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Potential Effects of Invasive Mussels on Clear Lake
Likely Changes _ Clear Lake.png

There are numerous factors to consider when estimating the potential effects of invasive mussels on a water body. System-wide effects of quagga and zebra mussels depend on water mixing rates, lake morphology, and turnover rates (Karatayev et al. 2015). Likely changes to Clear Lake (Figure 1) include:


A disrupted food chain that negatively affects fisheries and wildlife

Quagga and zebra mussels (Dreissena spp.) are known as ecosystem engineers because they control the availability of resources to other organisms by the physical changes they cause in the environment (Jones et al. 1994) and have profound effects on lake and river ecosystem function and structure (Zhu et al. 2006). The ecological effects of these mussels are considered the most far-reaching relative to other aquatic invasive species (AIS), causing local extinction of many native mollusks (Strayer and Malcom 2007; Burlakova et al. 2014), changing the structure of food webs and fish assemblages, and contributing to the collapse of valuable sport fish populations (Kelly et al. 2010; Bossenbroek et al. 2009; Strayer 2009; Pimentel et al. 2005).  Increased occurrences of harmful algal blooms (Higgins and Vander Zanden 2010) can contribute to declines in fish populations (Knoll et al. 2008). Once established, invasive mussels commonly reach densities in excess of 10,000 individuals per square meter (Depew 2021).


System-wide effects of quagga and zebra mussels depend on water mixing rates, lake morphology, and turnover rates (Karatayev et al. 2015). Quagga mussels can be found in all regions of a lake, form larger populations, may filter larger volumes of water and may have greater system-wide effects (especially in deep lakes) compared to zebra mussels, which are restricted to shallower portions of lakes (Karatayev et al. 2015). After initial invasion, invasive mussels will primarily have direct effects on ecological communities whereas post-invasion, less predictable impacts will likely be indirect effects that cause ecosystem changes (Karatayev et al. 2015). Proactive, pre-invasion management investments that emphasize the importance of strong prevention and early detection programs have been shown to be much lower than re-active, post-invasion expenditure (Cuthbert et al. 2022).


Quagga and zebra mussels filter particles from the water, resulting in improved water clarity (Karatayev et al. 1997, 2002), and corresponding increases in benthification, (Mills et al. 2003).  Scientists refer to this as "turning ecosystems upside down" because of the transfer of energy to littoral areas with concurrent increases in benthic biomass (Mayer et al. 2014; Rumzie et al. 2021).

Boats, engines, docks, and other infrastructure (e.g., water delivery supply lines) encrusted with invasive mussels

Dreissenid mussels grow on a variety of infrastructure systems, including water intake pipes for drinking water, irrigation, power plants, locks, and dams and canal systems, greatly impacting operation and maintenance costs (ISAC 2016). Continual attachment can increase corrosion rates of steel and concrete (USGS 2016), leaving equipment and infrastructure vulnerable to failure. Additionally, the mussels grow on navigational buoys, docks, and hulls of boats and ships—increasing drag, affecting steering, and clogging engine intakes—all of which can lead to overheating and engine malfunctions (ISAC 2016).

Beaches and shoreline encrusted with sharp shells

The shells from dead dreissenid mussels can wash ashore, smothering beaches and potentially injuring swimmers and other water recreationalists from cuts sustained from the shells’ sharp edges (Nelson 2019).

Boating restrictions to reduce spread of mussels

Mandatory watercraft inspections prior to launch, closures of boat ramps, restrictions on shore launching, mandatory Mussel Fee Stickers, and closed water bodies are examples of boating restrictions that have been implemented as a result of dreissenid introductions.

Increased cost and maintenance for clogged, fouled, or contaminated water delivery infrastructure

Clear Lake is a source of water for the Clear Lake Oaks County Water District, which treats and provides municipal drinking water for 4,700 people. In addition, many individual homeowners with individual water systems, draw water from the lake.

Invasive mussels pose serious threats to water resources hydropower infrastructure and operations (Rumzie et al. 2021). Invasive mussels can affect all facility components exposed to raw water; mussels can clog pipelines and water intakes and disrupt operations at hydroelectric power plants, municipal water supply facilities, and conveyance systems used in irrigation, resulting in water lines incapable of supplying a consistent and reliable source of water (Vissichelli 2018). Smell, bacteria, and decay are other key issues associated with a mussel infestation; management response is continual cleaning, treatment, mitigation filters, and other actions. A 2021 study of costs associated with invasive mussel impacts and management at 13 hydropower facilities in Canada and the United States (Rumzie et al. 2021) documented costs associated with established invasive mussels in both preventative control measures and increased maintenance. 

  • Preventative control capital costs (one-time costs) ranged from $100,000 to $200,000 per facility

  • Preventative control annual costs ranged from $4,000 to $141,700 per facility

  • Increased maintenance reoccurring costs ranged from $22,000 to $505,000 per facility

  • Increased maintenance annual costs ranged from $26,000 to $112,000 per facility

  • Annual monitoring costs ranged from $1,970 to $47,245 per facility

  • Unplanned outages cost per occurrence ranged from $44,000 to $80,000 per facility

  • Unplanned outages total cost was $849,000


Examples of preventative and maintenance costs include treating with chlorine, cleaning generator coolers 3-4 times per year to remove mussel debris, and increased labor costs to maintain all hydropower equipment.

The cost to remove mussels and manage drinking water intakes at Hoover, Davis, and Parker Dams, three facilities with invasive mussel infestations on the Colorado River, was more than $6,026,100 in 2016. Mussel-related costs at Hoover, Davis, and Parker Dams through 2016 totaled $6,025,100, and expected costs from 2017 to 2026 totaled $10,372,108 (Boyd 2016). The State of Washington estimated direct impacts to dams from invasive mussels is $42.9 million (Community Attributes 2017). The cost for the management response is passed to the consumer (Vissichelli 2018).

Fishing tournament restrictions

The California Department of Fish and Wildlife (2020) produced Guidance for Developing a Dreissenid Mussel Prevention Program in 2020. Included in the document is an acknowledgement that fishing tournaments are a common human-mediated pathway of dreissenid mussel introduction, and that “conditions on fishing tournaments” are a potential management action to prevent a dreissenid mussel introduction.

Loss of revenue to Lake County communities

The State of Montana calculated estimated per day expenditures for resident anglers multiplied by the number of days of fishing, total angler expenditures for 2013 amounted to approximately $193 million (Swanson 2016). percent and 10 percent reduction in fishing

To date there are no studies estimating the impact of invasive mussels on tourism (Nelson 2019). The State of Montana used a scenario-based approach for recreational fishing to estimate the economic damages – 2 percent, 5 percent, and 10 percent reductions in visitation as a result of dreissenid establishment. Tourism spending was assumed to be proportional to visitation. They documented a 2-10% range of percent reductions in visitation and the corresponding reduction in spending. If visitation is reduced by two percent, the most conservative scenario, the amount of money spent by nonresident visitors would decrease by $17.8 million, a half of a percent reduction in total tourist spending in 2017. At the 10 percent reduction in visitation, tourism spending would decrease by $89 million or 2.6 percent of total tourist spending in 2017.  The 2 percent and 10 percent reductions in visitation were used for the lower and upper bound estimates, respectively.

Reduction in property values

The effect of dreissenid mussels on property values has not been explicitly estimated, however, the economic impacts of invasive aquatic plants, algal blooms, and degraded water quality due to excess nutrients on home sale price have been well documented (Horsch and Lewis 2009, Zhang and Boyle 2010, Baron et al. 2016, Walsh et al. 2011; Bingham et al. 2015; Ara et al. 2006).

  • Results from multiple studies in Minnesota, New Hampshire and Maine demonstrated a 1-meter decrease in water clarity decreased property values from 3.1 to 8.6 percent with a median value of 5.8 percent (Jakus et al. 2013).

  • In an assessment of the economic impact of harmful algal blooms to property values on Lake Erie, Bingham et al. (2015) used a 10 percent reduction in value to shoreline properties. 

  • A study of Ohio lakes found harmful algal blooms with microcystin levels in excess of 1 µg/L reduced lakefront property values by 22 percent (Wolf and Klaiber 2017). In northern Wisconsin, lakefront property values decreased by 8 percent, on average, after invasion of Eurasian milfoil (Horsch and Lewis 2009). 

  • The presence of milfoil and native aquatic vegetation in Vermont lakes decreased property value ranging from 0.3 percent to 16.4 percent depending on the degree of total macrophyte (aquatic plant) coverage (Zhang and Boyle 2010). 

  • Montana State General Fund and county governments where affected properties are located will also experience a decrease in property tax revenue from the lowered property values (Nelson 2019). Predicted losses in property tax revenue from decreases in lakefront property value ranged from $2.2 to $3.8 million per year.

Risk Assessments
Clear Lake is at a high risk for introduction of dreissenids because of the volume of out-of-county boaters that use the water body, the reputation nationally as a blue ribbon warm water fishery, numerous and free access points for visiting boaters, and water chemistry conducive to invasive mussel establishment (Lake County Watershed Protection District 2019).

FireShot Capture 133 - Google Earth -

Google Earth map of Clear Lake.

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