A very important piece of information comes from watching how the fish are behaving.
Fish have a tiny brain compared to humans.
Most of what they do, and how they do it are not from deductive reasoning, but from instinct. Fish also must practice conservation of energy. They cannot swim all over looking for food. The small amount of nutrition from an insect has to be balanced with the amount of energy used to get the food. Food comes to them, usually floating in or on the surface of the water.