Why do sharks attack surfers when they are laying down on a surf board?

Sharks have a very keen sense of sight. With this ability, they are able to tell if a person has just eaten or when the person last ate. Sharks are known to attack people with a full stomach to feed of that nutrients as well. It is advisible to not enter the water immediately after eating and to wait a period of 30 minutes to and hour before resuming oceanic activities.