On the surface level, "The Fish" recognizes the age, experience, and strength of the creature that has been caught. This "tremendous fish" has the appearance of one who has lived a long and active life and has earned a certain respect simply for having survived for that length of time. Upon closer examination, the speaker spies the "five big hooks grown firmly in his mouth" - testaments to past escapes from other anglers. In tribute to the determination and strength and beauty of her catch, the speaker releases the fish to freedom once again.
On a deeper level, Bishop is portraying the experiences of every woman, including herself. As a strong believer in the rights of women to earn and receive respect based on their efforts, Bishop is using the metaphor of the fish's life experiences to meditate on the experiences of women. Women acquire beauty and experience and strength through their lives; others may attempt to capture and restrain them, but the hope is that they will be released to continue in their lives as determined and strong individuals in their own right.