they have become almost totally blind, 他们的双眼几乎完全失明

156. A newly identified species of river dolphin
     这种淡水豚是一个新种
157. found nowhere else on Earth.
     在地球上其它地方均无分布
158. In these black, tangled waters,
     由于生活在黑暗而复杂的河水里
159. they have become almost totally blind,
     他们的双眼几乎完全失明
160. so they move slowly,
     所以他们游得很慢
161. carefully using their sonar to scan the forest floor for fish.
     仔细地用声纳扫描林地表层寻找鱼类为食
162. If this forest can hide a new species of dolphin...
     如果这片森林里能隐居着新种江豚

来自《行星地球 第二季 第三集 Planet Earth S02E03 丛林》第 159 行