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Sharks in freshwater
By MIKE SCARANTINO © St. Petersburg Times, published April 28, 2001 It's a fact bull sharks swim upstream and exist in freshwater for extended periods of time. In certain places around the world, they've been known to travel thousands of miles upriver and inhabit inland lakes for long periods. Bull sharks have what is known as an osmoregulatory system, which allows them to exist in a wide range of salinity levels from hyper-saline to totally freshwater. Other occurrences of these sharks migrating between inland lakes and the Caribbean are well documented. One such place is Lake Nicaragua in Central America. It holds a fair population of this unique species. Bulls have been reported swimming up well known rivers such as the Gambia, Tigris, Amazon and Mississippi. Closer to home, a pair of small bull sharks have made their way to the headwaters of the Homosassa River. They've been residing around the spring at the state wildlife park, and they don't seem to want to leave. They were first spotted on New Year's Day, just as old man winter was blowing his icy breath on Florida. "For the past four months, they've been co-existing happily with the other residents that live over the spring," said Art Yerian, Homosassa State Park wildlife care supervisor. Yerian has been documenting their behavior. "They seem quite content, though I wish they would travel back to the Gulf." During manatee encounters and other necessary functions, park employees have to wade or swim into the spring area. The sharks add an element of discomfort to the experience, though they haven't exhibited any kind of aggression toward those entering the water. Of the more than 360 species of sharks worldwide, bulls top a list of four sharks known to be aggressive toward humans. In order, they are: bull sharks, tiger sharks, the great white and oceanic whitetip. Bulls head the chart by virtue of their shallow-water existence. Bull sharks are aggressive by nature. Being coastal sharks and common to water less than 30 feet deep, they come in contact with humans more than certain other relatives. Characterized as non-descript looking fish, bulls are gray, heavy-bodied sharks with a distinctly short, blunt snout. They can be found in nearly all tropical and subtropical seas throughout the world and reportedly grow to a maximum length of 11 feet. The average life span of a bull shark is 24 years. Females reach a larger size than the males, and sexual maturity is between the ages of 6 and 15 years. Pupping or birthing of their pups happens in the early summer. Adults will come into shoreline lagoons and shallow coastal estuaries and begin the mating dance during the spring of each year. After having their young, adults leave those coastal nursery grounds and head back to deeper water. Juveniles remain behind until reaching maturity. Studies show that this separation is a natural means of protecting the young from predation by their parents. It limits threats to their existence to other predators. When bull sharks are near the shoreline, they pose the greatest threat to humans. Recent attacks, though most are accidental occurrences, are awe-inspiring events. Particularly horrific was the latest case in Pinellas County, where a swimmer went into the canal behind his home and was fatally attacked. It's the shark's shallow-water existence and the fact bulls swim the coastal estuaries and freshwater river systems to breed that brings them in closer proximity with humans for long periods of time. Like other sharks, bulls are opportunistic feeders consuming an array of bony fish, such as tarpon, cats, mullet, jacks, grunts and flats. Rays and skates are other important components of their diets. In the Homosassa spring, Yerian has noticed the heads of catfish and mullet rolling in the current. He thinks the sharks are feeding well. Humans are not on their daily diet, and most attacks are thought to be accidental or acts generated by threat or aggression. It widely is held that during pupping many sharks take on a more aggressive posture, more so when they exist in shallow water. Again, actions like these would lend themselves to the threat of reactive strikes. Ways to avoid accidental attacks are to swim in groups or more populated areas. More attacks occur to solitary swimmers. Don't swim in darkness or at twilight times, when sharks are most active. Avoid water that is murky, or near deep drop-offs. Don't go into the water with an open wound or freshly-speared fish. Lastly, erratic movements made by swimmers are more attractive to sharks. Typically, prey foods that exhibit such behavior bring in the sharks like a dinner bell ringing. You have more chance of being struck by lightning or winning the lottery than being attacked by a shark. Worrying about possible attacks will take the joy out of recreational water sports, though it is nice to know who and what you're swimming with. If you have a question or comment, call Capt. Mike Scarantino at (352) 683-4868.
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