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Quagga and Zebra Mussels

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Zebra and quagga mussels are nonnative Dreissenid mussels from Europe. They reproduce prolifically, especially in warm western waters. Despite efforts to control their western expansion, they have begun to rapidly invade California fresh water systems. Zebra mussels were first discovered in Lake St. Claire in the Great Lake Regions in 1988. In January 2007, quagga mussels were found in Lake Mead, west of the continental divide, despite the passage of federal legislation and targeted strategic efforts by U.S. Fish & Wildlife to prevent their westward movement. Since January 2007, quagga mussels have been found in reservoirs in Nevada, Arizona and throughout Southern California. Most Southern California waterways receiving raw water from the Colorado River are suspected of being infested. By January 2014, 33 water bodies in California were infested. In January 2008, the closely related zebra mussel was found in San Justo Reservoir, which is located in San Benito County near the Santa Clara County line. In December 2013 Lake Piru in Ventura County was confirmed to be infested with quagga mussels. The Lake Piru infestation is significant because it is the first reservoir in Southern California that does not receive Colorado River water to have quagga mussels.

In May of 2010, the MCWRA, County Parks and SLO County Flood Control & Water Conservation District (SLO), began implementing a Mussel Prevention Program at Lakes Nacimiento and San Antonio which involves screening of all boats entering the reservoirs. The program standards are located here along with the Vessel Screening Permit form.

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Nacimiento And San Antonio Reservoirs

For more than 50 years, Nacimiento and San Antonio reservoirs have provided agricultural, residential and industrial water supply as well as recreational benefits. The lakes attract thousands of boaters, swimmers and fishermen every year. It has been a valuable investment that needs to be protected.

That's why we're asking you, before you visit Nacimiento and San Antonio reservoirs, to take a few moments to learn about how you can prevent an invasion of Quagga and Zebra mussels to these valuable resources. If we're all vigilant, we can stop these invasive aquatic species from spreading to our reservoirs.

Take Note!

ALL VESSELS AT SAN ANTONIO and NACIMIENTO REQUIRE A VALID VESSEL SCREENING PERMIT PRIOR TO LAUNCH, unless they are currently enrolled in the Resident Vessel Program at Nacimiento.

Vessel Screening Form

2019 Quagga and Zebra Mussel Prevention Program at Lake San Antonio and Lake Nacimiento

The Problem

Quagga and Zebra mussels are aquatic invasive species that are not native to Nacimiento and San Antonio Reservoirs. If these mussels get into the lakes, they are likely to cause economic and/or environmental harm. The impact of these mussels cannot be overstated. They will displace native species, disrupt ecosystems and harm recreational activities such as boating and fishing. They are likely to also cause considerable financial losses for water supply and commercial activities around the lakes. They have the potential to foul the workings of the dams and the new San Luis Obispo County water delivery system, and to reduce property values around the reservoirs.

Why The Concern?

Quagga and Zebra mussels grow in dense clusters, like barnacles. The microscopic larvae reproduce quickly and thrive in California's warm Mediterranean climate. Quagga mussels were found in Southern California waters in 2007. These mussels pose imminent risks to California's waterways. The costs of managing these mussels in California could reach hundreds of millions of dollars.

Solution (Don't Move Mussels From One Place To Another)

Currently, Zebra mussels are not known to occur in any other California water bodies, but efforts to educate the public and prevent more widespread introduction is underway. It is up to you as an individual boat or pet owner or fisherman to take preventative measures to ensure that the mussels don't travel with you.

What You Can Do

The mussels can be transported by recreational boaters and fishermen via boat hulls, motors, ballast water, live bait and other gear, including clothing. What you can do is thoroughly inspect and dry all of your equipment that contacts the water and don't bring in live bait from outside sources. Also, if your pets enter the water, they should be inspected and dried, too. And please use your imagination when cleaning your boat, gear and clothing. Where would you hide if you were a mussel?

Take Note!

Boaters are required to have an inspection and carry the vessel screening permit on the boat. Below are the areas where to inspect your boat.

More Info

For more information about inspection, the history of aquatic invasive species migration and how you can become involved in stopping the hitchhikers, here are some links:

  1. San Luis Obispo County Public Works
  2. Santa Clara County Parks
  3. Santa Clara Valley Water District
  4. San Benito County Parks
  5. Santa Barbara County Parks
  6. Pinto Lake
  7. California Department of Fish and Game
  8. 100th Meridian Initiative
  9. California Department of Boating and Waterways
  10. U.S. Fish and Wildlife Service


Mussel Hotline 1-866-440-9530

Monterey County Water Resources Agency
1441 Schilling Pl., North Bldg., Salinas, CA 93901

    Phone (831) 755-4860   |   FAX (831) 424-7935   |   After Hours (831) 796-1166