are the piranhas. 它们就是锯脂鲤

220. looking for a chance to strike.
     找准机会发动进攻
221. And waiting in the wings,
     有些鱼则守在一旁等待
222. ready to pick off any injured fish,
     时刻准备攻击任何受伤的鱼
223. are the piranhas.
     它们就是锯脂鲤
224. The feeding frenzy quickly develops.
     它们的进食速率极快
225. Piranha can strip a fish to the bone in minutes.
     锯脂鲤几分钟内便能 将一条活鱼啃得只剩下骨架
226. Great numbers of fish sustain vast flocks of water birds.
     大量的鱼养活着大批聚集的水鸟

来自《行星地球 第一季 第三集 Planet Earth S01E03 淡水》第 223 行