Control Methods

In locations in which ESA-listed salmonids, Bull Trout, and Steelhead, and their critical habitats exist, muriate of potash is a potential tool that can be used.

Potash is a common plant fertilizer which is largely comprised of potassium salts. Forms used to treat dreissenids include potassium chloride (KCl), potassium hydroxide (KOH), and potassium sulfide (K2SO4).

Potassium fertilizers used in agriculture have been shown to precipitate salts when applied in large quantities and/or through time, which can cause salinity problems in spoils (Magen 1996). There is little information on the effects of potassium applied directly to water, however, increased nutrient loading is the anticipated outcome. Irrigation systems cause compound leaching over time and allow precipitates to accumulate in soils (Burt and Isbell 2005).

Toxicity

Potassium ions interfere with the respiration of dreissenids at the gill surface (Fisher et al. 1991, Aquatic Sciences Inc. 1997). Acute lethal effects of potash on juvenile brook trout (Salvelinus fontinalis) and juvenile Chinook salmon (Oncorhynchus tshawytscha) are not expected at concentrations used to control dreissenids (Densmore et al. 2018). In fact, exposure concentrations of eight times greater than the dose of KCl used as a molluscide (800 mg/L) in a static system during a 96-hour period resulted in no mortality, and no behavioral, histological, or gross morphological effects on fish of either species (Densmore et al. 2018). Significant mortality among sensitive aquatic invertebrates, such as water fleas (Daphniidae), is not unexpected (Densmore et al. 2018). Other invertebrates, such as crayfish (Procambarus spp.), demonstrate some degree of sensitivity to KCl (Densmore et al. 2018). For example, crayfish exposed to KCl at higher concentrations (e.g., 800 mg/L–1,600 mg/L) for at least 24 hours experienced immobilization, but half were able to fully recover in fresh water within 24 hours (Densmore et al. 2018). Further analysis is needed to fully realize the threats to crayfish and other invertebrate species from KCl.

Liquid potash was successfully used, with 100% effectiveness, to eradicate zebra mussels from the Millbrook Quarry in Virginia, USA (Fernald and Watson 2014).

Potash Application

Potash consists primarily of potassium chloride (KCl). Potash is not a registered pesticide in the United States and requires a Section 18 FIFRA Pesticide Emergency Exemption from the EPA to allow its use in the four CRB states.

Target application rates are 95–115 mg/L (KCl), ≤ 10 mg/L (KOH), and 160­­–640 mg/L (K2SO4). Applications may be made at the surface, mid-depth, or deep waters to ensure appropriate mixing and to maintain the desired concentration throughout the treatment area. Potash can be applied up to 21 days after mixing to achieve desired effectiveness.

Equipment includes High Density Polyethylene storage tanks with spill containment to protect against spills and ensure a constant supply of stock solution. A stock solution of about 12% potassium is mixed by a chemical supplier and delivered to the site on an as required basis where it is transferred to the storage tanks and kept in solution by an electric tank mixer. The quantity of metric tons of KCl required to treat the site is estimated in advance based on the size of the contained portion of the water body.

Water-based operations use a work boat outfitted with a specially designed diffuser assembly. Stock solution from the shore-based storage tanks continuously feed the diffuser through a floating 3.8 cm (1.5 in.) diameter supply line and shore-based centrifugal pump transfer system. Proper diffusion of potassium is a critical element of the treatment method.

Treatment proceeds on a systematic basis by separating the cordoned off areas into segments or treatment zones delineated by water depth. The work platform-based retractable diffuser assembly consists of perforated vertical flexible hoses having capped and weighted ends attached to the horizontal section. This allows for an enlarged mixing zone to be achieved while the flexible hose reduces damage due to submerged obstacles. An echo sounder is used to monitor water depth and the depth of the submerged diffuser assembly to maintain an optimum height above the bottom of the water body. This system also reduces the risk of entangling the diffuser assembly on bottom features.

To ensure the potassium diffusion system is operating efficiently and is attaining target potassium concentrations throughout the treatment zone, potassium spot monitoring is completed during each charge operation. This provides personnel with information on how quickly and how well the potassium is dispersing through the treatment zone. This information can be used to modify the treatment protocol, either by increasing or decreasing the dosing rate to achieve target concentrations. Following the “charge” activities, a final sampling exercise is conducted throughout each cordoned off area to characterize potassium concentrations at various depth profiles. Monitoring points at each enclosed area are spaced depending on the width of the enclosed area at each transect location. Sites are monitored along each transect to ensure feasible and maximum monitoring coverage of the treated transect area. Duplicate samples are collected and analyzed for every tenth sample for quality assurance and quality assurance/quality control (QA/QC) purposes.

To determine the potassium concentrations, water samples are obtained by two different methods. Surface grabs are conducted where water depths are less than 2 m and are collected at least 0.15 m below the surface. A peristaltic pump, or Kemmerer bottle, is used to collect samples from each thermocline present in the sectioned off area and at depths greater than 2 m. Samples are analyzed with a concentration meter, in combination with a potassium probe. Sample identification, location, depth, date, GPS coordinates for each monitoring point, and other pertinent information is recorded in a field logbook and on reporting log sheets. The field instruments are calibrated prior to use every day with standards of known value. Monitoring is conducted daily throughout a 12-hour shift.

EarthTec QZ™ EarthTec QZ™ is a copper-based algaecide/bactericide (a formulation of copper sulfate pentahydrate) labeled to control zebra and quagga mussels. EarthTec QZ™ is registered in all 50 states as an algaecide/bactericide and in Montana and Washington as a molluscide. EarthTec QZ™ is documented as achieving 100% mortality of mussels when exposed to the product for 96 hours (Watters et al. 2013). The product can be spread on the surface of a water body or pumped into a water body, and disperses rapidly.

EarthTec QZ™ is a liquid formulation that is miscible in water and has ionic diffusion properties that cause it to readily disperse throughout the water column. The product’s active ingredient is delivered in the cupric ion form—a biologically active form of copper (Watters et al. 2013). EarthTec QZ™ does not have any degradation byproducts, and no adjuvants or surfactants are used in the application.

Toxicity

Lethal dose and exposure time of zebra mussels to EarthTecQZ™ had been identified under laboratory conditions (Watters et al. 2013, Claudi et al. 2014).

The cupric ion (Cu2+) form of copper is considered the most toxic form of copper to aquatic life because it is the most bioavailable (Eisler 2000, Solomon 2009). In addition, the cupric ion form of copper is more lethal in soft water compared to hard waters rich in cations because cations reduce its bioavailability (Pagenkopf 1983, Paquin et al. 2002). The toxicity of copper to fish and other aquatic life depends on its bioavailability, which is strongly dependent on pH, the presence of dissolved organic carbon (DOC), and water chemistry, such as the presence of calcium ions.

• Juvenile rainbow trout (Oncorhynchus mykiss) were exposed to either hard water or soft water spiked with copper for 30 days (Taylor et al. 2000). Fish in the hard-water, high dose (60 µg/L) treatment groups showed an increased sensitivity to copper.

• The mean 96-hour LC50 (with 95% confidence limits) for copper exposure in alevin, swim-up, parr and smolt steelhead (Salmo gairdneri) is 28 (27–30), 17 (15–19), 18 (15–22), and 29 (>20) µg/L of copper, respectively (Chen and Lin 2001). The mean 96-hour LC50 for copper exposure in alevin, swim-up, parr, and smolt Chinook salmon (Oncorhynchus tshawytscha) is 26 (24–33), 19 (18–21), 38 (35–44), and 26 (23–35) µg/L of copper, respectively. The experiments were done by adding copper as copper sulfate.

• Aquatic snails (Biomphalaria glabrata) had a 24-hour and 48-hour LC50 (with 95% confidence intervals) of 1.868 (1.196–3.068) and 0.477 (0.297–0.706) mg/L Cu, respectively (de Oliveira-Filho et al. 2004).

• 1-day-old freshwater snail eggs (Lymnaea luteda) were exposed to copper at concentrations from 1 to 320 µg/L of copper for 14 days at 21 °C in a semi-static embryo toxicity test (Khangarot and Das 2010). Embryos exposed to copper at 100 to 320 µg/L died within 168 hours. At lower doses from 3.2–10 µg/L, significant delays in hatching and increased mortality were noted.

EarthTec QZ™ Application

Application methods vary depending on the scale of project. It is applied at a rate of up to 2 mg/L, not to exceed 0.1 mg/L total copper. Concentrations may be held constant up to 30 days (depending on dose) to achieve effective treatment for all dreissenid life stages. EarthTec QZ™ copper is highly water soluble and does not precipitate. The product remains suspended until uptake by bacteria and algae occurs (Master Label for EarthTec QZ™, EPA Reg. No. 64962-1). Dispersion into the water body quickly reduces concentrations to below effective levels outside of the isolated treatment area.

EarthTec QZ™ is applied near the water surface and allowed to disperse, or is delivered via hose and pump to the depths, sites, and surfaces of the area of infestation. When applying to large areas, it is dispensed along a route with gaps no greater than 200 feet. Generally, when fish are present, no more than one-half of the body of water is treated at a time, starting near one shore and moving outward in bands to allow fish to move away. When treating half of a body of water, the second half must not be treated within 14 days from the last treatment. For effective control of adult and juvenile mussels, it is applied at the recommended rate of 2–16 parts per million (i.e., 2–16 gallons of EarthTec QZ™ per million gallons of water) to yield a rate of 0.120–0.960 mg/L (ppm) metallic copper. A total of at least four days is required for mortality of dreissenids to occur. Colder water temperatures may require longer exposures and doses closer to the high end of the allowable range. Within the half of the water body being treated, repeat applications may be needed to maintain lethal concentrations of copper for a sufficient time period. The second half of the water body is not treated within 14 days of the last treatment of the first half. Effective control can also be achieved by longer exposures (e.g., 5–30 days) at lower doses (1–5 parts per million EarthTec QZ™, to yield a rate of 0.06–0.30 mg/L (ppm) metallic copper.) When reapplying, a concentration of 1.0 mg/L (ppm) metallic copper in the treated water is not exceeded.

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Copper products

From Dahlberg et al (2025): Zebra mussels (Dreissena polymorpha), an invasive dreissenid mussel, have been established and caused considerable effects in many North American aquatic ecosystems. In response, copper-based pesticides have been used to manage zebra mussel populations. We evaluated the effects of a low-dose copper-based molluscicide for zebra mussel suppression on nontarget species in Lake Minnetonka (Minnesota, USA). Our study evaluated nontarget effects before and after treatment. Chlorophyll-a concentration increased in both the treated and reference bays 1 and 14 d posttreatment. Zooplankton community composition changed in both bays over the course of this study; zooplankton abundance and diversity initially decreased in the treated bay but gradually recovered and was back to pretreatment and reference bay levels after one year. We observed no signi cant differences in benthic invertebrate abundance or diversity between the treated and reference bays, although abundance and diversity estimates were dynamic and uncertain. Among caged organisms, copper bioaccumulation was higher in both mussel species than in sh, and among sh, was highest in fathead minnow (Pimephales promelas). These ndings contribute to our understanding of the potential effects of copper-based pesticides on aquatic ecosystems and provide insights for zebra mussel management.

Zequanox® is a biopesticide consisting of the dead bacterial cells of Pseudomonas fluorescens strain CL145 A that, when ingested by zebra and quagga mussels, destroy the digestive lining (https://marronebioinnovations.com/molluscicide/zequanox/). All treatments are undertaken by state-licensed applicators. Prior to beginning chemical treatment, the area to be treated is sealed off using non-permeable geotextile membranes, creating a contained open water body.

Zequanox® is maintained at a rate of 100 mg/L for up to eight hours; treatments are often repeated, although the label recommends no more than four Zequanox® applications annually.

Toxicity

Zequanox® is a potential tool for controlling dreissenids in shallow water habitats in lakes without significant long-term effects on water quality (Whitledge et al. 2014). However, this biopesticide does cause temporary, but substantial, reductions in dissolved oxygen because of the barriers that prevent well-oxygenated water from circulating into treatment zones (Whitledge et al. 2014).

Exposure to Zequanox® caused no mortality to blue mussels (Mytilus edulis) or any of six native North American unionid clam species (Pyganodon grandis, Lasmigona compressa, Strophitus undulatus, Lampsilis radiata, Pyganodon cataracta, and Elliptio complanata) (Bureau of Reclamation 2011). Exposure of duck mussel (Anodonta spp.), non-biting midge (Chironomus plumosus), and white-clawed crayfish (Austropotamobius pallipes) to Zequanox® in a 72-hour static renewal toxicity test at concentrations of 100–750mg active ingredient/liter resulted in LC50 values for Anodonta: >500mg active ingredient/liter, C. plumosus: 1075mg active ingredient/liter, and A. pallipes: >750mg active ingredient/liter, demonstrating that Zequanox ® does not negatively affect these species at concentrations required for greater than 80% zebra mussel mortality (i.e., 150mg active ingredient/liter) (Meehan et al. 2014).

Nicholson (2018) conducted a replicated aquatic mesocosm experiment using open-water applications of Zequanox® (100 mg/L of the active ingredient) to determine the responses of primary producers, zooplankton, and macroinvertebrates to Zequanox® exposure in a complex aquatic environment. Short-term increases occurred in phytoplankton and periphyton biomass (250–350% of controls), abundance of large cladoceran grazers (700% of controls), and insect emergence (490% of controls). Large declines initially occurred among small cladoceran zooplankton (88–94% reductions in Chydorus sphaericus, Ceriodaphnia lacustris, and Scapheloberis mucronata), but abundances generally rebounded within three weeks. Declines also occurred in amphipods (Hyalella azteca - mean abundance 77% less than controls) and gastropods (Viviparus georgianus - survival 73 ±16%), which did not recover during the experiment. Short-term impacts to water quality included a decrease in dissolved oxygen (minimum 1.2 mg/L), despite aeration of the mesocosms.

Zequanox® Application

Products are mixed in tanks and injected at the water surface. Following treatment, monitoring occurs every 1–2 days for 14 days post-treatment. Monitoring consists of collecting surface water samples at various locations inside the treatment area. Samples are submitted for analysis by mass spectroscopy, with results reported within 1–2 days. Portable meters are used to inform bump applications in the field.

During the Zequanox® application, concentrations are estimated using turbidity measurements, on the first and last day of treatment application. Monitoring of concentrations is of limited utility because the active agent in Zequanox® is degraded within 24 hours after it is added to water (Molloy et al. 2013).

Product Label | Product Safety Data Sheet

Natrix (EPA Registration No. 67690-81) consists of Copper Ethanolamine Complex and other ingredient, and is labeled for use in some states for the control of mollusks in still or flowing aquatic sites, including: golf course ponds, ornamental ponds, fish ponds, irrigation and fire ponds, and aquaculture, including fish and shrimp; fresh water lakes, ponds, and fish hatcheries; potable water reservoirs; and crop and non-crop irrigation and drainage systems (canals, laterals and ditches) and chemigation systems. Natrix controls invasive/exotic aquatic mussel, snail, oyster or clam species, such as: zebra mussels (Dreissena polymorpha), quagga mussels (Dreissena rostriformis bugensis), Asian clams (Corbicula fluminea), and island applesnails (Pomacea insularum). Natrix may also be applied to control nuisance mollusks, such as snails that are vectors for parasites (e.g. swimmers itch or schitosomes).

Fish Advisory Statement: This copper product is toxic to fish and aquatic organisms. Unlike most organic pesticides, copper is an element and will not break down in the environment and will therefore accumulate with repeated applications. Copper is a micronutrient, but its pesticidal application rate exceeds the amount of copper needed as a nutrient. Waters treated with this product may be hazardous to aquatic organisms. Treatment in areas with dense aquatic weeds and algae can result in oxygen loss from decomposition of dead biomass. This oxygen loss can cause fish and invertebrate suffocation. To minimize this hazard, do not treat more than ½ of the water body and wait at least 14 days between treatments at avoid depletion of oxygen due to decaying vegetation (excluding water infrastructure and constructed conveyances such as drainage canals, ditches and pipelines or intakes and aqueducts for drinking water or irrigation use). Begin treatment along the shore and proceed outwards in bands to allow fish to move into untreated areas. Consult with the State or local agency with primary responsibility for regulating pesticides before applying to public waters, to determine if a permit is required. Application of algaecides to high density blooms of cyanobacteria can result in the release of intracellular contents into the water. Some of these intracellular compounds are known mammalian hepato- and nervous system toxins. Therefore, to minimize the risk of toxin leakage, manage cyanobacteria effectively in order to avoid applying this product when blooms of toxin-producing cyanobacteria are present at high density. In situations where rapidly reproducing toxic algal species pose a public health threat to drinking or recreational water resources, applicators must receive authorization from applicable state, local or tribal water resources authorities to apply copper at intervals shorter than 14 days should the circumstance demand. Certain water conditions including low pH (≤6.5), low dissolved organic carbon (DOC) levels (3.0 mg/L or lower) and “soft” waters (i.e. alkalinity less than 50 mg/L) increases the potential acute toxicity to non-target aquatic organisms. The application rates on the product label are appropriate for water with pH values > 6.5, DOC levels >3.0 mg/L, and alkalinity greater than 50 mg/L. Avoid treating waters with pH values <6.5, DOC levels <3.0, and alkalinity less than 50 ppm (e.g., soft or acid waters), as koi, trout and other sensitive species of fish may be killed under such conditions.

APPLICATION DIRECTIONS Natrix®, 67690-81 Rates and exposure times will vary based on the species and life stage of the species being controlled, as well as treatment conditions. For effective control of adult life stages and depending on site conditions, repeated or extended applications may be necessary. Use lower rate and frequency of application in softer water (<50 ppm alkalinity) or when treating species with greater susceptibility to Natrix; use higher rate and frequency of application in harder water (>50 ppm alkalinity) and when treating adult mollusks and/or less susceptible species. To control invasive species under the direct authorization of a state resource agency, repeat applications may be conducted when necessary to maintain an efficacious concentration of 0.25 to 1.0 ppm for up to 96 hours. Monitoring of in-water copper concentrations must be completed during this application program to guide the rate and timing of repeat applications. Contact a SePRO Specialist for species and site specific recommendations. For the control of mollusks, do not exceed a concentration of 1.0 ppm copper (3 gallons of product or 2.74 lbs metallic copper per acre-foot) during any single application. For the control of schistosome-infected freshwater snails, the maximum application rate may be increased to 1.5 ppm copper (4.5 gallons of product or 4.11 lbs metallic copper per acre-foot) during any single application. Do not apply at a concentration greater than 1.0 ppm of copper in areas containing an active potable water intake unless the water intake can be turned off until copper concentrations are less than 1.0 ppm at the intake.

Whole Waterbodies Maximum annual application rate of 21.9 lbs of metallic copper per acre-foot (8 applications per year at up to 1 ppm). This rate/frequency is calculated based on staggering the treatment of each half of the water body every 14 days (at a rate of 2.74 lbs. metallic copper per acrefoot = 1 ppm) for eight months (244 days).

Water Management Units For large waterbodies such as lakes and reservoirs that support aquatic habitat, this product may be applied in multiple individual treatments to different, discreet sections of a waterbody, or water management units, within the 14-day retreatment interval, provided that the sum of those areas together constitute no more than half of the total area of the entire waterbody. Maximum annual application rate of 46.6 lbs. of metallic copper per acre-foot per year (17 applications per year at up to 1 ppm). This rate/frequency is calculated based on the maximum number of possible applications allowed based on a 14-day minimum (at a rate of 2.74 lbs. metallic copper per acre-foot = 1 ppm) retreatment interval for eight months (244 days). Do not apply more than 46.6 lbs. of metallic copper to a water management unit, regardless of the pest(s) targeted by applications.

Still and Quiescent Waters Natrix®, 67690-81 For treatments to whole waterbodies, administer copper at a rate of up to 1 ppm (2.74 lbs copper/acre-foot) at a maximum annual rate of 21.9 lbs metallic copper per acre foot. Monitor the copper concentration and when it falls below the desired concentration, apply additional copper to bring the concentration back up to the desired concentration. Monitor mussel populations and terminate the additional applications once mussels are dead or 14 days have passed since the initial application. Applicators must wait at least 14 days after the last application before making any additional applications. Apply Natrix diluted or undiluted. Dilution with water may be necessary at the lower application rates to ensure uniform coverage of the area to be treated. Dilute the required amount of Natrix with enough water to ensure even distribution in the treated area with the type of equipment being used. To achieve a concentration of 1.0 ppm, apply 3 gallons per acre foot.

Application Rate Calculation: acre feet X desired metallic copper (ppm) X 3 = gallons of Natrix to be applied

Example: The amount of Natrix needed to provide the desired concentration of 1.0 ppm of active ingredient in a 1 surface acre waterbody with an average depth of 4 feet may be calculated at follows:

1 surface acre* X 4 foot average depth = 4 acre feet

4 acre feet X 1.0 ppm X 3 = 12 gallons of Natrix *

1 surface acre = 43,560 ft2

Flowing Waters Apply Natrix diluted or undiluted. Accurately determine water flow rates prior to treatment. In the absence of weirs, orifices, or similar devices, which give accurate waterflow measurements, volume of flow can be estimated by the following formula: Cubic feet per second (cfs) = average width (feet) x average depth (feet) x average velocity† (feet/second) x 0.9 †:The velocity can be estimated by determining the length of time it takes a floating object to travel a defined distance. Divide the distance (feet) by the time (seconds) to estimate velocity (feet/seconds). This measure should be repeated 3 times at the intended application site and then used to calculate the average velocity. After accurately determining the water flow rate in cfs or gallons/minute, find the corresponding rate in Table 1 or use the below formula. cfs X desired concentration of copper (ppm) = quarts/hour of application. Applications for up to 96 hours may be necessary to achieve control of the targeted mollusks. Calculate the amount of Natrix needed to maintain the drip rate for the targeted exposure period (hours): Quarts per CFS x CFS x # exposure hours. For example, to achieve a desired concentration of 1.0 ppm copper; 1.0 quart per CFS x 5 CFS x 10 hours of exposure = 50 quarts of total product. Rates will target 1.0 ppm copper concentration in the treated water for the treatment period; 1 quart per cubic foot per second (cfs) per hour in flowing water. Use lower rate on highly susceptible species or if longer exposure times are maintained. Apply Natrix in the channel at weirs or other turbulence-creating structures to promote the dispersion of the chemical. Use a drum or tank equipped with a valve or other volume control device that can be calibrated to maintain a constant drip rate. Use a stopwatch and appropriate measuring container to set the desired drip rate. Readjust accordingly if the canal flow rate changes during the treatment period. A small pump or other metering device may be used to meter Natrix into the water more accurately. SePRO recommends consulting a SePRO Aquatic Specialist to determine optimal use rate, location of treatment stations and treatment period under local conditions.

Natrix is currently registered in the following states depicted in light blue (as of 27 January 2025):

There are water tracers that are carcinogenic, genotoxic, or ectoxic[1]. Fluorescent dyes that demonstrate no effect on genotoxicity or ecotoxicity are classified as safe for use in water tracing (Behrens et al. 2001). Rhodamine dyes (aminoxanthenes) are used as hydrologic tracers in surface water systems (Runkel 2015). Rhodamine dyes are synthesized by reacting 3-dialkylaminophenols with phthalic anhydride (Ismael et al. 2013). Rhodamine WT is water soluble, highly detectable, and fluorescent in a part of the spectrum not common to materials commonly found in water, harmless in low concentrations, and reasonably stable in aquatic environments (USGS 1986). Domenico and Schwartz (1990) described rhodamine WT as a conservative, ideal tracer because it does not react with other ions or the geologic medium to any appreciable extent.

Toxicity

Molinari and Rochat (1978) concluded there is relatively low ecotoxicological risk from rhodamine WT. Smart (1984) concluded rhodamine WT is a severe irritant to the eye and moderately irritating to the skin. Nestmann and Kowbel (1979) documented rhodamine WT was mutagenic in the Salmonella typhum/mammalian microsome Ames test. Douglas et al. (1983) concluded rhodamine WT does not represent a major genotoxic hazard because it was weak in vitro mutagenicity using very high dye concentrations.

In aquatic ecosystems, larval stages of shellfish and algae are most sensitive to fluorescent dyes (Smart 1984). However, Rhodamine WT does not affect development nor cause mortality in shellfish eggs and larvae after 48 hours exposure, and dye concentrations as high as 1 mg/l can be tolerated for two days without damage to aquatic organisms (Smart 1984). Fairy shrimp, Thamnocephalus platyurus, had a toxicity of EC50 24 hours: 1,698 mg/L-1. A total of 48-hour exposures at 24° C of 11,000 Pacific oyster (Crassostrea gigas) eggs per liter and 6,000 12-day-old larvae per liter, in sea water with concentrations of rhodamine WT ranging from 1 μg/l to 10 mg/l, resulted in development of the eggs to normal straight-hinge larvae and no abnormalities in the larvae development (Parker 1973). Coho salmon (Oncorhynchus kisutch) and Donaldson rainbow trout (Oncorhynchus mykiss) held for 17.5 hours in a tankfull of sea water with a dye concentration of 10 mg/l at 22°C showed no mortalities or respiratory problems (Parker 1973). A concentration of 375 mg/l, and extended time of an additional 3.2 hours resulted in no mortalities or abnormalities (Parker 1973). The fish remained healthy in dye-free water when last checked one month after the test. J.S. Worttley and T.C. Atkinson (reported as personal commun., 1975, in Smart and Laidlaw 1977) exposed a number of freshwater and brackish water invertebrates, including water flea (Daphnia magna), shrimp (Gammarus zadIlachl), log louse (Asellus aquaticus), may fly (Cloeon dipterum), and pea mussel (Visidium spp.), to water containing up to 2,000,000 μg/L of rhodamine WT for periods of up to 1 week. No significant differences in mortality between the test and control animals were observed.

Dye concentrations for water tracing purposes are low enough to exert almost no toxic impacts on water fauna, including fairy shrimp, water fleas (Daphnia magna), horned planorbis snail (Planorbis corneus), and guppy fish (Poecilla reticulata) (Rowinski and Chrzanowski 2011).

The lethal dose of rhodamine WT in rats is 25,000 mg kg-1 (Field et al. 1995). The oral lethal dose for humans is estimated to be 25,000 mg kg -1 d-1, which would require an adult to ingest 875,000 mg l-1 of rhodamine WT for a dose of 25,000 mg kg-1 d-1 to be achieved (Field et al. 1995). Field et al. (1995) tested the possible ecotoxicity effects of 12 water tracer dyes, including rhodamine WT, on human health. They concluded rhodamine WT has no skin absorption, has limited oral uptake, has inadequate data on carcinogenicity, and poses little concern for both oncogenic and mutagenic effects as well as little concern for chronic toxicity, including liver and kidney effects.

Ecological toxicity structure-activity relationship (SAR) concerns for rhodamine WT are as follows:

Fish (96 hours LC50) > 320 mg 1-1a

Cladocera (48 hours LC50) 170 mg l-1a

Green algae (96 hours EC50) 20 mg l -1

The high LC50 demonstrated for aquatic organisms indicate unlikely serious effects on groundwater fauna from 1-2 mg 1-1 dye concentrations in the water (Field et al. 1995).

When used at recommended dosages, rhodamine WT does not constitute an environmental hazard associated with manmade nitrosamines in the environment (Steinheimer and Johnson 1986). However, it should be noted that Field et al. (1995) emphasized their focus on acute toxicity relative to lethal doses, noting that other toxicological effects, such as developmental toxicity, were not calculated. Concentrations below 910 µg/L for Rhodamine WT is not expected to pose a risk to aquatic freshwater life in the case of intermittent discharges, e.g. tracer experiments released in streams (Skjolding et al. 2021).

Rhodamine WT Application and Best Management Practices (from Field et al. 1995)

The maximum recommended concentration of rhodamine WT is 2 mg 1-1. Individuals using tracers should be experienced or well trained in their use, and tracer concentrations should not exceed 1–2 mg 1-1 persisting for a period in excess of 24 hours in groundwater at the point of groundwater withdrawal, or discharge. Such concentrations are well below toxicity levels, allows for easy recognition by the naked eye, and is above persistent dye concentrations traditionally recommended for tracer tests.

Sudden water-level drawdowns during winter conditions can temporarily reduce dreissenids in impounded river sections, although this type of control is considered a method to temporarily reduce large numbers of adults (Leuven et al. 2014).[2] Freezing air temperatures are highly lethal to zebra mussels within a matter of hours (Grazio and Montz 2002). Water drawdowns occur when managers decrease the maximum depth in a body of water that has adequate water level control structures (Grazio and Montz 2002). Winter water drawdowns were used to treat Lake Zumbro, Minnesota, and Edinboro Lake, Pennsylvania, in 2000 and 2001. Although complete mortality of invasive mussels was observed in drawdown areas (1.5-meter drawdowns), mussels successfully overwintered in waters deeper than the maximum drawdown depth (Grazio and Montz 2002). A drawdown of Ed Zorinsky Reservoir (Zorinsky Lake), Nebraska, in the winter of 2010 resulted in the eradication of zebra mussels within the lake, and the lake was refilled and re-opened for recreation in 2012 (Hargrave and Jensen 2012). Zebra mussel veligers were detected in May 2016, however, adult mussels have not been observed. Total elimination of dreissenids with this management technique is unlikely, and the potential costs and benefits before attempting fall/winter lake drawdowns for zebra mussel control should be evaluated on a site-by-site basis.

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Winter drawdowns can decrease taxonomic richness of macrophytes and benthic invertebrates and shift assemblage composition to favor taxa with r-selected life history strategies and with functional traits resistant to direct and indirect drawdown effects (Carmignani and Roy 2017). Fish assemblages, though less directly affected by winter drawdowns (except where there is critically low dissolved oxygen), can be indirectly negatively affected via decreased food resources and changes in spawning habitat (Carmignani and Roy 2017). Drawdowns modify abiotic conditions, cause sediment desiccation and freezing, place stress on vegetative root structures (Siver et al. 1986), displace plants as a result of erosion of frozen sediment during spring refills (Beard 1973, Mattson et al. 2004), and stifle species growth by increasing acidity and cations to toxic concentrations (Peverly and Kopka 1991). Annual winter drawdowns can, through time, coarsen sediment texture and remove nutrients in the exposure zone, making these sites unsuitable for macrophyte colonization and growth, particularly in steep-sided basins (Hellsten 1997). Other adverse impacts of drawdowns (New Hampshire Department of Environmental Services 2019) may include:

• Large amounts of aquatic plants and organisms that succumb to the drawdown begin to decay shortly after drawdown, but nutrient release to the water body may not occur until full-pond level is achieved. Nutrients released from decayed material will quickly be used by algae and cyanobacteria, leading to increased cell production. Shallow lakes have shown shifts from clear, plant-dominated conditions to turbid, algal dominated systems.

• Algal or cyanobacteria blooms may follow.

• Aquatic food web changes may result in shifts in plant and animal structure.

• Oxygen concentrations throughout the water column may be impacted.

• Changes in the bottom sediment may also occur. Softer sediments may become compacted, or frozen segments that are lighter than water could loosen and float around in large masses, or as floating islands in the water body, only to settle once again in a new location.

Impacts to aquatic animal species can be significant. These impacts range from stranding animals to food chain modifications, or stressors associated with the drawdown. Fish, frogs, salamanders, turtles, aquatic insect larvae, mussels, and others can be affected by a drawdown. Agile and faster moving organisms may be able to move upstream or downstream to other unimpacted habitats, however, these fish may be confined to smaller, shallower areas where they become easy prey to consumers, or suffer from oxygen deprivation. Slower moving, more sedentary organisms have a greater risk to negative impacts. Freshwater mussels, snails, insects, and crayfish may not be able to find suitable habitat, and may succumb to the drawdown.

Maintaining water levels is important for populations of Northern Leopard Frogs (Lithobates pipiens) throughout all of their seasonal activities (IUCN-CMP Threats 1, 2, 3, 4, 7). Exposure to cold temperatures via winter drawdowns could have negative, and potentially lethal, effects on species, such as the Northern Leopard Frog, which overwinters in torpor at the bottom of large rivers (CMP 2010). This is specifically a concern in the Kootenai River near the Idaho/British Columbia border (Washington Ecological Services - USFWS).

Winter drawdowns could negatively affect all life stages (adults, metamorphs, and larvae), of the invasive American Bullfrog (Lithobates catesbeianus), which overwinters at the bottom of large water bodies. Breeding populations of American bullfrogs disappear following pond drying (Maret et al. 2006) and metamorphosis of bullfrog larvae can be intercepted by selective draining to reduce the length of time a water body has water (Govindarajulu 2004).

Information in this section is from Culver et al. (2013).

Removal, either by hand or another mechanical method, can potentially eradicate dreissenid mussels when 1) the structure from which mussels are being removed lends itself to this technique, and 2) when mussels are concentrated within specific areas of a water body or on particular infrastructure within it. Mussel populations can successfully be eradicated using this strategy only if 1) no additional larval or juvenile/adult mussels are entering the water body from infested waters (aqueduct or reservoir) and/or boat traffic, and 2) if enough mussels are removed to reach the point where the population can no longer sustain itself. Achieving the latter can be difficult, due to the mussels’ ability to inhabit inaccessible places, limiting removal efforts and increasing chances that individuals will survive. Where there are many inaccessible areas, a combination of tactics will likely be most effective.

Even when eradication is not possible, this strategy offers an effective method for controlling the population when applied appropriately, and when used in combination with other control tactics. Likewise, if the infested area is large (>20,000 square feet), a combination of oxygen deprivation using tarps and manual/mechanical removal may be useful.

The steps to be taken in manual removal include organizing divers, training divers, determining the distribution of mussels, conducting pre-implementation surveys, preparing the target site, manually removing the mussels using hand-held tools, collecting the mussels, disposing of the mussels, decontaminating persons and gear, and evaluating tactic success. For more information on the specific steps associated with manual and mechanical removal of aquatic invasive species, California Sea Grant has developed an information sheet (2013) for educational purposes (https://caseagrant.ucsd.edu/sites/default/files/3%20Manual%20Mechanical%20Individual_121418.pdf)

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Physical harvesting of dreissenids can reduce the diversity and abundance of soft-sediment benthic community taxa (Wittman et al. 2012). Following best management practices for manual removal minimizes any effects on non-target organisms (Culver et al. 2013). Steps involved in manual removal (Culver et al. 2013) include: organize divers, train divers, conduct pre-implementation surveys, prepare target site, manually remove mussels using hand-held tools, collect removed mussels, dispose of removed mussels, decontaminate persons and gear, and evaluate efficacy of effort. Effort to remove mussels manually can be minimized by using a suction pump made from PVC and a SCUBA tank to vacuum the mussels into collection bags, however, use of this technique can significantly disrupt benthic macroinvertebrate community structure (Wittman et al. 2012). ​Suction harvesting side effects can include high turbidity, reduced clarity, and algae blooms from nutrient release caused by disturbance of bottom sediment, which can reduce oxygen conditions and ultimately affect ecosystem communities (New York State Department of Environmental Conservation 2005). Suction harvesting also has the potential to release sediment-bound heavy metals into the water column, which can affect the food chain in the water body (New York State Department of Environmental Conservation 2005).

Benthic mats are large, dark tarps anchored to the bottom of a water body to control invasive mussels by restricting water flow, oxygen and food from the mussels beneath the mats, and blocking light to prevent photosynthesis from producing oxygen beneath the mats.[3]

The first demonstrated use of benthic mats to successfully eradicate a dreissenid mussel population occurred in Lake Waco, TX after zebra mussels were discovered in 2014. Placement of benthic mats for 5 months, followed by monitoring over five years, resulted in a declaration of "eradication" by 2021 (Conry et al. 2024).

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Bottom/benthic barriers or mats can be installed on portions of lake bottoms and weighted, resulting in oxygen deprivation. This tactic is used for low to moderate mussel infestations in difficult to access locations, and can be enhanced by combining it with tactics that target larval stages (Culver et al. 2013). This method is not as effective in locations with large infestations. ​Steps involved in oxygen deprivation (Culver et al. 2013) include: organize divers and boat operators, locate needed supplies, review the need for area closures, determine mussel distribution, conduct pre-implementation survey, conduct a pilot study, install tarps, add chemicals/biocides if needed, monitor during installation, remove tarp, decontaminate persons and gear, and evaluate efficacy of effort. Benthic barriers interfere with respiration in fish and macroinvertebrates. Benthic barriers comprised of anchored textile/plastic are generally placed over vegetation to prevent the growth and establishment of plants whereas benthic barriers can be created by depositing silt to smother bottom-dwelling organisms (US Army Corps of Engineers 2012). Response to silt barriers can include feeding inhibition, reduced metabolism, avoidance, or mortality (Collins et al. 2011). ​Although studies have shown that benthic barriers may impact non-target organisms, especially benthic dwellers, and will affect chemistry at the sediment-water interface, impacts are limited to the area of installation, and because only a small percentage of lake bottoms are typically exposed to benthic barriers, lake-wide impacts are not expected and have not been observed (Mattson et al. 2004).

Ultraviolet (UV) radiation is an effective method for controlling zebra mussels in all life stages, although veligers are more sensitive than adults. Complete veliger mortality can be obtained within four hours of exposure to UV-B radiation, and adult mortalities can also be obtained if constant radiation is applied. UV radiation can be harmful to other aquatic species, and its effectiveness may be decreased by turbidity and high suspended solids loads (Wright et al. 1997). Doses as low as 26.2 mJ/cm2 and 79.6 mJ/cm2 can decrease survival of pre-settlement stage larvae by nearly 50% and 80%, respectively, within four days of exposure (Stewart-Malone et al. 2015).

The use of UV light to control larval dreissenids in industrial cooling water systems is well documented (Pucherelli and Claudi 2017). To reduce environmental effects, lower costs, and avoid the need for discharge permitting, UV light irradiation can be used to prevent or limit mussel colonization in industrial facilities, and can be used in water bodies in combination with treatments targeted at adult dreissenids. Site-specific characteristics, such as the ability of the water to transmit UV light, suspended solids, and flow conditions, affect the efficacy of this treatment (Pucherelli and Claudi 2017). This technique requires continuous UV light application for up to 120 hours, and is considered only partially effective in killing larval dreissenids.

A Hydro-Optic Disinfection (HOD) UV system was installed at Davis Dam, effectively reducing quagga mussel settlement at all doses tested (Morrell 2020). Mussel settlement was reduced by 88% at the lowest dose (20 mJ/cm2) and a 99% reduction was observed at the highest dose (100 mJ/cm2). Larvae exposed to UV experienced delayed mortality, and mortality rates were variable based on UV dose, monthly environmental conditions, and larvae size. The UV light is applied using watercraft and submerged UV light panels, which are raised and lowered in the water column to target larval dreissenids.