"The Fish" might be one of the most clear-cut examples of man vs. the natural world. The cool thing about this struggle, though, is that there is very little struggle at all. The speaker catches the fish (though the fish doesn't fight), then holds the fish out of the water for a bit (the fish is still not fighting), and ultimately lets him go. So while the scenario allows for the age-old man vs. wild battle, there is no real violence. The struggle happens within the speaker, and ultimately ends peacefully.
The fish let the speaker catch him because he knew all along that the speaker would release him.
The speaker releases the fish because she sees it as too human to kill.