Whereas prior to this the speaker sees the fish both as something to be feared and as something to catch, when the speaker notices the evidence of all the other fishermen that the fish has outwitted or survived, the attitude towards the fish is shown to change dramatically. Notice how the speaker talks about the fish after noticing these details:
Like medals with their ribbons
frayed and wavering,
a five-haired beard of wisdom
trailing from his aching jaw.
The speaker sees the various bits of fishing line and other evidence that the fish has escaped so many fisherman as "medals" for this fish, who is compared to some kind of old war-veteran with his "beard of wisdom." Suddenly, the fish in the mind's eye of the speaker is transformed from being merely a thing to catch to a figure that deserves respect and whose wisdom needs to be acknowleged. When this epiphany breaks upon the speaker, we are told that "victory filled up" the boat as the speaker decides to let the fish go and not to kill it for his or her own amusement.