绪论 虎鲸类型和种群(第5/10 页)

of one clan and three pods， and number less than 80 individuals and are listed as endangered.

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