Even record freeze couldn't eradicate Central Texas' toughest invasive species, experts say

Heather Osbourne
Austin American-Statesman

Not even a historic week of freezing temperatures and record-setting snowfall last month could mitigate some of the toughest and most threatening invasive species known to Central Texas, biologists from the Texas Parks and Wildlife Department say.

While many of Austin's beloved Mexican free-tailed bats were injured or killed by the winter weather, some not-so-cherished invasive critters such as zebra mussels, tilapia and maybe even apple snails fared just fine, said Monica McGarrity, a senior scientist at Texas Parks and Wildlife who specializes in aquatic invasive species.

Zebra mussels, which are small but prolific mollusks, first appeared in Austin-area lakes in 2017 and have since attached themselves to boats, pipes and docks, and even infested drinking water sources such as Lake Travis and Lake Austin. Their presence can affect the taste and odor of drinking water.

Texas Parks and Wildlife experts for years have worked to lessen the environmental effects of the mussels, but McGarrity said we have no way to eradicate them completely. 

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Some might have thought the mussels would die off in the freezing waters, exposed to the extreme frigid temperatures during the storm, but McGarrity said it's actually the opposite.

"Zebra mussels are basically living at the edge of their thermal tolerance here for heat, and so, during our hot summers, they are basically just trying to survive," she said. "So the cold really isn't a problem for them. I'd say the cold likely had no effect on the population." 

Similarly, McGarrity said the cold snap apparently had no effect on invasive tilapia in Travis County. However, some reports suggested tilapia deaths in other waterways, specifically Falcon Lake.

Tilapia were introduced to Texas lakes in the 1960s and are known to diminish diversity of native plants, fish and shrimp, according to Texas Parks and Wildlife officials. Both zebra mussels and tilapia have been linked to deaths of native mussels that don't harm the area's waters. 

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Zebra mussels kill native mussels by attaching to their shells, stealing their food source and eventually suffocating them, invasive species experts say. 

According to the Texas Invasive Species Institute, tilapia are tropical fish but can survive water temperatures as cold as 10 degrees or as warm as 86 degrees. 

Although the severe winter weather doesn't appear to have made a dent in the tilapia or zebra mussel populations, Texas Park and Wildlife officials say another invasive species plaguing one Austin-area lake might have been hurt by the cold. 

Apple snails, which can grow to the size of a fist, are a common invasive species in the Houston area but are found only in Bear Lake in Travis County, according to McGarrity.

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The snails have the potential to cause health problems for humans because they can carry parasites that infect people and other mammals. 

Researchers are working to see if the Central Texas winter weather might have had any large-scale effect on the snails. 

"It'll be interesting to see if any apple snails die off as a result of this," McGarrity said. "They aren't very cold-tolerant, but they are able to bury in the sediment. So if the lake is deep enough they might have not been affected much, but we expect there could be some cold impacts to this."

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