Giant Salvinia

Previously Funded Aquatic Invasive Species Research

Zebra Mussels

Near real-time detection and monitoring of invasive mussel species in Texas waterways

Baylor University

Early detection of zebra mussel veliger larvae requires microscopic analysis of plankton samples that can be time consuming and delay results. This project tested a novel and efficient process to more quickly detect and enumerate zebra mussel veligers, refine the technology, and explore spatiotemporal variability of veliger presence and density over time in the study areas.

Project Final Report

Assessing the Population Dynamics and Body Condition of Zebra Mussels Within and Between Two Texas Water Bodies with Different Population Trajectories: Lakes Belton and Stillhouse Hollow

Temple College

Long-term studies have indicated that some zebra mussel populations in Texas decline in density and growth rates over time, whereas others do not. This study sought to better understand population dynamics in two lakes with different population trends in conjunction with food availability and water quality parameters. This study evaluated potential explanations for population declines. Study results indicated the life span of zebra mussels in these lakes was approximately one year or less as seen in other Texas lakes. Additionally, results suggested high summer water temperatures negatively impacted mussel survival rates, particularly for older, larger individuals more subject to high temperature-induced starvation. Shell growth rate at 2 meter depth was similar to other lakes but declined at 8 meter depth suggesting a need for future studies to assess variation in growth at different depths. Although shell growth rates, densities, and survival did not vary between lakes, tissue dry weights were higher at Lake Belton where phytoplankton concentration (chlorophyll-a) was higher, suggesting better physiological condition. Future research could include long-term studies to better understand these populations and assessment of bacterioplankton to assess potential influence on differences in shell growth at different depths not explained by phytoplankton alone.

Project Final Report

Impacts of zebra mussels on native mussels and population dynamics and downstream dispersal of zebra mussels.

Texas State University

Zebra mussels have the potential to negatively affect Texas’ native unionid mussel species, many of which are listed as threatened or endangered species. This study surveyed native mussel populations to study zebra mussel fouling of their shells and the health of native mussels in infested areas. Laboratory tests were used to examine the impacts of zebra mussel die-offs and fouling on native mussels. Additionally, the project continued analysis of the population dynamics of zebra mussels in Canyon Lake and their spread downstream in the Guadalupe River. Lab experiments showed that both direct and indirect interactions with zebra mussels can significantly reduce unionid mussel glycogen storage, but zebra mussel infestation on shells had a significantly stronger effect than indirect competition. The study also found that increased temperatures in Texas likely exacerbated these effects. Decay of zebra mussels due to summer mortality was also found to cause considerable release of nutrients, including ammonia which is highly toxic to juvenile unionid mussels. Population dynamics of zebra mussels in Canyon Lake are explored in depth in the final report, but found that although higher summer temperatures seem to limit zebra mussels, the population has continued to increase and expand in Canyon Lake; monitoring of this population is ongoing to assess future trends.

Project Final Report

Growth, survival and reproductive success of zebra mussels in Texas lakes

Texas Christian University

Water chemistry plays an important role in the success of zebra mussel invasions. This research project evaluated how variable water chemistry conditions (for example, calcium concentration) in lakes across the state impact the growth and survival of zebra mussels. Previous studies have shown that calcium is critical for survival and growth of adult zebra mussels. This study examined whether calcium levels affect zebra mussel egg fertilization, development of larvae, and growth of juveniles and adults. Results of one experiment in this study found a significant relationship between calcium concentration and adult mussel survival after 84 days; results of veliger survival trials were not conclusive. However, calcium levels in experimental tanks may have increased during the study and research was hindered by the pandemic and pipe damage due to the winter freeze event resulting in trial periods that may not have been of sufficient length, and additional future research on this topic is needed. Notably, experiments with egg fertilization and development found failure of eggs to develop beyond fertilization in low calcium waters, supporting the theory that reproduction in waters below 12 mg/l calcium (e.g., many East Texas lakes) cannot occur.

Project Final Report

Dispersal of zebra mussels downstream of an invaded reservoir and assessing the risk of dreissenid mussel invasion into lakes of Texas

Texas State University

This project sought to predict zebra mussel invasion potential for Texas lakes using mathematical gravity models based on lake habitat suitability, lake attractiveness to boaters, and boater movements. An additional objective was to assess downstream dispersal of zebra mussels using plankton and settlement sampling. Results of the gravity model were instrumental in guiding early detection monitoring in Texas. Zebra mussel downstream dispersal assessment found settlement was greatest within six river kilometers (rkm) downstream of invaded lakes (less during summer months) but larvae were occasionally found as far as 97 rkm downstream and juvenile settlement as far as 55 rkm after a period of prolonged increase in river discharge. These findings suggest zebra mussels are dispersal limited in rivers but that long-distance dispersal may be facilitated by increased discharge.

Project Final Report

Assessing the risk of dreissenid mussel invasion in Texas based on lake physical characteristics and potential for downstream dispersal

Texas Tech University

This project sought to predict suitable habitat for zebra mussels in Texas using Maxent models and to refine them with lake physicochemical data. An additional objective was to assess the potential for downstream dispersal using zebra mussel environmental DNA (eDNA). Final habitat suitability model results were based only on global climate data and similar to past research and were ultimately not used to inform refinement of early detection monitoring strategy. Results of eDNA analysis found zebra mussel eDNA up to approximately 90 river kilometers downstream of infested lakes, suggesting or corroborating potential for long-distance downstream dispersal.

Project Final Report

Zebra mussel growth, reproduction, and invasion in Texas lakes.

University of Texas at Arlington

This research sought to evaluate trends in zebra mussel populations in Texas lakes. Research findings indicated that zebra mussels can decline in abundance in some lakes given certain environmental conditions exist, including extended periods of low pH and low water oxygen levels. Zebra mussel abundance can also be cyclical as their populations increase and decrease over time.

Project Final Report

Aquatic Plants

Using remote sensing to map Arundo donax populations in Native Fish Conservation Areas throughout Texas to better understand causal factors of invasion and set management priorities

Texas State University

Giant reed (Arundo donax) is a highly problematic invader of rivers and creeksides with significant impacts on both riparian and aquatic habitats and efforts to manage this species are ongoing in the Hill Country. This study tested and developed the use of remote sensing technology to identify infested areas and areas where infestation is increasing as well as examining landscape factors influencing infestations and identify areas at high risk of impacts. This technology was applied to Native Fish Conservation Areas across the state to aid in prioritizing areas for future control efforts.

Project Final Report

Combating invasive aquatic plants by manual removal and planting of native aquatic plants

University of Texas at San Antonio, Edwards Aquifer Authority, City of San Marcos, US Fish and Wildlife Service – San Marcos Aquatic Resources Center

Hydrilla is one of the most common invasive aquatic plants in the upper San Marcos River, which is home to endangered Texas wild rice—a submerged aquatic plant—and Fountain Darters. This research examined whether planting Texas wild rice and other native aquatic plants after hydrilla is manually removed can help to improve the effectiveness of hydrilla management efforts. This study also evaluated whether planting of native species can suppress or outcompete the invasive hydrilla. The results of laboratory studies found that water stargrass and Illinois pondweed planted collectively or alone cannot suppress or outcompete hydrilla if planted in smaller ratios than hydrilla but may have greater relative growth rates when planted in higher numbers. In field plots where up to 75% of hydrilla was removed, neither water stargrass nor Texas wild rice gained a competitive advantage. When 100% of hydrilla was removed, Texas wild rice exhibited equal or greater coverage to hydrilla. Regardless of removal percentage, hydrilla was found in all field plots at the end of the study, but was greatest in plots with 100% removal, indicating a great ability to quickly reinvade a disturbed site. Additional studies are needed to determine the optimal percentage of hydrilla to be removed and the number of native aquatic plants required to suppress hydrilla.

Project Final Report

Development of several new biological compounds in the laboratory to be used in management of giant salvinia.

Stephen F. Austin University

Giant salvinia is currently one of the most problematic aquatic plants in Texas due to its ability to develop dense mats that impact habitat for native fish and wildlife and impede boater access. This research developed natural ‘endocide’ compounds extracted from the giant salvinia plant itself. Testing showed that the endocides poisoned or inhibited growth of other giant salvinia. More research is needed to assess whether endocides have potential for helping to manage giant salvinia. However, although the university holds patents for several compounds, there were difficulties maintaining them in solution at ambient temperatures as would be needed for field use. Additionally, the principal investigator has retired and this promising work has not been continued.


Assessing abundance, sex ratio, and space use by suckermouth armored catfish to enhance control efforts

Texas A&M University; Texas State University

Non-native suckermouth armored catfish compete with native species, alter food webs, and cause habitat degradation through burrowing into banks, and efforts to remove these invasive fish are underway in the San Marcos River (SMR) to protect imperiled species. This study assessed seasonal abundance of this species in the SMR as well as assess movement and population sex ratios and tested potential new control augmentation techniques. Results of this study will aid in guiding and enhancing the efforts of ongoing removal efforts. Estimates of total fish in the 1-km study reach of the SMR were approximately 4,600 individuals; abundance declined as water levels dropped but was consistently higher near artificial structures, at shallower depths, and in faster velocities. The population sex ratio was skewed toward females (60%) and sexual dimorphism (i.e., males larger) and size data from past tournaments suggest removal efforts may be more biased toward removing males. Sexual dimorphism evaluation was used to successfully create a simplistic, model-base chart that can be used to predict sex from external characteristics (i.e., total length to dorsal and anal fin length ratios) which may aid in data collection from harvested individuals. The results also indicate that armored catfish in the SMR are largely restricted in their movement, suggesting suppression is likely to be successful but eradication unlikely due to recolonization by a small number of dispersing individuals. Experimentation with habitat exclusion devices suggests this may be a complementary management strategy if implemented at a broad scale at the sites of aggregations. This work forms the foundation for current research into the potential use of genetic biocontrols to manage this species in the SMR.

Project Final Report

Introduction of non-native fishes in Texas streams

Stephen F. Austin University, Texas A&M University – College Station

Bait-bucket introductions of non-native fishes are widespread in freshwater ecosystems in the US. This may be how two coastal fish species—the Sheepshead Minnow and the Gulf Killifish—were introduced into inland freshwaters outside of their native range in Texas where they may have negative effects on imperiled native fish species. Researchers conducted bait shop surveys in Texas to learn more about the fish species being sold and whether this is a potential introduction pathway for these two invasive species. Field studies in the Brazos and Red rivers evaluated the status of populations of these invasive fishes and how they may affect the native aquatic ecosystem. Additionally, an ecological model was developed to map current and possible future introductions of these species across Texas river basins. Although unverified anecdotal reports suggest that these two fish species were historically sold and introduced as live bait, surveys and visits to bait shops during this study found no evidence of these species being sold or the bait industry being a current pathway for invasion. Although in low abundance, these two species were found in the wild in sites of the Middle Brazos River where they were collected in past surveys. These detections were in areas with likely high traffic for recreational fisheries and modeling suggested sites near lakes/reservoirs are most suitable for these species, which could link introduction to recreational fishing in these places. Continued monitoring of wild introduced populations is needed to evaluate potential upstream distribution expansion and ecological impacts.

Project Final Report

Evaluating research gaps related to tilapia in Texas

Texas Tech University

Invasive tilapia have been found in some Texas waters and can escape from ponds built on creeks. Researchers assessed the risk this invasive species poses to imperiled native fishes by reviewing published science and developing a computer model of areas in Texas that would be suitable for them to survive. Results regarding which imperiled fishes could be impacted by tilapia study contributed to further conservation prioritization assessment research that led to revision of regulations for exotic species that seek to prevent harmful impacts on native ecosystems to require department approval for stocking tilapia in a designated conservation zone since early 2021.

Mapping the geographic range of bigheaded carp in Texas

Texas Tech University

Current regulations prohibit movement of live nongame fish in areas in Texas where invasive bigheaded carp have been found in the past to prevent them from being spread by bait bucket transfers. Researchers and TPWD biologists surveyed for these invasive fish using electrofishing and tested for their DNA in the environment. No live bigheaded carp or their environmental DNA were found in new areas. This suggests that these regulations and outreach efforts as well as manmade barriers (i.e., dams) have been effective to prevent these invasive fishes from expanding their range in Texas waters.