Attacks have also frequently occurred when humans were spear fishing in ocean waters. Sharks are attracted to the signs given off by dying fish -- the smell of blood in the water and the electrical impulses given off as the fish struggles. Sharks detect these signals with their ampullae of Lorenzini, a set of "detectors" under the skin on a shark's snout. The ampullae are electrically sensitive cells that connect to the skin's surface through small tubes. Once a shark arrives on the scene, it may become agitated and aggressive in the presence of so much food. A hungry, excited shark can easily mistake a human for its usual prey.
There are cases in which sharks seem to attack out of aggression, rather than hunger. Very little is known about shark behavior, but it is believed that some species, including great whites, display dominance behavior over other sharks. This behavior can take the form of "punching" with the snout, or bites that don't do much damage to the tough skin of a shark. Unfortunately, when a shark makes a dominance display toward a human, these "gentle" bites can still cause horrendous damage.
Sometimes, the cause of a shark attack is simple to determine -- the shark is responding to human aggression. Nurse sharks, for example, are generally placid fish that lie still along the bottom of the ocean floor. For some reason, this makes some divers think that it's a good idea to pull their tails. Irritated nurse sharks have taught several divers to keep their hands to themselves. For this reason, shark attack statistics are divided between provoked and unprovoked attacks.