The wicked hydrilla is in check, for now.
None of the invasive, boat-choking plants were found on the surface of Lake Austin during a recent survey by the Texas Parks and Wildlife Department, though officials said some hydrilla still exists on the floor of the lake. This was cause for celebration among city officials and the Lower Colorado River Authority, which have spent a combined $210,000 in recent years stocking the lake with sterile Asian grass carp, an exotic fish that loves to munch on the problematic plant.
“It’s not gone, but it is controlled at a level better than we’ve ever done before,” said Mary Gilroy, environmental program coordinator for the city of Austin’s watershed department.
Marcos De Jesus, a fisheries biologist for the Texas Parks and Wildlife Department, said small sprigs that can’t be seen are still growing on the floor of the lake.
“These are hydrilla roots called tubers that can re-sprout,” he said. During the six-hour survey in September, officials used sonar equipment on a boat to look for the plant and also tossed rakes on long ropes to the bottom and dragged them across areas where hydrilla was suspected. “Not a single plant was found,” De Jesus said.
The study of the vegetation in the lake revealed 600 acres of hydrilla in February, falling to 330 acres in June and then to none in September. The most recent survey also found 203 acres of other aquatic vegetation, mostly Eurasian watermilfoil, a much more beneficial plant that increases oxygen, which is good for aquatic habitat and fish.
“The milfoil makes for good fishing, because fish like to hide in the plant. It’s good for the ecosystem,” said Gilroy.
Credit the drop in hydrilla to the 48,000 Asian grass carp that have been stocked in Lake Austin since 2003 when hydrilla was out of control.
“Carp have kept the hydrilla mowed to the bottom of the lake, much like cows chewing on a pasture. But carp don’t kill the plant entirely. They can’t get to the roots,” Gilroy said.
The aggressive and invasive hydrilla, hated by boaters because the plant can ruin propellers, was discovered on Lake Austin in 1999. The plants caused problems when heavy rains fell in July 2002, said Gilroy.
“The floodwaters hit a wall of hydrilla, and that caused flooding on homes on the lake,” she said. “Also, 100 acres of hydrilla swept downstream and ended up in the grates of Tom Miller Dam.”
LCRA spokeswoman Clara Tuma said the hydrilla damaged trash racks in front of the dam’s hydroelectric unit. The LCRA spent $384,000 repairing and replacing the racks, she said.
The city and LCRA paid to stock the lake with hydrilla-hungry carp. The Texas Parks and Wildlife Department and the nonprofit Friends of Lake Austin bought some of the fish, too. The LCRA has also lowered the lake at times to remove hydrilla and give dock owners a chance to build or repair docks.
Officials are aware the hydrilla can make a comeback, De Jesus said. A naturally diminishing population of carp, warm temperatures and sunlight all contribute to plant growth, he said.