04-03 | Lake Nasworthy ‘Infested’ with Invasive Zebra Mussels

April 3, 2025

Media Contact: TPWD News, Business Hours, 512-389-8030

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AUSTIN – The Texas Parks and Wildlife Department (TPWD) has designated Lake Nasworthy, near San Angelo, as “infested” with invasive zebra mussels. This designation signifies there is an established reproducing population present in the lake.

TPWD confirmed the presence of zebra mussel larvae (veligers) in mid-March from plankton samples that were collected in Nov. 2024 during the mussels’ fall spawning period. Zebra mussel larvae were detected in samples collected at two sites on the lake, near the Knickerbocker Road bridge and near the dam.

TPWD fisheries biologists subsequently searched the shoreline and substrate at numerous sites around the lake. The biologists found a single settled adult zebra mussel along the shoreline near the bridge. The detection of larvae at multiple sites and the presence of an adult mussel indicates the presence of an established, reproducing population.

“Unfortunately, the zebra mussel infestation of Lake Nasworthy is part of a trend in recent years of spread of this highly invasive species to lakes in the western part of the state,” said Monica McGarrity, TPWD senior scientist for aquatic invasive species. “All boaters taking steps to properly clean, drain, and dry boats to prevent the spread of invasive species is critical to stopping the further spread of zebra mussels.”

Plankton samples were also taken in the fall at the nearby Twin Buttes Reservoir and, fortunately, zebra mussel larvae and DNA were not detected. These results suggest that Twin Buttes remains free of this invasive mussel at the present time. However, TPWD will continue to monitor Twin Buttes Reservoir and collect additional plankton samples this spring.

Zebra mussels, a non-native shellfish that attaches to hard surfaces, pose a significant threat to our lakes. These invasive mussels damage boats and water supply infrastructure, alter lake ecosystems and cause harm to native species. They also litter shorelines with hazardous, sharp shells that impact lakefront recreation.

Because zebra mussels are most often transported on or in boats, boaters play a critical role in preventing them from spreading to new lakes. Zebra mussels attach to boats and anything left in the water, including anchors, and can survive for days out of water, often hiding in crevices where they may escape notice. Their microscopic larvae are free-floating and invisible to the naked eye, meaning they can be transported unknowingly in residual water in boats.

“Invasive mussels have now spread to 38 Texas lakes, with 34 fully infested, but there are far more lakes that still haven’t been invaded and are at risk,” said Brian Van Zee, TPWD Inland Fisheries Regional Director. “Each boater taking the necessary actions to clean and drain their boat before leaving the lake and allowing compartments and gear to dry completely when they get home can make a big difference in protecting our Texas lakes.”

TPWD urges boaters to follow these three simple but crucial steps to clean, drain and dry boats and gear before traveling from lake to lake. Remove plants, mud and debris; drain all water from the boat and gear; once back home, open compartments and allow everything to dry completely for at least a week, if possible.

If a boat or other equipment, such as barges, docks, lifts, pumps, etc., have been stored in the water at a lake with invasive mussels, they are likely infested and pose an extremely high risk for transporting these invasive species to a new lake unless they are properly decontaminated. Before moving a boat or other equipment that has been stored in the water, call TPWD at (512) 389-4848 for guidance.

In addition to the harm that invasive species can cause to aquatic ecosystems, water infrastructure and the recreational experience at lakes, the transport of these organisms can result in legal trouble for boaters. Transporting prohibited aquatic invasive species in Texas is illegal and punishable with a fine of up to $500 per violation. It’s also the law that boaters must drain all water from their boat and onboard receptacles, including bait buckets, before leaving or approaching a body of fresh water. They must also remove all invasive plants from the boat and trailer before leaving a lake.

For more information on how to properly clean, drain and dry boats and equipment, visit the TPWD YouTube channel for a short instructional video. To learn more about zebra mussels and other invasive species in Texas, visit tpwd.texas.gov/StopInvasives.

TPWD and partners monitor for invasive species in Texas lakes, but anyone who spots them on boats, trailers or equipment that is being moved can help prevent new introductions by reporting the sighting to TPWD at (512) 389-4848. Anyone who finds invasive species such as zebra mussels in lakes where they haven’t been found before can help identify new introductions by emailing photos and location information to aquaticinvasives@tpwd.texas.gov.