The many, many, shock arrows in every tree nearby.
I think the old man even said to just collect 20 shock arrows.
Because nobody says you: "Defeat it and something good will happen". Even one Zora only asks for a picture, nothing more.
I know he says only collect arrows. Now the moment I went there, the fact that the ''boss'' is there, I thought I had to defeat him in order to get these 20 shock arrows.
After countless death did I see that there are arrows just lying about, and kicked my self for repeating that fight.
The game does not make it clear.