mostly within Vietnam. Unlike bighead and silver carp, largescale silver carp have not been widely introduced around the world for use in aquaculture, although at least one introduction was made to some waters of the Soviet Union, where they hybridized with the introduced silver carp. In North America Silver carp were imported to North America in the 1970s to control algal growth in aquaculture and municipal wastewater treatment facilities, but escaped from captivity soon after their importation, and are now considered a highly invasive species. Silver carp, with the closely related bighead carp, often reach extremely high population densities, and are known to have undesirable effects on the local environments and native species, including the bigmouth buffalo. They have spread into the Mississippi, Illinois, Ohio, Missouri, Tennessee, Wabash Rivers, and many of their tributaries in the United States, and are abundant in the Mississippi catchment from Louisiana to South Dakota and Illinois. Dams seem to have slowed their advance up the Mississippi River, and until late November 2008, silver carp had not been captured north of central Iowa on the Mississippi. Dams that do not have navigation locks are complete barriers to natural upstream movement of silver carp, unless fishermen unintentionally assist this movement by the use of silver carp as bait. In 2020, Alabama Department of Conservation and Natural Resources found silver carp in Alabama's Pickwick and Wheeler reservoirs on the Tennessee River, but the species has not expanded its range in Alabama’s waterways. The Tennessee Valley Authority (TVA) has considered several methods to control the spread of Asian carp, including fish barriers at 10 locks controlled by the TVA. One is a bioacoustics fish fence, which uses a combination of sound, light and air bubbles. These barriers are installed at Barkley Lock and Dam in Kentucky, and are currently being studied for their effectiveness in deterring Asian carp. Other types of barriers used for Asian carp include carbon dioxide and electricity. The TVA has conducted environmental impact studies to minimize the impact of the barriers on native species. The TVA has also considered adjusting flow rates during Asian carp spawning periods, which are usually during high-water events, as Asian carp eggs are only semibuoyant and will sink to the bottom and die with low river flow. The silver carp is sometimes called the "flying" carp for its tendency to leap from the water when startled; it can leap