Introduction 

The zebra mussel (Dreissena polymorpha) is an 

invasive species that has posed appreciable risks to 

the waters of the United States since its accidental 

introduction into Lake Saint Clair, Michigan, in 1988 

(Hebert and others, 1989). Broad physicochemical 

adaptability, prolific reproductive capacity, and rapid 

dispersal methods have enabled zebra mussels, within a 

period of about 20 years, to establish populations under 

differing environmental conditions across much of the 

eastern part of the United States (Strayer, 2009).

Overland transport of zebra mussels was likely 

responsible for the rapid spread of the species in 

the Eastern United States (Carlton, 1993). Zebra 

mussel adults and juveniles can attach to boat hulls, 

trailers, and other submerged equipment, facilitating 

their introduction into noninfested water bodies far 

removed from their current distribution (Carlton, 

1993). Microscopic zebra mussel larvae (veligers) can 

be transferred between water bodies through overland 

transport in vessel bilge water, ballast water, live wells, 

and cooling systems (Ram and McMahon, 1996). In 

regions where zebra mussels have already become 

established, downstream transport might increase the 

dispersal rate (Horvath and Lamberti, 1999). Since no 

large-scale, environmentally safe eradication method 

has been developed for zebra mussels, it is difficult to 

remove established populations (Strayer, 2009).

Zebra mussels reproduce externally, with 

fertilization occurring in the water column. In warmer 

climates, reproduction commonly occurs semiannually 

during two distinct spawning events (typically in the 

spring and fall). Zebra mussels have a microscopic, 

free-swimming planktonic larval stage that lasts 

approximately 4 weeks (Mackie, 1991). Late-stage 

veligers settle out of the water column and attach to 

substrates by using proteinaceous byssal threads (fig. 1). 

Once attached, the mussels metamorphose into juveniles. 

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2012–3077

August 2012 Printed on recycled paper

Prepared in cooperation with North Texas Municipal Water District, Dallas Water Utilities, Greater Texoma Utility Authority, and 

City of Sherman Water Utilities

Under ideal conditions, zebra mussels reach adulthood 

within 3 months (Mackie, 1991).

The presence of large zebra mussel populations 

often causes undesirable economic and ecological 

effects. Dense zebra mussel aggregations clog water 

intake and transfer pipes (Mackie, 1991). In the United 

States, zebra mussel infestations cause millions of 

dollars in damage each year to water-processing 

infrastructure and hydroelectric powerplants, with an 

estimated 10-year cost of $3.1 billion (Cataldo, 2001). 

Zebra mussels displace native mussels by outcompeting 

them for food resources, thus threatening the viability 

of native mussel species (Schloesser and others, 1997). 

Selective rejection of harmful organisms during feeding 

by zebra mussels can concentrate certain species of 

cyanobacteria that produce microcystin (a cyanotoxin 

associated with harmful algal blooms) and can increase 

concentrations of the organic compound geosmin, which 

results in taste and odor issues in water (Vanderploeg 

and others, 2001). 

Figure 1. Byssal threads on an adult zebra mussel (Dreissena 

polymorpha), an invasive species that has posed appreciable 

risks to the waters of the United States.

USGS Zebra Mussel Monitoring Program for North Texas
In Texas, the presence of zebra mussels was 

first confirmed in April 2009 in Lake Texoma in the 

Red River Basin along the Texas–Oklahoma border 

(fig. 2) (Texas Parks and Wildlife Department, 2009). 

They were most likely introduced into Lake Texoma 

through overland transport from an infested water 

body. Following this local introduction, the presence 

of zebra mussels has been reported in both the Red 

River and Washita River arms of Lake Texoma. A 

pipeline leading south from Lake Texoma past the City 

of Sherman water treatment plant to West Prong Sister 

Grove Creek enables the interbasin transfer of water 

from the Red River to the Trinity River Basin. The 

transferred water flows through Sister Grove Creek 

and into Lavon Lake, where it is then utilized as a 

drinking water supply for residents in the Dallas–Fort 

Worth metropolitan area of north Texas. The pipeline 

thus provides a direct route for interbasin transfer of 

zebra mussels, but since the discovery of the species in 

Sister Grove Creek (August 2009), the transfer of water 

from Lake Texoma to Lavon Lake has been halted. In 

July 2012, however, the presence of zebra mussels in 

Ray Roberts Lake was verified. Ongoing studies are 

being conducted to determine if this is a reproducing 

zebra mussel population.

Water managers tasked with supplying the 

6.6 million residents of the Dallas–Fort Worth 

metropolitan area must ensure that the area receives a 

continuous supply of water that meets both the needs 

of the current (2012) and the projected (doubling in 

number by 2050) populations (North Central Texas 

Council of Governments, 2011). This metropolitan area 

depends on surface water captured in area reservoirs, 

including those in the Trinity River Basin, for the 

primary source of drinking water (North Central Texas 

Council of Governments, 2011). Major area reservoirs 

used for drinking water supplies include Lavon Lake, 

Ray Hubbard Lake, Ray Roberts Lake, Lake Lewisville, 

and Grapevine Lake (fig. 2). Interbasin transfers of 

water from reservoirs outside the Trinity River Basin 

originate from Lake Texoma on the Texas–Oklahoma 

border and from two additional lakes in north Texas: 

Lake Tawakoni and Lake Fork Reservoir (withdrawals 

from Lake Palestine are planned). The presence of an 

established zebra mussel population in a reservoir in the 

Trinity River Basin could result in increased operations 

and maintenance costs for water resource managers and 

could potentially serve as a source population leading to 

further expansion of this aquatic nuisance species. 

Description of the USGS Zebra Mussel 

Monitoring Program

The U.S. Geological Survey (USGS) Zebra Mussel 

Monitoring Program for north Texas provides early 

detection and monitoring of zebra mussels by using 

a holistic suite of detection methods. The program is 

designed to assess zebra mussel occurrence, distribution, 

and densities in north Texas waters by using four 

approaches: (1) SCUBA diving, (2) water-sample 

collection with plankton tow nets (followed by laboratory 

analyses), (3) artificial substrates, and (4) water-quality 

sampling. Monitoring under this program began in 

April 2010 at Lake Texoma, the City of Sherman plant, 

Sister Grove Creek, and Lavon Lake. Ray Hubbard Lake 

was added to the program in October 2010 and Lake 

Lewisville in June 2011. Grapevine and Ray Roberts 

Lakes were added in October 2011, and Lake Fork 

Reservoir and Lake Tawakoni were added in November 

2011. Surveys at Lake Palestine began in April 2012. Data 

collected during this type of monitoring can assist rapid 

response efforts and can be used to quantify the economic 

and ecological effects of zebra mussels in the north 

Texas area.

SCUBA Diving

USGS personnel certified as SCUBA divers search 

for juvenile and adult zebra mussels during dives to 

determine the occurrence and densities of zebra mussels 

on water intake structures, boat ramps, marina flotation 

and support devices, or other submerged structures 

(fig. 3). Dive efforts are focused on collecting zebra 

mussel occurrence and settlement data by using typical 

veliger spatial-temporal settlement patterns. At least 

once each year at each monitored lake, a dive is done to 

assess change in zebra mussel presence or settlement from 

the previous spawning event. Additional dives are done 

as necessary or as requested. Zebra mussels identified 

during SCUBA diving are enumerated either by the 

divers underwater or by analysis of high-definition video 

footage, which is collected during each dive. If zebra 

mussels are present, their density (mussels per square 

meter) is determined.

Water-Sample Collection and

Laboratory Analyses

Water samples are collected by using fine mesh 

(64-micron) plankton tow nets to monitor for the 

presence of zebra mussel veligers (fig. 4). Collection 

of water samples with plankton tow nets is targeted 

toward introduction vectors of veligers (boat ramps and 

marinas), water-processing infrastructure, and areas 

conducive to typical zebra mussel dispersal patterns. Tow 
Figure 2. Study area for the U.S. Geological Survey Zebra Mussel Monitoring Program in north Texas. 

Trinity River

EXPLANATION

Interbasin

pipeline

Lake with known reproducing zebra mussel population

Lake with ongoing investigation

Other lake

Red River Basin

Trinity River Basin

Approximate location of interbasin pipeline

Base from U.S. Geological Survey digital data,1:24,000 

Universal Transverse Mercator projection 

Zone 14, North American Datum of 1983

Watersheds from Texas Water Development 

Board, 2002 

Lake Texoma

Lake

Tawakoni

Lavon

Lake

Lake

Palestine

Lake

Lewisville

Ray

Roberts

 Lake

Ray Hubbard

Lake

Lake Fork

Reservoir

Grapevine

Lake

Red River

Red River

arm

Washita

 River

arm

Washita River

Sister

Grove Creek

97°30' 97°00’ 96°30' 96°00’ 95°30'

34°00’

33°00’

32°00’

Dallas

Sherman

Denison

Fort Worth

LOVE

COUNTY

MARSHALL

COUNTY

COLLIN

COUNTY

GRAYSON

COUNTY

HUNT COUNTY

DENTON

COUNTY

FANNIN COUNTY

COOKE

COUNTY 

ROCKWALL 

COUNTY

TARRANT

 COUNTY

BRYAN

COUNTY

DALLAS COUNTY

ELLIS COUNTY

JOHNSON

 COUNTY

KAUFMAN

 COUNTY

RAINS

COUNTY

VAN ZANDT

COUNTY

HENDERSON

COUNTY

NAVARRO

COUNTY

HILL COUNTY

WISE

 COUNTY

HOPKINS

 COUNTY

DELTA

 COUNTY

LAMAR

 COUNTY

TEXAS

OKLAHOMA

TEXAS

OKLAHOMA

Study

area
net sampling is done in spring and fall (approximately 

May and October), when veliger densities are expected 

to be highest. Tow net sampling is done from boats at 

probable areas of infestation in lakes, at selected bridge 

crossings on Sister Grove Creek, and in a raw water 

metering vault at the City of Sherman plant. Each study 

area has an area-dedicated plankton tow net to prevent 

accidental cross contamination by sampling equipment. 

Replicate water samples (1,000 liters each) are collected 

at each site with plankton tow nets (either vertical or 

horizontal, depending on depth at sampling location) 

according to established methods (Denise Hosler, Bureau 

of Reclamation, written commun., 2010). At Lake 

Texoma, where zebra mussels are established, discretedepth veliger sampling, which allows spatial-temporal 

profiling, is done by using a centrifugal pump. The 

pump intake hose is lowered to discrete depths during 

sample collection while the outlet water is run through a 

plankton tow net. 

Each water sample collected by plankton tow 

net is funneled into a collecting cup, rinsed into a 

250-milliliter polyethylene bottle, and preserved in 

50-percent ethanol. Samples are transported to the 

USGS North Texas Program–Fort Worth laboratory 

and analyzed by using cross-polarized light microscopy 

(fig. 5) and methods described in Johnson (1995) 

to identify veliger presence. If present, veliger 

concentrations (number of veligers per liter) are 

determined by using methods described by the Bureau 

of Reclamation (Hosler, 2011). A replicate sample is sent 

to a Bureau of Reclamation genetic analysis laboratory 

in Denver, Colorado, that uses the methods described 

in Hoy and others (2010) for polymerase chain reaction 

(PCR) analysis, which targets species-specific DNA to 

verify the presence and species of veligers that were 

detected during microscopy. 

Artificial Substrates

Colonization and growth of zebra mussel juveniles 

and adults are also monitored by using artificial 

substrates (fig. 6). Each sampler is composed of a 

series of stacked 4- by 4-inch hardboard tiles that are 

separated with 0.25-inch spacers. Samplers are deployed 

where zebra mussel introduction is most likely (boat 

ramps and marinas), near vulnerable water processing 

infrastructure (pumps and intake structures), and 

in other areas of expected zebra mussel occurrence. 

Samplers are suspended from structures that are in or 

hang over the water body. The samplers are routinely 

visually inspected in the field by USGS personnel 

for the settlement of zebra mussels. Samplers used 

during this type of monitoring remain in the water for 

at least 3 weeks before being inspected to allow time 

for colonization of naturally occurring algae, which 

promotes veliger settlement. If zebra mussels are present, 

their density (mussels per square meter) is determined, 

and the shell lengths of a subset of individuals are 

recorded. A handheld digital microscope with attached 

camera is used to detect and document the shell lengths 

(usually greater than 400 micrometers) of recently 

settled mussels. 

Figure 4. U.S. Geological Survey personnel sampling for zebra 

mussel (Dreissena polymorpha) veligers by using a plankton tow 

net, Lavon Lake, Texas.

Figure 3. U.S. Geological Survey personnel SCUBA diving for 

the Zebra Mussel Monitoring Program in Lake Texoma, Texas 

and Oklahoma. 
Figure 5. Cross-polarizing microscope, camera, and 

associated computer equipment, U.S. Geological Survey North 

Texas Program–Fort Worth laboratory.

Water-Quality Sampling

Conducive temperature, pH, concentrations 

of calcium and dissolved oxygen, and other 

physicochemical water properties are critical for zebra 

mussel survival, growth, and reproduction. Collection 

of water-quality data assists in risk assessments for 

zebra mussel infestation. Water-quality samples are 

collected by using standard field collection protocols 

(U.S. Geological Survey, variously dated) at two discrete 

points in the water column—at lowest depth and at 

surface—at selected lakes twice per year (approximately 

May and October). Discrete water-quality samples are 

stored on ice and shipped to the National Water Quality 

Laboratory in Denver, Colo., by following laboratory 

protocols described in Alex and Grano (2011). 

Discrete water-quality samples are analyzed 

for alkalinity and selected constituents by using the 

methods described in Fishman and Friedman (1989), 

Fishman (1993), U.S. Environmental Protection Agency 

(1993), American Public Health Association (1998), and 

Patton and Kryskalla (2003, 2011). Constituents that 

are analyzed include common ions (calcium, chloride, 

fluoride, magnesium, potassium, silica, sodium, and 

sulfate) and nutrients (nitrogen, nitrites, nitrates, 

phosphorus, and orthophosphate). Quality assurance is 

addressed through the analysis of field and equipment 

blanks and replicate samples, which together provide a 

quantitative description of precision and bias. 

Four-parameter water-quality data sondes are used 

to measure the physicochemical properties temperature, 

dissolved oxygen concentration, specific conductance, 

Figure 6. Artificial substrate with water temperature data 

logger for monitoring colonization and growth of zebra mussels 

(Dreissena polymorpha) in Lake Texoma, Texas and Oklahoma.

and pH. Physicochemical properties are measured and 

recorded at 5-foot depth intervals—from lowest depth 

to the surface—providing a profile of water-quality 

conditions in the water column at the time of sample 

collection. Additionally, water temperature data are 

recorded every 15 minutes at 5-foot depth intervals at 

three locations near a Lake Texoma intake structure 

by submersible data loggers attached near the artificial 

substrate samplers (fig. 6) and are stored in the USGS 

National Water Information System (NWIS) database 

(U.S. Geological Survey, 2012).

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proper shipping of samples to the National Water Quality 

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—Christopher J. Churchill and Stanley Baldys III

For additional information, contact 

Director, Texas Water Science Center 

U.S. Geological Survey 

1505 Ferguson Lane 

Austin, Texas 78754–4501 

http://tx.usgs.gov/ 

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