Dr. Robert McMahon, Professor Emeritus of Biology at the University of Texas at Arlington, has been monitoring 14 North Texas reservoirs for the presence of zebra mussels using three techniques. One technique looks for zebra mussel DNA in the water, another uses a microscope to look for zebra mussel larvae (veligers) in the water, and the third uses a submerged monitor to look for newly settled juvenile mussels.

Lakes involved in the study include Eagle Mountain, Lewisville, Lavon, Ray Hubbard, Ray Roberts, Arrowhead, Bridgeport, Tawakoni, Caddo, Wright Patman, Fork, Lake O' the Pines, Caddo and Texoma.

Of those lakes, only Lake Texoma is known to be infested with zebra mussels.

Despite rigorous and repeated sampling in North Texas, Dr. McMahon has detected no zebra mussels of any life stage in any Texas lake other than Lake Texoma. Nevertheless, the most recent tests, carried out in October 2011, showed low levels of zebra mussel DNA in six lakes: Eagle Mountain, Lewisville, Ray Roberts, Arrowhead, Bridgeport and Caddo.

"Lakes Lavon, Ray Hubbard, Tawakoni and Wright Patman had no detected zebra mussel DNA in the samples," said McMahon. "Lavon and Ray Hubbard tested positive for zebra mussel DNA in the spring of 2011, so the absence of any veliger DNA in the fall samples suggests that mussels have not become established in those lakes. Lakes Fork, Lake O' the Pines and Bob Sandlin were not examined for mussel DNA in the fall of 2011, because they were considered inhospitable to zebra mussels based on high summer temperatures and low calcium levels."

The presence of zebra mussel DNA in the water is not an indication that zebra mussels have become established in a lake or that they will become established. "There are cases in the western U.S. where positive DNA results indicated the presence of zebra mussels, but those results could never be confirmed," said Brian Van Zee, TPWD Inland Fisheries regional director. "In fact, Lakes Ray Hubbard and Lavon tested positive in the spring of 2011 for zebra mussel DNA, but the presence of zebra mussels could not be confirmed by microscopy or settlement samplers. The spring 2011 samples were taken right around the time that contaminated boats were found on both lakes, so that may be why they tested positive. Both boats were removed and decontaminated, and subsequent monitoring on these two lakes, by three different entities, has not confirmed the presence of zebra mussels."

McMahon noted that he was surprised by the positive result for Caddo Lake. "I consider the lake's calcium levels to be too low and the summer water temperatures too high to support zebra mussels," he said.

Microscopic examination of water samples from the 14 lakes in both June and October 2011 showed zebra mussel veligers present only in Lake Texoma. "During June juvenile mussels were found on settlement monitors only in Lake Texoma, while no juvenile zebra mussels were found on settlement monitors in any of the 14 lakes in October, including Texoma," McMahon added.

McMahon suspects that boats being transported from Lake Texoma to other lakes are the source of the DNA found in the six lakes. "The data suggest that mussels and/or mussel larvae are being carried into these lakes by recreational boaters but are not becoming established as a sustainably reproducing mussel population," he said. "This is a sign that mussels are being introduced to Texas lakes, and if such introductions continue, some of these lakes may eventually become infested with zebra mussels."

"TPWD will continue monitoring these lakes for the presence of zebra mussels and doing everything it can to encourage boaters and anglers to Clean, Drain and Dry their boats for at least a week before moving them to another lake," Van Zee said. "We encourage boaters and anglers to visit http://www.texasinvasives.org/ to learn more about how they can help protect the waters they enjoy."

Concern over the possible transport of zebra mussels between bodies of water prompted TPWD to propose new regulations governing movement of boats or fish between lakes in North Texas. Details on the proposed regulations can be found at http://www.tpwd.state.tx.us/newsmedia/releases/?req=20120126b.

Zebra Mussel Background Information

Robert F. McMahon
Professor Emeritus
Box 19498
Department of Biology
The University of Texas at Arlington
Arlington, Texas 76019

David K. Britton
Aquatic Invasive Species Coordinator
Fisheries & Aquatic Resource Conservation
U.S. Fish & Wildlife Service, Region 2

How should people interpret the "weak positives" findings for these lakes?

Britton: Weakly positive results from a PCR assay can have many explanations. Such results can be explained by errors such as contamination or insufficiently specific sequence primers. Or they could be a true detection of the presence of target DNA. Conclusions must be based on all available lines of evidence. At this point, we cannot completely rule out the potential for contamination, but many of us believe that it is unlikely that samples were contaminated. We also do not have evidence to suggest that the PCR primers were not specific enough to amplify DNA of species other than zebra mussels. These primers have been tested on closely related species and are specific enough to distinguish zebra mussels from quagga mussels. Thus, it is possible that these results truly reflect the presence of zebra mussel DNA in Texas waters. This does not mean that there are established populations of zebra mussels in these waters.

The DNA could come from dead mussels attached to a boat hull, or from feces of waterfowl that have eaten mussels during a recent visit to Lake Texoma, for example. It is too early to jump to conclusions regarding the presence of live mussels in these waters. Another researcher has sampled many of the same waters, using equivalent methods, and has not found any evidence of zebra mussels in these waters. Therefore, we should interpret the current "weak positives" as suggestive--but certainly not conclusive. Remember, if you were to receive a weakly positive test result for a disease, in all likelihood, your doctor would prescribe additional tests before offering a definitive diagnosis. Likewise, we have reason to be concerned about these results, but it would be prudent to gather additional information before drawing conclusions. McMahon: It is important to consider that neither zebra mussel veliger larvae nor settling zebra mussel larvae have not been found in any of the 14 studied lakes except Lake Texoma, where a self-sustaining zebra mussel population is clearly established. Even presence of zebra mussel veligers in the other 13 lakes (which has not occurred) is not conclusive proof that a self-sustaining zebra mussel population has been established, although it is stronger evidence than veliger DNA. In contrast, settlement of mussel juveniles on settlement monitors is strong evidence of a self-sustaining zebra mussel population. So far zebra mussel veligers and juvenile settlement have only been recorded at Lake Texoma, where it was expected.

How have similar findings turned out in other locations, in Texas or elsewhere?

Britton: Test results last spring indicated the presence of zebra mussel DNA in Lake Lavon and Lake Ray Hubbard. This is not surprising considering that there is a confirmed population of zebra mussels in Sister Grove Creek, upstream of the sampling sites. Plus there have been boats found on both lakes that were contaminated with zebra mussels. However, we have no evidence for an established population in either lake. In fact, follow-up tests at Lake Lavon and Lake Ray Hubbard this fall came back negative for zebra mussel DNA. The states of Colorado, New Mexico, and Utah have all detected zebra mussels in lakes previously thought to be free of them. Tests were either by PCR assay or light microscopy or by both. Follow-up tests, however, have come back negative for reservoirs in all three states. We may have evidence that zebra mussels are reaching these lakes, but it may be that they have not reached a threshold, or have not been introduced under the conditions necessary to start a new self sustaining invasion. Not every cigarette flicked out of a passing car will ignite a forest fire.

We are only beginning to get an idea of how often zebra mussels are being moved from one water body to another. This evidence, gathered in an attempt for early detection, is suggesting that zebra mussel introductions may be much more common than the record of successful invasions suggests. Thus, preventing boats from transporting mussels (and other aquatic invasives) would go a long way to protect our waters. McMahon: It is almost certainly the case that zebra mussels and/or their larvae have been carried by boaters from Lake Texoma to other Texas lakes for the last two years and from infested water bodies in Oklahoma and Kansas over an even longer time period. That no adult populations have been found in any Texas lakes other than Lake Texoma suggests that the rate of mussel introduction has not been high enough to allow self-sustaining mussel populations to develop. It is no surprise that mussel DNA has been found in a number of Texas lakes after the summer boating season as boaters are known to transport adults and veligers between infested and uninfested water bodies. However, the very low densities at which that DNA was found suggests that not enough veligers were transferred to establish a sustainably reproducing adult population. I suspect that when sampling is repeated at these 13 lakes in the spring of 2012, after an extended period of reduced boater movements during the preceding winter, many of the sampled lakes found to have mussel DNA in the fall of 2011 will not have a positive DNA result.

Why are three different monitoring techniques used, and how do they complement each other?

Britton: The three main monitoring techniques for zebra mussel larvae are 1) cross-polarized light microscopy, 2) imaging flow cytometry, and 3) polymerase-chain-reaction based assays. The first two look for whole organisms, specifically the veliger larval stage, in concentrated water samples. The first is performed by trained technicians, the second by a robotic machine. The third method, PCR, looks for DNA. The whole organism need not be present. Thus, positive PCR results by themselves cannot be used to conclude that a zebra mussel population exists. The most accurate method for early detection is cross-polarized light microscopy. Imaging flow cytometry is the next most accurate. And, PCR is the least accurate. PCR, however, can be used to conclusively distinguish between zebra mussels and quagga mussels, two very similar species. This is very difficult to do with either of the other detection methods. Thus, the best evidence for an existing population of mussels (in the absence of finding a full-grown adult) is a combination of cross-polarized light microscopy and PCR. Some western states (e.g. Colorado and Utah) require evidence from some combination of techniques before concluding that zebra mussels are present. No veligers have ever been detected in any Texas reservoir (except for Lake Texoma) by cross-polarized light microscopy. A workshop discussing this topic was held in Denver in 2009. See http://www.musselmonitoring.com/2009workshop.asp.

On the same web site you will also find an executive summary of a report (now in publication) that discusses the three primary early detection methods. McMahon: I used three differing techniques each with a differing potential to detect a sustainably reproducing zebra mussel population in order to avoid falsely indicating that a lake was infested with a sustainable zebra mussel population. Thus, the PCR testing, which is the most sensitive of the tests I used, indicates only that veligers may be present in the water column but is not indicative of a reproducing mussel population because the veligers could have been carried into a water body by a number of means. Detection of zebra mussel veliger larvae by microscopic examination of plankton net tow samples indicates that mussel larvae are present in a lake and is a stronger indication of a reproducing mussel population, but is still not unequivocal evidence of infestation, because the origin of the veligers is not known (i.e., could be brought in by boaters or water transfers), nor can their potential to settle in great enough densities to develop into a sustainably reproducing adult population be assessed. In contrast, settling of juvenile mussels on a settlement monitor is very strong evidence of a sustainable adult mussel population. Thus, detection of low levels of mussel DNA can only be interpreted as a water body potentially being infested with a sustainable mussel population which requires further monitoring for confirmation. Detection of actual zebra mussel veligers is stronger but not conclusive evidence of zebra mussel infestation requiring more intensive monitoring and inspection for confirmation. In contrast, settlement of juveniles is strong evidence of a sustainable mussel population requiring even greater efforts to confirm the presence of adults. Only in mussel-infested Lake Texoma did all three means of mussel detection prove positive. In eight of 13 other lakes examined only mussel DNA was detected (2 in June and 6 in October) indicating the possible presence of veligers but not a sustainable adult population. Thus, these lakes can only be considered to be "suspect" for zebra mussel infestation and should continue to be closely monitored to confirm an actual sustainable adult infestation.

What is the USFWS doing to combat the spread of zebra mussels in North Texas?

Britton: The USFWS contributed funds to TPWD last year to help an eradication attempt in Sister Grove Creek. We assisted in providing watercraft inspection and decontamination training for TPWD last year and have expressed an interest in providing additional training when it is convenient to TPWD. We have provided educational/outreach materials in the form of the national Zap-the-Zebra brochures. And, most importantly, we have pursued a cooperative relationship with TPWD to help bring together partners and stake holders to more effectively and efficiently reach common goals. Most recently, the USFWS has helped TPWD to seek approval for a state management plan that would allow Texas to receive additional funding through our agency for invasive species work, including zebra mussel prevention efforts. However, approval of this state management plan is dependent on the Governor's signature. Unless the Governor (or a proxy from his office) signs this plan (already tentatively approved by the Federal Aquatic Nuisance Species Task Force), Texas will remain ineligible for this federal assistance. We are hoping that the Governor will sign this soon so that Texas can begin receiving funds in 2012.

What are the possible/probable consequences if zebra mussels do spread to other North Texas reservoirs?

Britton: They will not stop in North Texas. The entire Trinity basin is in jeopardy. The Dallas/Fort Worth Metroplex has approximately 6 million residents dependent on the Trinity River for drinking water. Downstream, the Houston metropolitan area has another 6 million residents, most of whom are dependent on the Trinity River basin as well. The combined populations of these two metro areas exceeds 12 million. That is more people than live in any of 44 of our 50 states. Zebra mussels have a well-documented record of clogging raw water intakes, trash screens, filters, and outboard motor cooling systems. They are known to sink navigational buoys and even floating docks. They discourage recreational use of waters they infest. They litter beaches in huge numbers and stink. Their sharp shells cut swimmer's feet. They undermine the established food web by removing phytoplankton while simultaneously promoting unwanted blue green algae that is harmful to fish (as well as any other species that eats the fish). They have even been linked to outbreaks of avian botulism. They pose a serious risk to our fisheries and to our state parks. Meanwhile, boaters who used to use Lake Texoma are moving their boats elsewhere, places that currently do not have the nuisance of zebra mussels or restrictions associated with blue-green algal blooms. This exodus could, in part, explain the sudden findings of zebra mussel DNA uncovered in Dr. McMahon's latest preliminary report. From the Trinity, zebra mussels will likely (and quickly) spread to other river basins within the state. Interbasin pipelines are numerous and prevention technologies are not being utilized.

What is the main message you would like to convey to the public, to boaters, and to anglers?

Britton: Boaters and anglers are an important part of our fight. There are simple steps that they can take to protect our waters. Clean, Drain, and Dry is the mantra we'd like to convey. Boaters should make a habit of cleaning their boat every time it is removed from a water body. All areas that can hold water must be completely drained. And, everything should be allowed to dry for as long as it takes, in fact, the longer the better. If boaters are unsure or have questions, they should contact TPWD. And, they should be aware that it is against state law to introduce zebra mussels into waters in Texas. It is also against federal law to transport zebra mussels across state lines. Boaters and anglers should expect to hear more about zebra mussels in the near future, because this is a serious problem with serious consequences. But, with their help, we can prevent further invasions.\