Hoping to combat an aquatic invasive plant that has marched across a third of Lake Austin, the City of Austin is releasing one of the plant’s mortal enemies into the water: Carp.

Tonight, Austin officials will supervise the dumping of 3,000 sterile grass carp into the waters of Lake Austin.

Hydrilla occupied a record 566 acres of the 1,600-acre lake, according to an April survey by the state’s park and wildlife department.

Growing in thick mats on or just below the surface, the plants grow rapidly and entangle just about everything they can get their stems on: boat propellers, swimmers and other plants. They choke oxygen from the water, and they clog the water intake stations at LCRA dams, hindering power generation.

The persistence of the plant is captured by its name, named after Hydra, the multi-headed water-serpent that grew two new heads each time one was cut off.
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The plants first appeared in Lake Austin in 1999. By 2002, they were covering 320 acres. A couple of years later, the city and partners began introducing carp.

The recent uptick is a lingering consequence of the drought, according to Mary Gilroy, an environmental scientist for the City of Austin.

Shallow waters in Lake Travis, which feeds Lake Austin, has meant warmer waters. Meanwhile, the cutoff of water to downstream rice farmers has meant less flow of upstream Highland Lakes water through Lake Austin. Taken together, the effect is the relatively warm and stagnant conditions the plant favors, Gilroy said.

The 3,000 carp, which are each one foot long, will join the estimated 10,000 currently in the lake from previous releases.

The city is paying $6.90 per fish. The city is authorized by the state parks and wildlife department to release as many 12,200 more fish in coming months, Gilroy said.