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 Thanksgiving flood in the lower Texas Colorado River basin washed away 160 acres of hydrilla in Lake Austin, but this spring aquatic scientists are monitoring and treating infestation of the weed elsewhere along the river basin in Central and South Texas.
Hydrilla, an invasive Asian water weed imported to the United States in the 1960s as greenery for aquariums, has been a chronic problem on Lake Austin, and this spring the noxious weed has been reported on Lake LBJ in the Highland Lakes and Eagle Lake in South Texas. Hydrilla can choke power plant intake equipment, clog irrigation channels, strangle the motor on a power boat and terrify swimmers caught in their shadowy tendrils. Last September, hydrilla covered 240 acres of Lake Austin, a popular Central Texas recreational area and the site of a dam and hydroelectric power generation facility. Earlier this year, a Texas Parks and Wildlife Department (TPWD) survey conducted in late February turned up good news. Following the introduction of 8,000 grass carp (a sterile, weed-loving fish native to Southeast Asia), a winter drawdown of the lake and a flood that pushed more than 211 billion gallons of water through Lake Austin, there are only about two acres of hydrilla left in the lake. "The November flood scoured out about 160 acres of that weed. Going into the lake drawdown, there was an estimated 80 acres of hydrilla left," said John Wedig, a Lower Colorado River Authority aquatic scientist. The winter drawdown of the lake exposes the hydrilla to freezing temperatures, which kills the exposed vegetation. "We expected to have 40 acres of hydrilla left after the drawdown," but it was all but wiped out, Wedig said. Why? "It's a mystery. The theory is the grass carp consumed it." Grass carp will likely provide maintenance control as new growth re-emerges this spring, Wedig said. "Their favorite food is hydrilla." TPWD will do a vegetation survey every two months to monitor the presence of the weed. Lake LBJ
There are no grass carp waiting to munch down the two acres of hydrilla scattered over the surface of about 40 acres of Lake LBJ near Beaver Island at Granite Shoals. LCRA plans to use two types of herbicide to control the weed: a systemic herbicide that kills the entire plant, and a contact herbicide that kills the leafy part of the weed, leaving the root of the plant beneath the lake bottom. The systemic herbicide inhibits the production of chlorophyll, eventually killing the plant. There are no restrictions on fishing or swimming, but treated water can not be used for irrigation for 30 days. "We have learned from our experience with hydrilla on Lake Austin and Lake Bastrop that it is best to aggressively pursue control measures as quickly as possible before it takes over. The spring growing season is the time to do it," he said. LCRA suspected hydrilla was growing in Lake LBJ for about year. Staff at LCRA's Thomas C. Ferguson Power Plant began investigating after they found fragments of the noxious water weed on power plant intake screens. Lake residents also claimed to have seen hydrilla. Ferguson employees sent the plant remnants they'd pulled off intake screens to LCRA's aquatic scientists, who identified the weed as hydrilla. Eventually, LCRA staff found hydrilla intertwined with another weed called spiny naiad. "That was our first confirmation," Wedig said. When LCRA lowered LBJ during the winter, staff found additional patches of the weed. Lake Bastrop
Excessive growth of hydrilla has been a problem in Lake Bastrop for more than eight years, threatening the functioning of LCRA's Sim Gideon Power Plant there. LCRA has made periodic strides in cutting back the weed, which, in 1997, infested more than half the surface of the 900-acre lake. Hydrilla more than once has clogged the intake of the Sim Gideon plant, which draws lake water in to cool the generating facility. Last year staff at the plant removed 14 truckloads of hydrilla from intake screens. In 1999, LCRA used a mechanical harvester to cut about 60 acres of hydrilla near the intakes and in recreational areas on the north and south shores. These efforts were moderately successful, and in 2000 the hydrilla all but disappeared, probably the victim of a plant-specific pathogen rather than man-made methods of eradication. By 2003, the hydrilla was back, and the infestation began spreading rapidly. Last August, LCRA had modest results with herbicide use. This spring LCRA will use a systemic herbicide to control hydrilla near power plant intake screens if warranted and the harvester elsewhere in the lake. Eagle Lake
The November flood created the perfect conditions for hydrilla to flourish downstream in rice-growing country, where only limited herbicide use in a narrow window of time is possible for control. This spring, hydrilla is a concern at the Lakeside Irrigation District, which supplies the water that feeds irrigation canals. Hydrilla is one of several aquatic invaders, such as water hyacinth, that infest the 1,100-acre Eagle Lake. The lake is on average four to five feet deep with a bottom that is rich in nutrients. "These are very desirable conditions for the spread of hydrilla," said Randy Epps, area supervisor for the Lakeside and Garwood irrigation districts. LCRA has already made some attempts to control hydrilla with a herbicide added to the canals after the irrigation season concluded last fall, Epps said. Growing without control, hydrilla could shut down the plant pumps for as many as six hours while staff manually clears it away, a costly prospect. "The Lakeside plant is maxed out on acreage," Epps said, "Shutting down the pumps would be devastating." Because the use of herbicides is not possible during growing season — it will kill crops along with hydrilla — grass carp may be a viable option. "Aquatic herbicides are not very selective. That's why grass carp are attractive — if they can survive the alligators," Wedig said. "You don't have to worry about them jumping a levee and climbing into a rice field." Grass carp
Grass carp can pose their own problems. Lake Conroe, north of Houston, serves as an example of what can happen if the fish are introduced in an uncontrolled manner. The species, introduced in large numbers, reportedly ate not only the hydrilla but also much of the desirable vegetation, almost turning the reservoir into an aquatic desert. Hungry carp have been known to munch grass on the lawns of lakeside residents and the leaves from overhanging trees. And the fish were not rendered sterile, enabling them to propagate future (and larger) generations of plant-eaters. LCRA and its partners took precautions to avoid the same situation along Lake Austin by introducing sterile fish at intervals into the lake, rather than stocking all 8,000 fish at once. But as the presence of hydrilla dwindles, aquatic scientists are concerned what the fish will do when they find themselves with dramatically diminished stores of their favorite food. Eurasian watermilfoil (milfoil) is the dominant vegetation remaining in Lake Austin but not a preferred food of grass carp. "Hydrilla is the ice cream plant and milfoil is the Brussels sprouts food," Wedig said. The oldest grass carp in Lake Austin are now two years old. With an abundance of hydrilla to feed on, the fish have gotten hefty, weighing 12 to 15 pounds each. With less hydrilla to eat, the fish may slim down considerably, or worse, escape the lake. "They'll likely use any flood event to take the opportunity to leave the lake and search for food," Wedig said. LCRA aquatic scientists hope the fish will be patient. Hydrilla will return to Lake Austin eventually, despite its dramatic disappearance this winter. The weed will be reintroduced into the lake when it hitchhikes on boat trailers. It is reported that fishermen, who believe the weed serves as habitat for fish, sometimes deliberately introduce it. "We will always have to fight hydrilla in Lake Austin," Wedig said. "There's no such thing as a little hydrilla." Lisa Trow is an LCRA writer specializing in water topics. Contact her at lisa.trow@lcra.org. Related links:
LCRA hydrilla watch page has more info and links to resources |
