battered and venerable (line 8)
The fact that the fish is beat up makes him worthy of the speaker's respect somehow. If you respect something/someone you're humbled by it, right?
I admired his sullen face, (line 45)
This line works the same way line 8 does. The speaker respects and admires this fish, and therefore is becoming humbled by him.
Like medals with their ribbonsfrayed and wavering,a five-haired beard of wisdom (lines 60-62)
OK, so he's gone from a fish to a straight-up wise old man. The respect for this fish has only grown over the course of the poem.