How to fish and how to get a pearl of light ?

During a quest of Nyok in the forest he needs you to build a "Fish Cabin".

Build it and when it's built you just touch the water near the shadow of the fish and move the fish bait by tilting your phone right to left... When the fish rise to the bait slide up with your finger and you get a fish !

To get the pearl of light you need to fish the glow fish during the night !