I caught a tremendous fishand held him beside the boat (lines 1-2)
Maybe the real-life alternative to this line would be: "I caught a tremendous fish/ and dragged him into the boat to club him dead." Because the speaker hangs the fish there, we immediately sense her hesitation and a choice being made.
While his gills were breathing inthe terrible oxygen. (lines 21-22)
Here we see the speaker's choice is a matter of life and death. The fish can't breathe out of water and she is choosing to keep the fish hanging there, aware of what's happening and what is ultimately at stake.
I stared and staredand victory filled up the little rented boat (lines 65-67)
This is the tipping point in the speaker's choice. This victory/rainbow business seems to be what determines her decision to let the fish go.