AUSTIN, Texas — Years of work to control the nuisance exotic plant hydrilla in Lake Austin on the Colorado River have reduced the hydrilla infestation to its lowest level since it was discovered here in 1999, a result that bodes well for dozens of other Texas lakes facing similar threats.

Hydrilla is currently present in only about nine acres of Lake Austin, according to the July survey by the Texas Parks and Wildlife Department. This is a decrease from 26 acres in May and 201 acres in July 2004. At the highest point in 2002, hydrilla covered more than 300 acres of the lake.

The TPWD survey also showed that Lake Austin has about 75 acres of other vegetation, primarily pondweed and Eurasian milfoil. These plant species provide beneficial cover for fish and are not posing a nuisance to lake users.

“An important lesson from Lake Austin that applies statewide is that we did not rush the solution,” said Phil Durocher, TPWD inland fisheries director. “Our goal from the beginning has been to address the problems caused by hydrilla for landowners, boaters, swimmers and others while protecting the interests of anglers who enjoy the enhanced fishing opportunity provided by vegetation cover.”

TPWD, the City of Austin, the Lower Colorado River Authority and Friends of Lake Austin (a lakeside homeowner group) in 2002 agreed on a plan to battle hydrilla. Tactics agreed on included lake drawdowns, stocking of sterile grass carp and the release of hydrilla flies to eat the hydrilla, approved pesticides, mechanical harvesting and bottom barriers.

“The temptation in situations like this is to over-react,” said Earl Chilton, TPWD’s leading expert on exotic aquatic plants. “It might have been tempting to stock thousands of grass carp immediately. But, the partners created a plan incorporating several strategies, including research to make sure the carp would work as intended. The result is we still have adequate vegetation of other kinds and a healthier fishery and ecosystem. The steady, deliberate approach is better in the long run.”

First discovered in Texas three decades ago, hydrilla has since been found in nearly half of the 200 public water bodies managed by Texas Parks and Wildlife Department fisheries biologists, particularly those in the eastern part of the state.

Hydrilla was originally introduced in the United States as an aquarium plant, and has since spread throughout the South. Because it grows rapidly from fragments, hydrilla can be unintentionally transported from lake to lake attached to boat trailers or boat motor propellers. It can grow up to four inches a day in clear water, so it can establish quickly and spread rapidly.

A certain amount of vegetation, especially native plants that evolved in Texas waters, is healthy. However, exotic plants like hydrilla can get out of control, and can limit boating and swimming and other recreational access, restrict flows in canals and rivers, interfere with power plant intakes and other industrial water uses, and harm fish and wildlife.

In 2001, floods made worse by hydrilla created new and expensive problems on Lake Austin. Homes that had never been flooded were inundated by hydrilla that slowed water flow along the Colorado River. Huge mats of uprooted hydrilla made their way downstream and clogged hydroelectric power generators operated by the Lower Colorado River Authority, resulting in an estimated loss of $300,000 due to plant shutdowns.

Lake managers or communities with exotic aquatic plant problems have several control options available. A law passed earlier this decade by the Texas Legislature requires individuals or organizations to submit an Aquatic Vegetation Treatment Proposal to TPWD for approval. The department recommends a comprehensive approach known as Integrated Pest Management, which advocates choosing the best tools for the situation. Complete information is on the TPWD Web site.