Few Austin-area residents, or many people in the rest of Texas for that matter, paid much attention in the summer of 2017 when state fisheries scientists announced that zebra mussels had been discovered colonizing Lake Travis.

After all, the 19,000-acre Colorado River reservoir a bit upstream from the capital city and popular with recreational boaters and anglers was just the latest in a growing string of Texas reservoirs and river systems that the fingernail-size, non-native mollusks had invaded since first being discovered in the state eight years earlier. And while fisheries biologists and water managers offered somber warnings about the environmental and economic devastation the small bivalves could wreak, such threats easily could be ignored as overblown or immaterial to the average Austinite’s life. It was just a little mussel. What damage could it do?

They got a hint earlier this month when folks in much of Austin cracked their kitchen faucets or turned on their showers and the reality of this invasive species’ wide-ranging, multifaceted negative impacts was, quite literally, brought home. Instead of the clear, clean water they expected, out poured a stream of foul-smelling, rancid-tasting liquid.

Zebra mussels were to blame. Water supply lines tied to Lake Austin, immediately downstream from Travis and invaded by zebra mussel barely a year ago, were coated with a thick layer of millions — perhaps billions — of the bivalves. When some of those water lines were temporarily taken out of service for repairs, the mussels living in miles of those drained lines died and began deteriorating. When the lines were placed back into service, the flowing water carried the detritus of those rotting corpses directly into homes. The water, users said, smelledlike sewage effluent. It took well over a week and considerable expense for Austin Water, the water supplier, to solve the problem.

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Precautions a must

The episode illustrates just one of the myriad problems posed by these invasive bivalves — problems that cause hundreds of millions, if not billions, of dollars in economic costs, alter whole freshwater ecosystems, potentially devastating native fisheries and other aquatic life. Most crucially, it underscores how important it is that Texas boaters and anglers — the people directly responsible for the spread of zebra mussels — to take actions that can prevent or at least slow the spread of these malicious mollusks.

Boats brought zebra mussels to North America and have spread them to more than 30 states, hitching rides in bilges, live wells, bait wells, engine cooling systems, on hulls and engines and anything else that can hold water. Native to deep, cold lakes in Eurasia, the mussels were first documented on this continent in 1986 in Lake St. Clair, a lobe of Lake Erie where they are believed to have arrived in ballast water of commercial ships, introduced to the lake as adults or microscopic larvae (called veligers) when that water was pumped out of the ballast tanks.

Absent natural enemies and injected into a forage-rich environment, the small mussels exhibited explosive population growth. A single zebra mussel produces about a million eggs a year, and populations grow to billions within a year or less. And the mussels proved devilishly adaptable; the mollusks have thrived in waters scientists once thought too warm for an invasive whose native range is confined to cold waters.

Once established, zebra mussels have proven impossible to eradicate.

Zebra mussel populations in infected waters grow at an astounding rate, quickly coating any exposed, hard surfaces, clogging water pipes and other water transportation infrastructure, out-competing native species, altering ecosystems and even changing the chemical composition of water.

The invasive mussels arrived in Texas less than a decade ago, when they were documented in Lake Texoma on the Red River. It was there they showed just how much economic havoc they could cause.

When zebra mussels showed up in Lake Texoma in 2009, the North Texas Municipal Water District was drawing water from the Oklahoma side of Lake Texoma and piping it to Lake Lavon on the Trinity River system. That water was carrying zebra mussels veligers, so small filters can’t block them. Not only did this provide an avenue for new infestations, it violated federal law forbidding transport of an invasive species across state lines. To address the problem, the water district built a 46-mile pipeline and infrastructure for a treatment plant that could treat the water for zebra mussels. The effort cost $300 million and was paid for with a 14 percent increase in customers’ water rates.

Submerged equipment and structures in zebra mussel-infested waters are a battleground in a constant, costly war against the mollusks, which coat any hard surface including water-intake pipes and the screens and filters designed to block foreign objects from the systems.

On Lake Austin, the mollusks quickly covered and blocked as much as 50 percent of the 24-foot-by-30-foot screens on water intake pipes within a year of the invasives’ arrival. It cost Austin Water (and its customers) $212,000 a year to contract divers to clean those intakes.

The economic costs of zebra mussels are daunting, and the environmental costs can be devastating.

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Filtering out life

Zebra mussels are filter feeders, with each inch-long adult sifting as much a liter of water per day. They strain calcium and other minerals used to build their shells, removing these nutrients from the water. They also out-compete beneficial native mollusks, denying them needed nutrients and even smothering the natives, some of them endangered species, by coating them in a suffocating carpet of scores of smaller zebra mussels.

More widely damaging, the billions of mussels in an infested water body consume vast quantities of phytoplankton, zooplankton and algae. The most visible result, as seen on many lakes where the mussels have thrived, is incredibly clear, nutrient-poor water as the mussels remove the suspended minerals and microscopic plant and animal life.

This clear water can help trigger blooms of some forms of algae that deplete life-giving dissolved oxygen from waters, resulting in fish kills. It also encourages growth of invasive aquatic plants, further damaging the ecosystem.

But the most wide-ranging negative effect of zebra mussels to a fishery almost certainly is its effect on the aquatic food chain. Threadfin and other shad species, as well as other aquatic life, depend upon an abundant supply of plankton on which to feed. The vast amounts of plankton the invasive mussels strip from the water removes that food source from shad, causing their numbers to decline.

Those shad are primary components of the diet of larger fish, especially open-water predator species such as white bass and striped bass but also more littoral species such as crappie and largemouth bass.

A collapse of the shad population caused by zebra mussels hogging all the plankton quickly ripples up the food chain to those fish species dependent upon those forage species. Fewer shad equals fewer white bass, stripers, crappie, largemouths and other fish that Texas’ million-plus freshwater anglers target each year.

The effect of zebra mussels on a lake’s fishery can be insidious, taking years to manifest. The incremental collapse can be hard to recognize until it is impossible to miss.

Even terrestrial life can suffer zebra mussels’ effects. 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 and are among the few animals, aquatic or terrestrial, that do so.

Boats offer free ride

Those waterbirds have been blamed by some as vectors responsible for the spread of zebra mussels, the claim being that birds transport either adult mussels or larvae between water bodies. Research has empirically disproved that speculation.

Boats, not birds, are responsible for spreading mussels to new river systems. Floods can spread zebra mussels down a river system, injecting them into previously “clean” reservoirs. But those mussels — adults and the microscopic veligers — travel in boat bilges and bait wells, on hulls and outboard lower units and cooling systems from infected lakes in one river system to a new river system by boaters. A liter or less of water in a bilge or bait well can hold a couple og hundred of the near invisible veliger, a self-contained breeding colony of the mussels that can survive for weeks until accidentally released when the boat is launched in a new water body.

That is how zebra mussels have been spread to at least 22 lakes in five Texas river basins. So far. Those lakes include Austin, Belton, Bridgeport, Canyon, Eagle Mountain, Georgetown, Grapevine, Lady Bird, Lavon, Lewisville, Livingston, Ray Roberts, Richland Chambers, Stillhouse Hollow, Texoma, Travis, Waco and Worth. River systems are Brazos, Colorado, Guadalupe, Red and Trinity.

Texas, like all states where zebra mussels have been found, is scrambling to blunt that spread through a combination of education as well as imposing regulations aimed at forcing boaters to take preventive actions.

Texas law requires all boaters approaching or leaving reservoirs, rivers or other public freshwater anywhere in the state to 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.

Despite the education efforts and laws, zebra mussels continue to spread to new Texas waters. And it’s certain that spread will continue. More Texas lakes and rivers and the aquatic life in them — and more Texans — will experience the effects of zebra mussels. And as many Austin-ites discovered earlier this month, the consequences are ugly and expensive.

shannon.tompkins@chron.com

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