Adapted from The Shark Almanac by Thomas B. Allen

After some twenty years of studying sharks, Dr. Samuel Gruber, professor of marine science at the Rosenstiel School of Marine and Atmospheric Science at the University of Miami, once asked how anyone can predict anything about"something as wild and immune from investigation as an attacking white shark." We "simply cannot know what is in the mind of an animal," he added.

Research, however, is adding to our knowledge about the way sharks hunt and attack. Sharks, for example, can detect and will swim toward extremely weak electrical fields that surround their prey. This was demonstrated by A. J. Kalmijn, who showed that sharks would attack an electrical source that produced a field similar to that produced by a fish. Gruber suggested that a shark snapping at a nonfish was responding to a stimulus and was making a "reflexive bite." That is the kind of bite a shark might make in an attack on an object from which is emanating a stray galvanic field, such as a metallic boat or oceanographic gear. Some sharks have a reputation as "boat biters," a habit that researchers have attributed to responses to the boat's electrical field.

Don't try this at home — this
diver is wearing a chainmail suit.

Gruber estimated that"somewhat less than half of all attacks (excluding open-ocean and some other 'special' situations) are motivated by feeding or predation." Another researcher, Donald R. Nelson, a professor of biology at California State University, Long Beach, calls pelagic sharks "apex predators exquisitely tuned to their special sensory worlds." After studying shark behavior in natural surroundings, Nelson identified four general attack situations:

Unprovoked: A person in the water who has not stimulated the shark is nevertheless attacked (usually by a large shark). The assumption is that the person was regarded as prey and the shark took the opportunity to feed.

Unprovoked with "distress stimuli": One or more people in the water — typically, victims of an aircraft or ship disaster — are thrashing about. Perhaps there is blood in the water. Responding to "strong feeding stimuli," Gruber wrote, even small sharks "may attempt to bite pieces from the victim. The presence of abnormally intense feeding stimuli may cause a group of sharks to transcend their usual predatory behavior patterns and enter a state known as a feeding frenzy."

Provoked with physical contact: Typically, divers grabbing the tail of a shark and taking a ride. Most such "hobby horses" are nurse sharks. One study of attacks on divers showed that nurse sharks were responsible for eighteen attacks, compared to eleven involving white sharks. In many cases, the so-called "harmless" nurse shark took a "defensive bite" and rapidly swam away.

Provoked without physical contact: The victim and shark meet unexpectedly. Cornered or perhaps defending territory, a shark makes an aggressive gesture that the person fails to recognize. The shark then strikes and speeds away.