Tiny zebra mussels a Texas-size threat
They are only about the size of a fingernail, these miniature mollusks whose common name comes from the black stripes on the razorsharp shells formed from calcium and other minerals the animal they encase rapaciously strips from the water in which they live. But zebra mussels are an outsize threat to Texas’ waters, alien invaders bringing with them the prospect of significant ecologic and economic damage to the state’s inland lakes and rivers. At risk are the loss of productive fisheries, native mollusks and other aquatic life as well as damage to water-supply infrastructure that can — already has — cost Texans hundreds of millions of dollars.
The extent of that threat expanded over the past three weeks as biologists confirmed established, reproducing populations of the invasive mussels in two Texas river systems previously spared their unwelcome presence.
“It was disheartening, considering the effort everyone has put into preventing them from spreading,” Monica McGarrity said of the discovery in early June of zebra mussels in Canyon Lake on the Guadalupe River and the confirmation barely a week ago of a “full blown” infestation in Lake Travis on the Colorado River.
Those discoveries mean zebra mussels are now established in 11 Texas lakes on five river basins, all coming less than a decade after the first infestation was discovered on Lake Texoma on the Red River.
McGarrity knows only too well what the discovery of zebra mussels in the two reservoirs could — and almost certainly will — eventually mean to the lakes and the waterways on which they sit. McGarrity leads the Texas Parks and Wildlife Department’s aquatic invasive species team, a group of scientists focused on the invasive, non-native organisms that threaten the state’s waters and the life in them.
“When you look at their effects, it’s a concern any time you see zebra mussels show up in a new river system,” she said. Spread from Eurasia
Texas has seen that concern spread quickly as zebra mussels’ range has expanded at a depressingly quick rate across the state, invariably enabled by unwitting or uncaring boaters who transport the invasives from infested lakes to new waters.
That zebra mussels are even an issue in Texas came as somewhat of a surprise to biologists and fisheries managers.
Native to Eurasia, where they are found on deep, cool waters and do not exhibit the behaviors that cause so much trouble in North America, zebra mussels were not thought to be able to survive in the state.
The mussels were first documented on this continent in the late 1980s when they were found in Lake St. Clair, a water body near Detroit and connected to Lake Erie. The mussels almost certainly arrived in ballast water of commercial ships, introduced as adults or larvae (called veligers) when that water was pumped out of ballast tanks.
With no natural enemies and a forage-rich environment, the small mussels exhibited explosive population growth. A single zebra mussel produces about 1 million eggs a year, and populations grow to billions within a year or less. Once established they are impossible to eradicate. Zebra mussel populations in infected waters grow so high and so quickly that the mollusks coat any exposed, hard surfaces, clog water pipes and other water transportation infrastructure, outcompete native species, alter ecosystems and can even change the chemical composition of water.
They did just that in the Great Lakes and much of the upper Midwest through the early 1900s, expanding their range with the help of humans. Adult zebra mussels attached to boat hulls or motors or microscopic larvae surviving in bilges, bait bucket, livewells and even the water in outboard cooling system were transported from infected waters to new rivers and lakes, where they thrived and wrought their considerable damage.
But zebra mussels, native to a region where water temperature seldom if ever rose into the 80s, were not thought to be able to thrive in warmer waters.
“Based on everything known about them, it was thought (Texas) was too hot for them,” McGarrity said. “But they seem to have been able to adapt.”
Zebra mussels marched south through the Midwest, arriving in Texas in 2009. Over the next seven years, they spread to lakes in the Brazos and Trinity river basins, establishing populations in lakes Belton, Bridgeport, Eagle Mountain, Lewisville, Randell, Ray Roberts, Stillhouse Hollow and Dean Gilbert, a 45-acre lake near Sherman. Larvae have been found in a couple of other reservoirs, including Lake Livingston.
The mussels seem to thrive best in waters with an alkaline or neutral pH level, McGarrity said. Lakes in north and central Texas tend to fit that profile, while lakes in eastern Texas, where zebra mussels have not taken hold, are more acidic. Dire consequences
When the mussels invade, they can have dramatic economic and ecological consequences.
The mussels reproduce so quickly and in such dense concentrations they carpet any hard surface. That includes water management and transportation infrastructure such gates and pump parts, intake screens and pipes. The mussel concentrations are so thick they clog and close those vital systems.
This damage to water infrastructure systems has cost billions nationwide and well into the hundreds of millions in Texas.
The most breathtaking single example of economic damage zebra mussels have caused in Texas sprung from the infestation on Texoma. There, the North Texas Municipal Water District, which provides water for more than 1.5 million Texans in the Dallas area, was forced to construct a new 46-mile pipeline and other infrastructure to transport and treat water from the zebra musselinfested reservoir. That project cost the water district $300 million and resulted in a 14 percent jump in customers’ water rate.
The mussels’ ecological consequences can be just as dramatic; they can collapse a fishery and change a water body’s chemistry.
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 as well as consuming 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 now lacks fertility to support other aquatic life.
“They directly compete with species such as shad for plankton,” McGarrity said.
Threadfin and other shad, especially, depend greatly on rich plankton supplies to thrive. Without it, shad populations decline. Without shad, predator species such as white bass, striped bass, which provide a large percentage of the fishing opportunity on many of the mussel-infected lakes, decline.
Zebra mussel infestations also benefit blue/ green algae, increasing toxic blooms of the algae resulting in fish kills, McGarrity said.
Zebra mussels also can mean death to any other mollusks in affected waters. Along with simply outcompeting native mussels for resources through their overwhelming numbers, zebra mussels will coat the natives’ shells with hundreds of individual zebra mussels, smothering the native. This is a particular concern with the latest infestations on the Colorado River and, especially, the Guadalupe River, McGarrity said. The Guadalupe holds a fragile population of native mussels, including three species listed as threatened under Texas law. Boaters’ responsibility
Zebra mussels’ damage isn’t limited to aquatic life. Evidence suggests the filter-feeding mussels are susceptible to concentrating a strain of bacteria responsible for botulism in birds. From 2002-06, botulism outbreaks on zebra mussel-infested Lake Erie resulted in the death of more than 50,000 waterbirds, mostly loons and ducks such as scaup which feed on mollusks.
A common canard among some of the public is that those ducks and other waterbirds are responsible for the spread of zebra mussels, claiming the birds transport either adult mussels or larvae between water bodies. That claim has been empirically disproved through research, McGarrity said.
What has been proved is that boats spread zebra mussels. And Texas, like the more than 30 other states where zebra mussels have been found, has scrambled to try blunting that spread through a combination of trying to educate the public about the threat posed by the mussels and how to avoid spreading them as well as imposing regulations aimed at forcing boaters to take preventative actions.
Using authority granted by the 2013 Texas Legislature, the Texas Parks and Wildlife Commission adopted regulations requiring 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 water-holding receptacles or face a citation carrying a fine of as much as $500. The rule applies to all boats, including paddlecraft such as kayaks and canoes.
But Texas holds almost a million boats in the state, a highly mobile boating public that regularly uses their vessels on multiple bodies of water, non-residents trailering their boats to Texas waters from other states, hundreds of freshwater water bodies and an estimated 1,500 boat ramps. It is almost certain zebra mussels will continue to spread downstream of infected water bodies, adding to the growing list. The odds are stacked against preventing the spread of the invasive mussels.
McGarrity acknowledges the daunting challenge zebra mussels present. But keeping the invasives bottled up as effectively as possible is a responsibility all Texas boaters should shoulder.
“The clean-drain-dry rule is very effective at preventing spread of zebra mussels,” McGarrity said. “It only takes a few minutes, and it can make a huge difference.”