From its very first line, "The Fish" gleams with relish and mystery. The speaker recalls catching not simply a big fish, but a "tremendous" fish—a word that suggests the fish was both huge and spectacular. This was a fish to remember, in other words, a fish to tell stories about. And the rest of the poem will indeed be the story of this speaker's encounter with that tremendous fish.
The speaker's first feeling about this fish is surprise. The fish doesn't seem to be struggling to get free, but hanging heavily from the line, looking "battered and venerable" as some grand old man. And, indeed, the speaker personifies the fish, calling it "him"—already seeming to feel not just pride in having caught such a monster, but connection and fellow-feeling with this peculiar creature.
As it will turn out, it's not just the speaker that catches the fish, but the fish that catches the speaker. Right away, the speaker seems fascinated by the fish: its strangeness snags the speaker's attention just as firmly as the speaker's hook holds "fast in a corner" of the fish's mouth.
As the poem's free verse takes shape, forming one long column of short lines, it reflects the speaker's intense, minute attention to all this fish's details. This will be a poem about the "tremendous" things that can happen when one really stops to look hard at the natural world.