a large quantity of fish 谓语怎么用?单数还是复数?答案上是单数 我想不通...fish不是单复

a large quantity of fish 谓语怎么用?单数还是复数?答案上是单数 我想不通...fish不是单复数同形吗?那two fish做主语呢?
注明:a large quantity of 可以加可数和不可数名词.
a large quantity of fish这里是指鱼,不是肉