Why does the speaker let the fish go?

In Elizabeth Bishop's famous poem "The Fish," the speaker spends most of the poem describing what she sees as she looks at the fish, and the poem ends, of course, with the speaker letting the fish go. 
Early in the poem, the speaker notes that the fish "didn't fight. / He hadn't fought at all" (5-6). This seems a bit unusual, but then the speaker goes on to record other details and observations about the fish's appearance. The fish is described as "venerable," with "skin . . . / like ancient wallpaper" (8, 10-11). The word venerable, paired with "homely" is an interesting choice on Bishop's part. To venerate something is to respect it, usually due to its age or wisdom. Homely is a less positive word, making the fish seem ordinary and not beautiful. However, it could also mean that the fish is at home or comfortable in that place.
The fish is "speckled" and "infested," which seems to further indicate that the fish is quite ugly and nothing to really be admired; however, his age is reiterated here and later in the poem. The speaker starts to imagine the make-up of the fish's body, the inside textures of the fish, in a way that is revelatory to the speaker. The speaker doesn't get much reaction when she looks into the fish's eyes, but she "admired his sullen face" (45). Here, the turning point of the poem seems to occur: the speaker notices the hooks of other fisherman in the fish's mouth. She writes, 

and then I saw
that from his lower lip
—if you could call it a lip—
grim, wet, and weapon-like
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. (47-55)

It is the speaker's observation that he has been hooked at least four other times and either thrown back or fought his way free, that seems to lead the speaker to her decision to let him go. The speaker goes on to describe the old hooks as "Like medals with their ribbons / frayed and wavering" (61-62). Here the speaker seems to interpret the hooks as trophies to the fish, that he has won over the fishermen who have tried to capture him before. As the speaker continued to look intently at the fish, "victory filled up / the little rented boat" (66-67). It is unclear whether the speaker is referring to the victory she interprets the fish to feel because of its past triumphs or the speaker's sense of victory at having caught a fish that had escaped several times before. The speaker gets excited looking at the colors in the water and then decides to throw the fish back into the water. 
Bishop's poem is ambiguous in the sense that we don't know exactly why the speaker made the decision she did. However, it's possible that multiple meanings can exist at once. It could be that the speaker feels the fish belongs in that water, that he is sort of the old, wise ruler of that environment. It is possible that she feels sympathy and has connected on a level with the fish that does not allow her to kill or eat it. It also could be that she feels having caught and held the fish for the time she did was enough "victory" for her, and she is satisfied with the experience even if she doesn't take her figurative trophy home with her.