The poet's attitude is an attitude of aloofness at first. The poem starts off with a simple declaration. "I caught a tremendous fish." The sentence falls very flat though. There is no exclamation point. There is no feeling of triumph. There is no vast description of the work that it took to catch the fish. In fact, the narrator says that it didn't take any work to catch the fish.
"He didn’t fight.
He hadn’t fought at all."
The middle third of the poem is a description of how gnarled and grisly the fish looks. It's brown, has algae growing on it, and has barnacles even. It's not a pretty fish. If anything, the tone mingles between aloofness and slight revulsion. I mean at one point the narrator even describes the white sea lice that the fish has. Gross.
The narrator's attitude begins to shift with this line:
"I admired his sullen face"
The word "admired" signals the shift in attitude. An aloof attitude doesn't admire anything. But with that one word, the narrator signals the reader of his/her attitude shift. It continues from there as the narrator describes how tough the fish must be to have survived five fish hooks in its jaw.
Like medals with their ribbons
frayed and wavering,
a five-haired beard of wisdom
trailing from his aching jaw.
The shift in attitude is so profound that the narrator decides to let the fish go.