Published August, 2005
Dear Sea Scoop!
While visiting Lake Nicaragua, I was told that the lake has the only freshwater sharks in the world. Can sharks live in fresh water and if so are they big enough to be dangerous to humans?
– Susie Curtis, St. Croix, VI
Sharks are in the class chondrichthyes (kon-DRICK-these), an ancient group of fish that are cartilaginous (kar-tih-LAJ-in-us). Cartilaginous fish have skeletons that are made of cartilage (like we have in our nose and ears) rather than bone. Members of the cartilaginous fish also include all rays and skates.
Sharks are mainly marine (saltwater) inhabitants, however many sharks and rays can and do spend part of their lives in freshwater. As many as 43 species can travel miles into brackish and freshwater. Some of these, such as the smooth dogfish and sandbar shark travel to estuaries to give birth or feed. Others can travel many miles upriver and can spend weeks, months or even years in freshwater. The "freshwater sharks" of Lake Nicaragua are bull sharks (Carcharhinus leucas), a marine species that has adapted to survive in freshwater. These sharks are capable of freely moving between the Caribbean Sea, a saltwater body, and Lake Nicaragua, a freshwater body, by way of the Rio San Juan. Bull sharks can be found in tropical and subtropical waters worldwide. Researchers have found that this fierce predator can travel 4,200 kilometers (2,600 miles) upstream, and can spend between 4-6 years in freshwater. One bull shark was even found in the Mississippi River in Illinois!
The key to this is the bull shark's amazing capacity for osmoregulation (AHS-moh-reg-you-LAY-shun) the exchange and balance of water and mineral salts in the blood. Saltwater fishes have adapted to ensure that they keep enough freshwater inside their body to avoid dehydration. When bull sharks move to freshwater, their osmoregulation must readjust to keep salts from moving out of their body to the less salty environment.
Bull sharks are considered one of the deadliest sharks in the world. They can grow quite large. The maximum reported length of a bull shark is 3.5 meters, or about 11 feet. They are found in freshwater as well as very shallow coastal waters, and their fearless nature makes bull sharks more of a potential threat to humans than either the great white or tiger sharks. According to the International Shark Attack File, bull sharks are historically responsible for at least 69 unprovoked attacks on humans around the world, 17 of which resulted in a fatality. Bull sharks are the likely culprit in the 1916 New Jersey shark attacks which resulted in 5 deaths in a two-week time period. Three of the attacks were in a shallow creek 15 miles away from the ocean.
Sharks are fascinating animals that have been around for millions of years. More and more people are beginning to understand that sharks are not the mindless feeding machines we once thought. There are approximately 350 species of sharks in the world, 80% of which are either incapable of or unlikely to cause harm to humans. In fact, in the United States the odds of being attacked by a shark are about 1 in 6 million. Compare that with the odds of a death resulting from falling down the stairs at 1 in 200,000 and you should feel much safer on your next trip to the beach – although maybe not as safe coming down the stairs!. Sharks play a very important role in the ocean ecosystem. As the apex predator they help keep fish and other marine animal populations in check by culling out the sick, weak or young. To me, the most frightening shark statistic is that human beings are responsible for killing between 20 and 30 million sharks per year. Sharks are fished for their meat and fins, which are considered a delicacy in some parts of the world. Many scientists say the number of sharks worldwide has dropped by 50 percent over the past fifteen years. Among some kinds of sharks, the number may have decreased by more than 70 percent. Only recently have they been afforded the protection they need 208 shark species have been identified and put on the IUCN Red List of Threatened Species as either endangered, threatened or in need of further research.
For more information on sharks:
Discovery Channel's Shark Week
Freshwater Sharks, Skates and Rays
International Shark Attack File
NOVA Online Adventure: Island of the Sharks
ReefQuest Centre for Shark Research
World Conservation Organization Shark Specialist Group
For more information on marine conservation:
The Ocean Conservancy
Marine Conservation Biology Institute
The Nature Conservancy Marine Initiatives
Have a question about the world beneath the waves? Write it down and send it to Sea Scoop! Please remember to include your name and where you're from.
For more information on marine science in the Virgin Islands, visit the University of the Virgin Islands' Center for Marine & Environmental Studies.
Elizabeth Ban is the University of the Virgin Islands Marine Adviser for St. Thomas and St. John. She works to inform and educate citizens about ocean resources and promote coastal ecosystem health. She is based at UVI's Center for Marine and Environmental studies on the St. Thomas Campus. For more information about UVI's Marine Advisory programs, please call 340-693-1392.
Note: The photo of the Bull Shark was provided by Kevin McGimpsey.Click Here to visit his Web site.
Sea Scoop! #9: Fresh Water Sharks
Published August, 2005
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