A suite of chemical and non-chemical options exists for controlling invasive mussels in the CRB; some treatments are appropriate solely for hydropower facilities and water delivery systems, in which fish are not present and the water can be treated before being released into a sewage system. Other treatments, which may have lower toxicity to fish and living organisms, are more appropriate for open water situations. Although the CRB plan outlines numerous potential control options, many treatments may not be appropriate, or feasible, for response in open-water systems (see Section D-1 of the CRB plan (Heimowitz and Phillips 2008). The scope of this manual includes the treatment options most likely to be used in open-water systems. For example, oxidizing biocides (i.e., chlorine, bromine, hydrogen peroxide, ozone, and potassium permanganate) and non-oxidizing compounds (proprietary molluscicides; i.e. Clam-Trol, Bulab, and Bayluscide) are listed as chemical treatment options in the CRB plan. Although these treatments may be effective at controlling invasive dreissenid mussels, they are highly toxic to other aquatic species, including fishes, and are not included in this manual as likely treatment options in open-water situations.
The most likely treatment options that would be implemented for any water body in the four CRB states would include both chemical and physical treatments.
Several bioassays would be employed to determine the effectiveness of each chemical or mechanical treatment.
If adult dreissenid mussels are present in a water body, mussel mortality would be assessed via in-situ cage bioassays (Lund et al. 2017). Four cages of ∼50–100 mussels per cage would be placed within the treatment area. Cages would be constructed of plastic canvas mesh sheets (1–2 mm openings), anchored to the lake bottom. If the water body is stratified (having distinct epilimnion, metalimnion, and hypolimnion), additional bioassays representative of the different layers may be appropriate. Live, gaping, and dead mussels would be recorded daily until all mussels are dead or until no additional mussels die over three consecutive days.
Bioassays may need to be conducted with proxy species because some jurisdictions may not allow the use of adult dreissenids, particularly if the initial detection was a veliger detection, and no presence of adult mussels was detected.
A. Chemical Methods
The use of chemicals requires knowledge of permitting, labeling, and chemical-specific application regulations (BOR 2015).
Muriate of Potash—used as a biocide; requires a Section 18 of the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) Pesticide Emergency Exemption from the US Environmental Protection Agency (USEPA).
EarthTec QZ™—used as a biocide; (only in water bodies with non-salmonid/trout species).
Zequanox®—the only EPA-registered biocide for mussels.
Rhodomine dye—used to evaluate water flow and containment effectiveness (not used as a biocide).
B. Mechanical and Other Methods
Intense Ultraviolet-B and Ultraviolet-C Radiation
Water level management
Combinations of treatments may be used, and retreatments may be necessary. Treatment areas would be isolated up to 45 days during treatment to maximize dreissenid mussel exposure time, incorporate variables, such as temperature variations (which affects efficacy of potash), and provide for re-treatment, if needed. The 45-day isolation period would incorporate two full treatments if a second treatment was necessary to achieve 100% mortality. For example, one treatment within a 45-day period may achieve 100% mortality. If 100% mortality does not occur, a second treatment may be necessary, and could be conducted within the initial 45-day period.
A. Chemical Methods
A1. Chemical Method – Muriate of Potash
In basin locations in which ESA-listed salmonids and their critical habitat exist, the most likely product to be used, based on least toxicity to aquatic life as well as cost, is potash.
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).
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 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.
A2. Chemical Method – 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.
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.
A3. Chemical Method – Zequanox®
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.
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.
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).
A4. Chemical Method – Rhodomine Dye
There are water tracers that are carcinogenic, genotoxic, or ectoxic. 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.
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.
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.
B1. Mechanical and Other Methods - Intense Ultraviolet-B and Ultraviolet-C Radiation
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.
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.
B2. Mechanical and Other Methods – Water Level Management
Sudden water-level drawdowns during several 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). 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.
B3. Mechanical and Other Methods – Physical Removal
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)
B4. Mechanical and Other Methods – Benthic Mats
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.
5. Summary of Application Rates and Contact Time for Dreissenid Treatment Methods
The Columbia River Basin Interagency Invasive Species Response Plan: Zebra Mussels and Other Dreissenids (Heimowitz and Phillips 2008) documents the chemical methods available for dreissenid control, including the ones documented in Table 1. Appendix D in the CRB Plan identifies the treatment, target age, efficiency, contact time/concentration, and comments relative to effects on the environment and other species. Information from that appendix is summarized here for the treatments included in this manual.
Table 1. Summary of application rates and contact time for dreissenid chemical treatments.
 Carcinogenic substances have the potential to cause cancer. Genotoxic substances have the potential to damage genetic information within a cell, causing mutations, which may lead to cancer. Ectoxic substances have the potential to place biological, chemical, or physical stressors on an ecosystem.
 In a study in the Netherlands, the overall density of dreissenids decreased, but six months after the water level was increased, the mussel density slightly increased. Within 18 months, the mussel density had recovered to pre-drawdown levels.