Zebra mussels are spotted at Lake Texoma. Credit: Courtesy / Texas Parks and Wildlife Department

A year ago scientists discovered zebra mussels just north of San Antonio at Canyon Lake for the first time, the furthest south the invasive invertebrates have been found in the United States. With boaters and other recreationists moving between waterways to keep cool this summer, there’s a high risk that the mussels will find a new home in other lakes and reservoirs.

The tiny mussels, usually about half an inch long, can start reproducing six months after birth and a single female can produce 1 million eggs a year. They feed on algae, which puts them in competition with many essential creatures in Texas waterways. Scientists have noted that the water becomes clearer as more zebra mussels infest a lake, and other fish and invertebrates disappear.

San Antonio resident Wayland Roed has noticed that the mussels in Canyon Lake seem to be more numerous.

“Last year I found a couple, but this year when I turned some rocks over, I found at least 30 mussels a piece,” said Roed, who fishes in the lake. “Seems like they’re moving quick.”

Zebra mussels have been sighted at 14 Texas lakes so far, with Canyon Lake featuring a small but growing population, said Monica McGarrity, Aquatic Invasive Species team leader for the Texas Parks and Wildlife Department (TPWD). San Antonio sits on the southern boundary of the species’ national territory.

2017 map of zebra mussel occurences in Texas Credit: Courtesy / Texas Parks and Wildlife Department

Zebra mussels multiply quickly and latch on to critical pieces of infrastructure like water intake openings and drains.

“Besides their ecological damage, they foul water intake structures, so any facility that is using raw surface water can be damaged,” McGarrity said.

Zebra mussels could affect such structures as the Olmos Dam and CPS Energy’s coal-based Calaveras Power Station, which relies on water to release pressure and cool its systems.

Once they attach to a boat, the rapidly reproducing mollusks can clog boat motors, and their sharp shells can cut bare feet. The mollusks and their larvae can spread to other bodies of water by hitching a ride when a boat is used on multiple waterways.

In some parts of the country, chemical pesticides have been introduced which successfully target the mussels, but trials are ongoing and have yet to be introduced in South Texas.

study by TPWD and researchers at the University of Texas at Arlington observed three Texas lakes – Texoma, Ray Roberts, and Belton – and found that a rapid increase in zebra mussel numbers was followed by a steep decline.

“We’ve seen that in some reservoirs, they’ll eat themselves out of house and home and the population will crash, but then it reaches a stage where they’ll come back and the population will stabilize at some level,” said Brian Van Zee, Inland Fisheries regional director for TPWD.

Slower growth may give birds and other natural predators of the mussels a chance to grow their numbers, too, helping to balance the ecosystem.

Zebra mussels are on display at a TPWD press conference at Austin’s 360 Bridge Credit: Courtesy / Texas Parks and Wildlife Department

For now, it’s up to diligent recreationists to help halt the spread of zebra mussels. Sightings of the mollusks can be reported to the Texas Invasives website, which helps map the spread of invasive species through reports from citizen scientists. These reports help determine where TPWD spends its resources to combat the mussels’ spread.

The State’s official prevention mantra is for boaters to “Clean, Dry, and Drain” their watercraft when they are taken out of the water. Boat anchors are particularly attractive to the mussels, and boats in wet slips are at the highest risk of spreading the species.

“We can’t do it alone,” McGarrity said. “We rely on boaters to help us prevent their spread and safeguard our waterways and lakes together.”

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Mitch Hagney

Mitch Hagney is a writer and hydroponic farmer in downtown San Antonio. Hagney is CEO of LocalSprout and president of the Food Policy Council of San Antonio.