Feared zebra mussels invade Texas lakes
When zebra mussels exploded in the Great Lakes region during the early 1990s, fisheries managers in Texas and many other southern states certainly noticed, but most weren’t overly alarmed.
Yes, the alien freshwater mollusks, native to northern Eurasia and introduced to North America through the ballast water of commercial ships, had quickly become a major environmental and economic problem. Able to reproduce at tremendous rates — a single, fingernail-size mussel can produce a million eggs during spawn — and lacking any significant predators, the mussels swarmed northern waters, triggering considerable negative consequences.
But, evidence suggested, the invasive mussels were likely to remain a regional problem. They were confined to the Great Lakes. The mussels couldn’t transport themselves across scores of miles to infect river systems not directly connected to the infected waters. And, even if they escaped to new waters, the mussels’ relatively small native range was cold-water lakes; the mollusks might be able to live in the upper Midwest but almost certainly would wither and perish in the sultry waters of a southern summer.
It hasn’t worked out that way. Zebra mussels have spread at an alarming rate, thanks mostly to human actions. And the mollusks have proven much more tolerant of warm water than just about anyone suspected. They now are found in at least 30 states. By 2009, they had made it to Texas, first taking hold in Lake Texoma on the Texas/Oklahoma border.
Last week, barely seven years later, Texas fisheries officials announced discovery of zebra mussels in three reservoirs, boosting the number of lakes hosting the potentially devastating invasive species to a dozen spread across the Trinity, Red and Brazos river systems.
One of the new reservoirs on the list is Lake Livingston, the 90,000acre lake on the Trinity River about 80 miles northeast of Houston. Livingston, a hugely popular fishing destination and a primary water source for the fourth-most populous city in the nation, is the southernmost and easternmost Texas waterbody in which zebra mussels have been documented.
“We knew Lake Livingston could be at risk for zebra mussels, but we were hoping they wouldn’t show up,” said Brian Van Zee, Waco-based regional director of Texas Parks and Wildlife Department’s inland fisheries division. “You don’t want to see any new infestation; there can be a lot of negative consequences.” Variety of consequences
Some of those consequences are economic. Zebra mussels reproduce so quickly and in such dense concentrations they can carpet lake bottoms and anything under the water. They attach themselves to water management and transportation infrastructure such gates and pump parts and, especially, intake screens and pipes. The concentrations are so thick they clog and close these crucial systems.
This damage to water infrastructure systems has cost billions nationwide. It has cost hundreds of millions in Texas.
The largest economic impact in Texas, so far, has come from the infestation on Texoma. The North Texas Municipal Water District, which provides water for more than 1.6 million Texans, was forced to construct a new 46-mile pipeline and other infrastructure to transport and treat water from zebra mussel-infested Lake Texoma. That project cost $300 million and resulted in a 14 percent jump in customers’ water rate.
That’s one reason to be concerned when zebra mussels show up in a reservoir such as Lake Livingston, which serves as a major supplier of drinking water to Houston.
But the mussels’ environmental consequences can be just as significant. They can and do alter ecosystems.
Zebra mussels outcompete or simply smother native mollusks. But that’s just one of the native aquatic species they impact.
Each adult zebra mussel filters about a quart of water per day, straining from the water the calcium and other minerals used to build its shell. The billions of mussels also consume vast quantities of plankton. The result, as seen on many lakes where the mussels have thrived, is incredibly clear water as the mussels remove the suspended minerals and microscopic plant and animal life. But that water is now much less fertile for other aquatic life.
“When they filter out the plankton, they’re reducing the food supply at the bottom of the food chain,” Van Zee said.
Threadfin and other shad, especially, depend greatly on rich plankton supplies to thrive. Without it, shad populations decline. Without shad, predator species that depend on shad as forage — white bass, striped bass, crappie, largemouth bass — decline.
Clearer water can mean blooms of some forms of algae that deplete life-giving dissolved oxygen from waters, resulting in fish kills. The cleared water also can encourage the growth of invasive aquatic plants, further damaging the ecosystem.
There are other negatives, too. Evidence suggests zebra mussels are susceptible to carrying a strain of bacteria responsible for botulism in birds. Botulism outbreaks on zebra mussel-infested Lake Erie claimed the lives of more than 50,000 waterbirds, mostly loons and ducks such as scaup which feed on mollusks, from 2002-06.
It’s hard to quantify zebra mussel’s negative effects on lakes Texoma, Ray Roberts, Lewisville, Bridgeport, Dean Gilbert (a 45-acre lake in Sherman) and Belton, the six Texas lakes with documented established, reproducing populations of zebra mussels.
And it’s equally tough to try predicting the effects on Livingston, Eagle Mountain, Lavon, Waco, Worth, and Fishing Hole Lake where adult zebra mussels or their larvae have been found but a reproducing population hasn’t yet been documented.
“It can take a few years for some of the effects to show up,” Van Zee said.
What is not hard is figuring out how the mussels get into Texas lakes.
The three recent discoveries of zebra mussels — in Livingston, Eagle Mountain and Worth — may have come from mussels or their microscopic larvae, called veligers, carried downstream from upstream lakes known to be infested with the invasives.
“We’ve had tremendous flooding this year and last year on the Trinity River, so they may have been carried downstream from lakes we know have zebra mussels,” Van Zee said.
But they also could have come from being transported to the lakes by boaters, the primary way zebra mussels find their way into new waters.
Boaters on water bodies infected with zebra mussels can, unintentionally, end up with water containing zebra mussel larvae in their vessels’ bilge, live wells, bait buckets or even outboard cooling systems. Those larvae, which are able to survive long periods in the trapped water, can be released days or weeks later when the boat is launched into another water body.
“There can be as many as a couple of hundred veligers in a liter of water,” Van Zee said. “You can have a self-contained breeding colony of zebra mussels in your boat.” Clean, dry, drain
It has so far proven impossible to eradicate zebra mussels from infected waters. So fisheries managers, water authorities and regulatory agencies have focused their efforts on preventing, or at least slowing, spread of the insatiable invasive mollusks by boaters.
Texas has an aggressive campaign to educate boaters about the threat posed by zebra mussels and steps boaters are now legally required to take to prevent their vessel from becoming the vector for new infestations.
Texas law has for the past two years required all boaters approaching or leaving reservoirs, rivers or other public freshwater anywhere in the state drain and dry all water from their boat, live wells, bilges, motors, bait buckets and any other waterholding receptacles or face a citation carrying a fine of as much as $500.
With the discovery of zebra mussels in three additional reservoirs this past month and the onset of he summer peak of boating across the state, it’s especially crucial for Texas boaters to follow the “clean, drain, dry” mandate, Van Zee said.
“It takes just a couple of minutes to pull the boat plug, drain water from the engine, empty the livewells and bait buckets and let everything dry,” he said.
To do otherwise is to not take the threat posed by zebra mussels seriously. And, as history has proven, that’s a mistake.