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Invasive hydrilla stops spreading in Lake Austin after grass carp program

Invasive hydrilla stops spreading in Lake Austin after grass carp program

New Capabilities Austin, TX local

City plans about 7,000 more sterile carp this fall to shrink weed coverage toward target

2 days ago: City memo reports hydrilla growth has plateaued

Overview

Updated 1 hour ago

The invasive weed that has been choking Lake Austin has stopped spreading. City data released Thursday shows hydrilla coverage has held steady at 575 acres since June, ending the rapid growth that began in summer 2025.

The halt follows a measured grass carp stocking program started in June 2025. Austin will add about 7,000 more sterile carp this fall, guided by a predictive model built on 25 years of lake data, aiming to shrink hydrilla without repeating the 2013 over-stocking that wiped out the lake's vegetation.

Why it matters

Lake Austin supplies drinking water to Austin; unchecked hydrilla can clog boat motors, crowd out native plants and degrade water quality.

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Key Indicators

575 acres
Hydrilla coverage in Lake Austin
September 2026 survey; coverage stable since June.
7,091
Grass carp planned for fall 2026 stocking
Would bring total stocked since June 2025 to 12,234.
99 acres
Eurasian watermilfoil coverage
Up from 76 acres in the June survey.

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People Involved

BB
Brent Bellinger
Conservation program supervisor, Austin Watershed Protection Department
Leading the city's hydrilla management response
 Patrick Ireland
Patrick Ireland
District fisheries biologist, Texas Parks and Wildlife Department
Monitors invasive aquatic species across Texas

Organizations Involved

AW
Austin Watershed Protection Department
City agency
Manages hydrilla and aquatic vegetation in Lake Austin

City of Austin agency responsible for flood management, water quality, and watershed protection.
TP
Texas Parks and Wildlife Department
State agency
Conducts vegetation surveys of Lake Austin

Texas state agency managing wildlife, parks, and aquatic resources.
LC
Lower Colorado River Authority
Public utility
Manages Highland Lakes dams and water levels

Public utility managing the Highland Lakes, including Lake Austin's water levels.

Timeline

2003 October 2026

11 events Latest: 2 days ago Showing 8 of 11
Tap a bar to jump to that date

  1. City memo reports hydrilla growth has plateaued

    Latest Monitoring

    Watershed Protection releases data showing the invasive species has stopped spreading in Lake Austin.

  2. LCRA lowers Lake Austin 10 feet

    Management

    Water level dropped for maintenance; property owners urged to manage nuisance vegetation.

  3. Survey finds 575 acres of hydrilla

    Monitoring

    Texas Parks and Wildlife survey shows coverage stable since June; Eurasian watermilfoil at 99 acres.

  4. 2,790 grass carp added

    Management

    Third stocking brings total to 5,143 fish.

  5. 1,995 grass carp added

    Management

    Second stocking of the resumption program.

  6. 358 grass carp added

    Management

    First small batch of sterile carp released in July.

  7. City resumes grass carp stocking

    Management

    Hydrilla rebounded to near-record levels; Austin restarts a cautious carp stocking program.

  8. Native vegetation begins rebounding

    Ecosystem

    Aquatic plants return to the lake after years of near-zero vegetation.

  9. Carp eat all vegetation; fishery collapses

    Ecosystem

    Over-stocked carp consume all aquatic plants, triggering algae blooms and habitat loss for nearly a decade.

  10. Carp stocking ramps up amid drought

    Management

    Extended drought fuels hydrilla growth; city sharply increases carp stocking to compensate.

  11. City first stocks grass carp in Lake Austin

    Management

    Austin begins using sterile grass carp to control hydrilla.

Scenarios

1

Fall carp stocking cuts hydrilla toward target range

Likely Resolves by Nov 1, 2027

Discussed by: Austin Watershed Protection Department (predictive model projections)

The planned stocking of about 7,000 sterile carp this fall brings the lake's total population to one-third of the 2013 peak. The predictive model indicates this should set the lake on a path toward the target of 10-30 percent vegetation coverage. If the model holds, the September 2027 survey would show hydrilla acreage declining from 575 acres while native plants survive.

2

Plateau breaks; hydrilla expands again

Possible Resolves by Nov 1, 2027

Discussed by: Patrick Ireland, TPWD fisheries biologist (environmental variability)

Environmental factors outside the city's control—rainfall, flows from Lake Travis, nutrient loads—could fuel continued hydrilla growth despite the carp. The model itself shows wide variation in how long reduction might take. If weather favors the weed, the plateau could give way to expansion beyond 575 acres.

3

Over-stocking repeats 2013 vegetation collapse

Unlikely Resolves by Nov 1, 2027

Discussed by: Brent Bellinger, Austin Watershed Protection (explicit caution against repeat)

If the fall stocking overshoots the lake's carrying capacity, the carp could consume native vegetation along with hydrilla—repeating the 2013 collapse. The city says the post-stocking population will be only one-third of the 2013 peak, but the model's wide error bars leave room for error. Native plants like eelgrass and coontail would be the first casualty.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

2011-2014

Lake Austin carp over-grazing (2011-2014)

To fight a hydrilla outbreak during a severe drought, Austin sharply increased grass carp stocking. By 2014, the fish had eaten all vegetation in the lake, including native plants.

Then

The lake had no aquatic vegetation for nearly a decade, causing plankton blooms, loss of fish habitat, and a collapse of the fishery.

Now

The episode became a cautionary tale that shaped the city's cautious, incremental approach to carp stocking.

Why this matters now

The current program explicitly aims to avoid a repeat of this over-stocking; the fall stocking will bring the total population to only one-third of the 2013 peak.

1990s

Water hyacinth in Lake Victoria (1990s)

Water hyacinth, a fast-growing floating weed from South America, spread across Lake Victoria in East Africa, covering large areas and disrupting fishing and transport.

Then

Mechanical removal and chemical spraying proved expensive and ineffective at large scale.

Now

Biocontrol with weevils beginning in the late 1990s gradually reduced the weed, though it remains a recurring problem requiring ongoing management.

Why this matters now

Shows biological control can work for aquatic weeds, but takes time and needs steady monitoring to avoid unintended consequences.

Sources

(3)
KXAN Austin City of Austin KUT Radio