are returning to these streams to breed. 洄游至这些溪流以繁殖

171. It's autumn, and the beginning of a spectacle that each year
     正值秋天 一场年度奇观拉开了帷幕
172. transforms this forest and all that lives in it.
     为这片森林及万千生命带来了巨大变化
173. After years at sea, thousands of Pacific salmon
     在海中生活数年后 成千上万条太平洋鲑鱼
174. are returning to these streams to breed.
     洄游至这些溪流以繁殖
175. And they attract the forest's rarest resident.
     这项盛事吸引了森林中最罕见的居民
176. A spirit bear.
     白灵熊
177. A white variant of the black bear.
     黑熊中的一种白色亚种

来自《行星地球 第三季 第五集 Planet Earth S03E05 森林》第 174 行