black colouring in residents. However， the saddle patches of transients are solid and uniformly grey. Transients roam widely along the coast; some individuals have been sighted in both southern Alaska and California. Transients are also referred to as Bigg's orca in honour of cetologist Michael Bigg. The term has bee increasingly mon and may eventually replace the transient label. The transient ecotype is estimated to have diverged 700，000 years ago. There are at least three different “stocks“ of transients off North America， the AT1 stock which occurs from Prince William Sound to Kenai Fjords， the Gulf of Alaska/Aleutian Islands/Bering Sea GOA/AI/BS stock and the west coast stock which ranges from southeast Alaska to California. AT1 is considered a depleted stock; it was affected by the Exxon Valdez oil spill and declined from 22 individuals to eight between 1989 and 2004. The GOA/AI/BS stock may number around 500 whales while the west coast transients number over 320 orcas with over 200 along southeast Alaska， British Columbia and Washington and over 100 orcas off California. California transients do not appear to intermingle much with those further north and west coast transients may be divided into sub-munities.

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Offshore: A third population of orcas in the northeast Pacific was discovered in 1988， when a humpback whale researcher observed them in open water. As their name suggests， they travel far from shore and feed primarily on schooling fish. However， because they have large， scarred and nicked dorsal fins resembling those of mammal-hunting transients， it may be that they also eat mammals and sharks. They have mostly been encountered off the west coast of Vancouver Island and near Haida Gwaii. Offshores typically congregate in groups of 20–75， with occasional sightings of larger groups of up to 200. Little is known about their habits， but they are genetically distinct from residents and transients. Offshores appear to be smaller than the others， and females are characterized by dorsal fin tips that are continuously rounded.

Separate fish-eating and mammal-eating orca munities also exist off the coast of the Russian Far East and Hokkaido， Japan. Russian orcas are monly seen around the Kamchatka Peninsula and Commander Islands. Over 2，000 individual resident-like orcas and 130 transient-like orcas have been identified off Russia. At least 195 individual orcas have been cataloged in the eastern tropical Pacific， ranging from Baja California and the Gulf of California in the north to the northwest coast of South America in the south and west towards Hawaii. Orcas appear to regularly occur off the Galápagos Islands. Orcas sighted in Hawaiian waters may belong to a greater population in the central Pacific.

North Atlantic and adjacent

Orca tail-slapping in Vestfjorden， Norway

At least 15，000 whales are estimated to inhabit the North Atlantic. In the Northeast Atlantic， two orca ecotypes have been proposed. Type 1 orcas consist of seven haplotypes and include herring-eating orcas of Norway and Iceland and mackerel-eating orcas of the North Sea， as well as seal-eating orcas off Norway. Type 2 orcas consist of two haplotypes， and mainly feed on baleen whales.

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In the Mediterranean Sea， orcas are considered “visitors“， likely from the North Atlantic， and sightings bee less frequent further east. However， a small year-round population exists in the Strait of Gibraltar， which numbered around 39 in 2011. Distinct populations may also exist off the west coast of tropical Africa， which have generalized diets.

The northwest Atlantic population is found year-round around Labrador and Newfoundland， while some individuals seasonally travel to the waters of the eastern Canadian Arctic when the ice has melted. Sightings of these whales have been documented as far south as Cape Cod and Long Island. This population is possibly continuous with orcas sighted off Greenland. Orcas are sighted year-round in the Caribbean Sea， and an estimated 267 as of 2020 is documented in the northern Gulf of Mexico.