An animal the size of a grain of sand with the growth potential of about an inch and half has infested Lake Travis. Its sharp shell can cut up lakegoers’ feet and its rapid reproduction can cause economic and ecological damage.
In less than a year, the population of zebra mussels in Lake Travis has boomed and found its way downstream to Lake Austin. Because of the size of the lakes, eradication is an unlikely option. Does this invasive species have an upside? At least one state expert says no.
Zebra mussels attach to any stable substrate, including the rock, sand, gravel and concrete in Lake Austin.
“Just because you don’t see them, doesn’t mean they’re not there,” said Monica McGarrity, aquatic invasive species team leader for the Texas Parks and Wildlife Department. Any perceived benefits that the mussels might have for the lakes come with drawbacks.
Because these voracious mussels filter and feed on plankton, a high density of them can make water in lakes look clearer. A single mussel is capable of filtering a liter of water in a day, which might sound like a benefit, but isn’t for the overall ecosystem, McGarrity said.
Plankton serves as the base of a lake’s food hierarchy. With less of the single-celled algae available for other animals, sport fishers angling for game fish like largemouth bass in Lake Austin can expect to see them get smaller in years to come. Largemouth bass feed on shad fish, which feed on plankton.
“No shad, no bass,” McGarrity said. “If you have this competitor eating their food, catfish may do better, but the bass game fish aren’t going to fare well.”
Clearer water also creates low oxygen levels at the bottom of the lake and even areas with no oxygen, which fish and other animals need to survive.
Some ecosystems, including the Hudson River ecosystem in New York, have seen an invasion of zebra mussels bring more sunlight to rooted plants in the water, which can provide shelter for organisms that fish prey on. This means bottom-feeding fish, including catfish and sunfish, could thrive.
However, native mussels in the lakes are at risk of being smothered, starved and put on the federal endangered species list because they are overmatched by zebra mussels for plankton.
Eradicating zebra mussels is difficult in large lakes because it would require chemical treatments, McGarrity said. She expects the mussels will stay in Lake Austin, but she doesn’t know yet how bad the infestation will get.
Periodic drawdowns of Lake Austin in the peak heat of summer could dry up a few mussels, but they might not die because of the way they can close their shells and retain water.
“Once they’re in a lake, that’s it,” she said.
Overall, McGarrity saw no positive outcomes that would apply to Lake Austin and Lake Travis.
“That’s why it is critical for boaters to take the right steps to clean, drain and dry their boats” to prevent their spread, she said.