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Ocean Watch
By Susan Scott
Monday, November 20, 2000
Last week, readers David and Karen Stoutemyer wrote that while snorkeling in Kaneohe Bay, they noticed several 12- to 18-inch hammerhead sharks swimming near the shoreline. The couple has never seen adult hammerheads while snorkeling, sailing or kayaking in other parts of the bay, but a Marine Corps pilot told them he sometimes sees large hammerheads when he banks over the bay for landings. Hammerhead
sightings no
cause for alarmThese sightings raised good questions for the Stoutemyers about the life cycle of hammerhead sharks and their potential danger to people. The answers should interest everyone using Hawaii's bays.
Hawaii hosts two species of hammerhead sharks, smooth and scalloped. By far the most common one, especially inshore, is the scalloped hammerhead. It grows to about 13 feet long. It gets its name from three indentations along the leading edge of its hammer-shaped head, giving it a wavy look.
When they aren't giving birth, adult hammerheads swim at depths of 600 feet or more off Hawaii's reefs. On the bottom, at night, they eat lobsters, octopus, crabs, bottom fish and their favorite, stingrays. Sometimes, hammerheads approach the surface hunting for reef fish.
When ready to give birth, female hammerheads leave their deep-water homes and enter shallow bays, such as Pearl Harbor and Kaneohe Bay. This occurs primarily in May, June and July.
Hammerhead mothers deliver from 12 to 30 1- to 2-foot-long pups in shallow water. The number of pups a female produces depends on her size; the bigger she is, the more babies she has. After pupping, the moms return to their deep-water territories, leaving the kids to fend for themselves.
Young hammerheads graze along the bay floors, mostly at night. As the youngsters grow, they gradually move to the mouths of the bays and eventually join their relatives in the deep water.
Hammerheads are probably the most abundant sharks in the world's warm waters, yet bites from them are extremely rare. If a bite does occur, it's likely an accident on the shark's part, and the injury is usually relatively minor.
Since 1779 there have been three reports of hammerhead bites in Hawaii. One occurred in 1953 in Pearl Harbor when a 5-foot hammerhead bit a man on the foot and leg while he was crabbing.
In a 1981 incident, a 4- to 5-foot hammerhead bit a girl in five feet of water in Kaneohe Bay. The girl had surgery to repair damaged tendons to her foot. The third bite was in 1989 to a boy riding a body board off Molokai. Eight stitches repaired the child's foot.
One Hawaii researcher, who works extensively with hammerhead sharks, wonders if hammerheads were actually the culprits in the above cases.
Everyone wonders why hammerheads have such odd-shaped heads. One theory is that the flat surface is an efficient forward rudder, making the shark highly maneuverable. Another idea is that eyes and nostrils on the outer edge of the wide head give these sharks a wider range of vision and smell.
One function of the hammer-shaped head may be as a tool to hold down stingrays, hammerheads' favorite meal. An observer once saw a hammerhead shark press a ray to the ocean floor with the front of its head while taking bites from the ray's body.
Hammerhead sharks are not aggressive, have small mouths compared to other sharks and eat animals much smaller than us.
Knowing all these facts makes me feel comfortable about swimming in Kaneohe Bay. I hope it does the same for the Stoutemyers.
Marine science writer Susan Scott's Ocean Watch column
appears Mondays in the Star-Bulletin. Contact her at honu@aloha.net.
