We know pretty early on in "The Fish" that having caught the fish, the speaker has to decide whether to keep it or release it. Either decision, of course, has consequences. If the speaker keeps the fish, the fish will die (and become dinner). If the speaker lets him go, then what? Well, from the outcome of the poem, it seems that the speaker feels quite satisfied and fulfilled with her decision to release the fish. It's up to you to figure out why.
The real reason the speaker let the fish go was because she was too afraid to kill it.