Living Sanibel: Bonnethead and scalloped hammerhead sharks found within inshore waters
The smallest shark found in the waters surrounding Sanibel and Captiva, the bonnethead seldom tops 15 pounds. It is the smallest member of the hammerhead family of sharks and is readily identified by its unusually shaped head. It is too small to deliver a serious bite, and there is only one unprovoked bonnethead shark attack on record. Most bonnetheads are caught while anglers are targeting other species. It fights hard on light tackle, but is not considered table fare.
Because of its small size the bonnethead has avoided the plague of finning. Throughout its range, which runs from northern South America to the Carolinas, as well as the southern California coast, the bonnethead population is not endangered.
The bonnethead feeds on clams, crustaceans, crabs, and some fish. It uses an electro-magnetic sensory mechanism, known as the ampullae of Lorenzini, to detect the minute electrical charges given off by living organisms. In essence, it is able to hunt by auras, which are the invisible electrical fields radiated by all living things. When a bonnethead senses one of these fields buried in the bottom, it bites down without knowing what creature lies below, be it a flounder or a scallop. This organ, which does not exist in humans, is effectively a sixth sense.
Very similar to the great hammerhead in size and appearance, the scalloped hammerhead differs only in the wave-like or scalloped pattern across the front of its large hammer-shaped head. Of the 10 species of hammerhead sharks worldwide, three are common to the waters around Sanibel and Captiva.
The range of both the scalloped and the great hammerhead covers all the major tropical and subtropical oceans and seas in the world. The exact function of the hammerhead’s strangely shaped head is still being debated among marine biologists. With its eyes located at either end of the hammer, its stereoscopic vision is extraordinary, but other sharks do fine without this configuration, so that alone does not explain this adaptation.
The scalloped hammerhead sometimes schools in vast numbers. The seamounts located off Baja, Mexico, have recorded massive schools numbering in the thousands. While large enough to be a threat to man, hammerheads overall are not likely to attack. Worldwide there have been only 21 unprovoked attacks, resulting in two fatalities.
Locally the scalloped hammerhead is seldom found inshore. Like its larger cousin, the great hammerhead, it tends to stay well offshore where it is a fairly common by-catch of tarpon fishermen. Its flesh is edible, but it is not considered a good eating shark. Commercially it is killed for its fin, liver, and excellent hide. Most are taken by long-lining.
The hammerhead dines extensively on stingrays. One adult specimen that was killed for research had 50 stingray spines embedded in its stomach. It also eats barracuda, kingfish, dolphins, parrotfish, and blacktip sharks. Nothing preys on the adult scalloped hammerhead, but the pups are vulnerable to an array of predators, including other sharks.
This is an excerpt from The Living Gulf Coast – A Nature Guide to Southwest Florida by Charles Sobczak. The book is available at all the Island bookstores, Baileys, Jerry’s and your favorite online sites.