Natural Inquiries: The Bull Shark's Double Life

Three of the planet’s nearly 400 species of shark are responsible for most of the unprovoked traumatic attacks on people; one of them can survive in freshwater

• Michael Tennesen
• Animals
• Nov 15, 2011

ANGLERS HAVE BEEN KNOWN to stretch the truth at times, but Willy Dean wasn’t exaggerating when he described the fish he caught in the Potomac River in the fall of 2010. “When I first seen it, it was like Jaws,” he told The Washington Post. The Maryland fishing enthusiast was referring to the nearly 8-foot, 300-pound bull shark that landed in one of his nets. What was a shark doing in freshwater, far from the Atlantic Ocean? As it turned out, the creature simply was doing what comes naturally.

“There’s nothing unusual about bull sharks in the lower Potomac and the Chesapeake Bay area,” says George Burgess, director of the Florida Program for Shark Research and curator of the International Shark Attack File. The species ranges worldwide in tropical and subtropical waters, according to Burgess, and “it is the only large shark that can survive in both freshwater and saltwater.”

Most ocean sharks need salt in their bodies to prevent their cells from expanding and rupturing. But bull sharks have a special gland near their tails that helps retain salt and they have kidneys that recycle the substance when they enter freshwater.

In recent years, the predators have been sighted in the Mississippi River, Florida’s Indian River Lagoon and more than 2,000 miles from the ocean in the Amazon River. But it is the creatures’ habit of ranging in shallow coastal waters that can cause problems for people.

Three of the planet’s nearly 400 shark species are responsible for most of the traumatic unprovoked attacks on humans: tiger sharks, white sharks and bull sharks. The first two “are essentially offshore species that occasionally venture near shore,” says Chris Lowe, head of California State University’s Shark Lab in Long Beach. “But bull sharks are a true near-shore species.”

Bull sharks rarely occur north of Mexico on the Pacific Coast, but they range from Massachusetts to Florida along the Eastern Seaboard. Thirteen of the planet’s 79 confirmed shark attacks in 2010 occurred in the Sunshine State, and bull sharks accounted for some of those, including one fatality—one of only two U.S. shark fatalities that year. (By comparison, 29 people died from lightning strikes in the United States in 2010.)

University of North Florida biologist Jim Gelsleichter currently is investigating the bull shark’s hormone levels to help understand its nature. Named for its blunt snout and habit of head butting prey, it is one of the few shark species that will tangle with creatures larger than itself. “An earlier study found high levels of testosterone in one of the three bull sharks it looked at,” says the scientist, who is attempting to validate those results. “In Florida, we suggest that people avoid swimming in murky waters when visibility is limited,” adds Burgess.

Female bull sharks frequently bear their young in estuaries, bays, harbors and river mouths, enabling their offspring to spend their early years in shallow areas where they are safe from marine predators.
Burgess and his associates have been tracking bull sharks in Florida’s Indian River Lagoon and have discovered that the predators often feed on stingrays. “We found one bull shark that had a stingray spine that went completely though its heart—in one side and out the other,” Burgess says. “Yet the shark appeared to be perfectly healthy. It is one tough animal.”

Still, the sharks’ habit of spending time near shore makes them susceptible to certain pollutants. In one study, Gelsleichter found trace amounts of prescription medications in 90 percent of the juvenile bull sharks he examined. He also is concerned about the potential effects of the 2010 Gulf oil spill on the predators. Oil can be a slow killer, cautions the biologist, and effects “won’t show up for years.”

While bull shark numbers in U.S. waters declined during the 1990s, stricter fishing regulations have helped the species recover along parts of the East Coast and in the Gulf of Mexico. However, increasing numbers should not be considered “a warning to run and get out of the water,” says Burgess. “We face much greater risks everyday in our lives. Believe me, shark recovery is a good thing for the future of the oceans.”


NWF Priority: Protecting the Marine Environment

As part of its efforts to address threats to marine ecosystems, NWF has formed a long-term partnership with the prestigious Mote Marine Laboratory in Florida. Initially, the two organizations will establish an innovative research, education and public outreach initiative focused on science-based coral reef ecosystem restoration in the face of increasing sea surface temperatures and ocean acidification.

California journalist Michael Tennesen wrote about white shark movements in the Pacific Ocean in the August/September issue.