At vero eos et accusamus et iusto odio dignissimos ducimus qui blanditiis praesentium voluptatum deleniti atque corrupti quos dolores et quas molestias excepturi sint occaecati cupiditate non provident, similique sunt in culpa qui officia deserunt mollitia animi, id est laborum et dolorum fuga.
Et harum quidem rerum facilis est et expedita distinctio. Nam libero tempore, cum soluta nobis est eligendi optio cumque nihil impedit quo minus id quod maxime placeat facere possimus, omnis voluptas assumenda est, omnis dolor repellendus.
Itaque earum rerum hic tenetur a sapiente delectus, ut aut reiciendis voluptatibus maiores alias consequatur aut perferendis doloribus asperiores repellat.
It seems fairly straight forward to me. She catches a fish. It's and old fish as evidenced by the the lack of fight in it and by it's beat up appearance and by his "yellowed, the irises backed and packed with tarnished tinfoil". By the end of the poem, she begins to understand that the fish had at one time been a tough one and had fought at least five very hard battles. She is filled with a sense of victory for having succeeded where five others failed. And in the end she releases the fish, not out of sympathy, but out of respect. She felt he deserved to live what little life he had left as free as he had ever been.
That's my take anyway.