Who is humbled by whom? At first we might think that the fish is humbled by the fisher, because he doesn't even bother to fight against being caught. We'd probably be wrong though. The rest of "The Fish" demonstrates how much respect the speaker has for the fish, and how humbled she is by the fish's heroic and violent past. Respect and humility go hand in hand. The speaker develops enough respect for the fish's wisdom and experience to be humbled by him.
The speaker's humility isn't a symptom of weakness. In fact, releasing the fish shows the speaker's courage.