He didn't fight.He hadn't fought at all. (lines 5-6)
Just when we think we're going to have a huge power struggle we find out the old fish isn't even going to put up a fight! At this point we think the speaker has all the power.
He hung a grunting weight,battered and venerable (lines 7-8)
Line 7 reiterates lines 5 and 6, but line 8 complicates things by assigning the fish to a position of respect. Think about it, those who you respect have some power over you (your parents, your teachers, etc.). It seems at this point the speaker has the physical power (his hook is in the fish's mouth), but the fish holds a different kind of power.
—the frightening gills,fresh and crisp with blood,that can cut so badly— (lines 24-26)
This is so strange. While the speaker is thinking about the gills which cannot help the fish breathe outside of water, she makes the fish seem powerful by pointing out that the gills are sharp and could potentially cut the speaker really badly.