fish can swim为什么不用fishes

fish的单复数相同。一条鱼：a fish 两条鱼：two fish。fishes表示多种鱼，指种类。例如：

There are many fishes in the river. 河里面有多种鱼。

fish它既可以做可数名词也可以做不可数名词

做可数名词时反复同形，做不可数名词时是鱼肉的意思

例句：He likes eating some vegetables and some fish for dinner.