why does the speaker let the fish go

Bishop’s poem gives a highly detailed picture of a
“venerable” heroic fish that, withits “medals” and its “beard of wisdom,” becomes a
symbol of courageous endurance.From the colors of the fish, seen and imagined (“brown
skin,” “darker brown,”“rosettes of lime,” “tiny white sea-lice,” “white flesh,”
“dramatic reds and blacks,”“pink swim-bladder,” “tinfoil”), and from the colors of the
old fish-lines, the poem moves to the rainbow in the oil in the bilge (the lowest part
of the hull). The rainbow—the sign of hope and of God’s promise to Noah to spare
humanity—grows in the imagination until it fills “the little rented boat,” illuminating
(we might say) the speaker, who, perceiving the heroic history of the captive, forbears
to conquer and returns the fish to the water.