To determine the theme of a poem, you first read the entire work just to get a sense of what the author is communicating. Then you look for a universal idea, something that people could understand and relate to no matter where they live or who they are. One thing you have to keep in mind is that the theme to anything is largely a subjective matter; if you asked ten people for the theme to this poem, you’d probably get ten different answers.
After reading The Fish, I would say the theme is something like “Sometimes we find beauty and value in places we don’t expect.”
Once you’ve identified your theme, you want to be able to support it with evidence from the poem. Let’s look at it.
At first the speaker tells us that she caught a “tremendous” fish. That’s a pretty simple description. So we know it’s a big fish. Then she gives a few details about its appearance, and these details aren’t particularly flattering. The fish is old with strips of skin “like ancient wallpaper.” At this point it’s just a fish, nothing special.
But then the speaker begins to examine the fish more closely. It has “frightening gills, fresh and crisp with blood,” and then she imagines what it looks like on the inside: “dramatic reds and blacks,” and a “pink swim-bladder.” The fact that she’s using her imagination in connection with the fish implies that it has a value beyond simply being caught.
Finally, she looks at its face and into its eyes. The poet resists the temptation to get sentimental here. She doesn’t say anything to imply that they were actually looking at each other or that there was any understanding between them. Then she sees the old hooks in his mouth and understands that this fish has been caught repeatedly before, but it was always able to break the line and get away. The poet creates a nice simile here, and says that the hooks are “like medals.” This is the dawning of the respect that the poet feels for the fish.
Then, at the end, she describes the boat that she is in. It too is old and worn, with a “rusted engine,” and “sun-cracked thwarts.” It seems that the fish and the boat represent the age-old struggle of the hunter and the hunted. She feels a sense of victory, she has caught the fish that always got away before. But her respect for the old fish overrules her desire to keep it, and she lets it go.