fish用作可数名词指“鱼的条数”时单数和复数形式相同(two fish两条鱼),指“鱼的种类”时复数形式才为fishes(two fishes两种鱼)
How can you tell what sex a fish is?
你怎样辨别一条鱼的雌雄?
I don't like fish.
我不喜欢鱼。
You need a licence to fish in this river.
你在这条河里钓鱼要有许可证。