Why does the speaker in the poem let the fish go?

I caught a tremendous fish and held him beside the boat half out of water, with my hook fast in a corner of his mouth. He didn’t fight. He hadn’t fought at all. He hung a grunting weight, battered and venerable and homely. Here and there his brown skin hung in strips like ancient wallpaper, and its pattern of darker brown was like wallpaper: shapes like full-blown roses stained and lost through age. He was speckled with barnacles, fine rosettes of lime, and infested with tiny white sea-lice, and underneath two or three rags of green weed hung down. While his gills were breathing in the terrible oxygen —the frightening gills, fresh and crisp with blood, that can cut so badly— I thought of the coarse white flesh packed in like feathers, the big bones and the little bones, the dramatic reds and blacks of his shiny entrails, and the pink swim-bladder like a big peony. I looked into his eyes which were far larger than mine but shallower, and yellowed, the irises backed and packed with tarnished tinfoil seen through the lenses of old scratched isinglass. They shifted a little, but not to return my stare. —It was more like the tipping of an object toward the light. I admired his sullen face, the mechanism of his jaw, and then I saw that from his lower lip —if you could call it a lip— grim, wet, and weaponlike, hung five old pieces of fish-line, or four and a wire leader with the swivel still attached, with all their five big hooks grown firmly in his mouth. A green line, frayed at the end where he broke it, two heavier lines, and a fine black thread still crimped from the strain and snap when it broke and he got away. Like medals with their ribbons frayed and wavering, a five-haired beard of wisdom trailing from his aching jaw. I stared and stared and victory filled up the little rented boat, from the pool of bilge where oil had spread a rainbow around the rusted engine to the bailer rusted orange, the sun-cracked thwarts, the oarlocks on their strings, the gunnels—until everything was rainbow, rainbow, rainbow! And I let the fish go. – See more at: http://www.poets.org/viewmedia.php/prmMID/22238#sthash.b0CGPOGK.dpuf

Why does the speaker in the poem let the fish go?

Look for specific word choices/lines in the poem to help you support your answer.