Unfolding the way how to attach bait in Coral Island here. First, go and make bait for yourself. You require any insect or a few of the bronze kelp, either. You should not use any insects if you are trying to save them. So, utilize the kelp only.
Move toward your inventory and then plunk it into your equipment. Now, when you utilize your fishing rod, you will also use the bait. That is how you can attach the bait in Coral Island.