"The bull shark is an amazing shark in so many ways," Cousteau said. "It can switch from salt to fresh water, which is a rare thing. For most sharks, that would be deadly."
Sharks need salt inside their bodies; without it, their cells can expand and possibly rupture, leading to bloating and death. If most sharks, including great whites, enter freshwater, their internal salt levels become diluted.
Bull sharks have special physiological features that enable them to live in freshwater. A gland near the tail helps them retain salt, and the kidneys are designed to recycle the salt already in the body.
So it may have been a bull shark in Matawan Creek that took the lives of Lester Stilwell and Stanley Fisher.
Shark expert Scott Davis, who tracks the movements of great whites, said he doesn't know what the salinity level of the creek was in 1916, but the presence of a great white in such an area would be unusual.
"In my tracking studies, I've never noticed great whites going upstream," he said. "There are a number of places on the West Coast where they inhabit right at river mouths, but whether they go upstream no one knows. My perspective is that great whites would not be able to tolerate a freshwater environment at all."
Bull sharks, in contrast, have been reported in waters thousands of miles up the Amazon River in Peru and in the Mississippi River as far north as Illinois. There is also a population of the sharks in Lake Nicaragua, which because of its great distance from the ocean was once thought to be landlocked.
Sharing the Water
Bull sharks, which are generally about 7.5 feet (2.3 meters) long and up to 285 pounds (130 kilograms), are big eaters. Their diet consists primarily of bony fishes and small sharks, including young bull sharks, but they have been known to feast on everything from seabirds to dogs.
That appetite could spell trouble for humans who enter the same coastal and freshwater areas as the sharks.
"Bulls have a certain pattern of survival, a way of doing things, that happens to overlap with the way people encroach on their realm," Cousteau said. "Bathers are in shallow, warmer coastal waters where these sharks are living. It's amazing that we don't have more instances, and it just reconfirms that they really aren't interested in us and usually an attack is a mistake."
Last summer a bull shark was involved in a highly publicized incident in which a shark severed a young boy's arm. But experts say bull sharks aren't inherently violent toward humans, and in some environments are even docile. In the clear waters of the Bahamas, for example, divers regularly interact with crowds of bull sharks.
Some researchers speculate that this non-threatening behavior may be because the sharks can clearly see people and recognize that they are not a typical food source. By such reasoning, incidents in murkier waters, such as Matawan Creek, might be regarded as cases of mistaken identity.
Whatever the motivation, shark attacks on humans are relatively rare. According to the International Shark Attack File (ISAF), hosted by the Florida Museum of Natural History, there were 76 unprovoked shark attacks and five fatalities worldwide in 2001, a fairly typical year.
Cousteau said lack of caution in human actions often contributes to the shark attacks that do occur, such as swimming at dusk or dawn, splashing in murky water, and entering known feeding waters or areas where people are fishing or chumming.
"We pin the few shark instances that occur around the world on the sharks," he mused, "and never on us for maybe putting ourselves in bad situations."
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