Invasive Mussel Mitigation
California Department of Water Resources Senior Environmental Scientist Specialist Brianne Sakata, with the Division of Operations and Maintenance, works with California State Parks staff to collect plankton samples to study alongside invasive golden mussels from buoys in O'Neill Forebay in Merced County, California. Photo November 13, 2024.
Invasive species such as golden, quagga, and zebra mussels can thrive in the California State Water Project as they can survive in a wide range of environmental conditions, posing negative impacts to water delivery systems and the environment. Once these invasive mussel populations are established, they are difficult to control.
Golden mussels were first discovered in North America in October 2024 at the Port of Stockton when DWR discovered the mussels while conducting routine water quality monitoring. The same month, golden mussels were discovered at O’Neill Forebay in Merced County.
Unlike quagga and zebra mussels, the golden mussel (Limnoperna fortunei) can tolerate brackish water and low calcium levels, which means that more waterbodies are vulnerable to infestation. These invasive mussels can:
- Alter aquatic ecosystems by filter-feeding on planktonic algae and changing the habitat structure
- Impact water delivery systems by clogging small-diameter pipelines, screens, and filters
- Impact recreation by fouling boat hulls and motors, and clogging intakes and cooling lines
To prevent these damaging mussels from spreading, DWR routinely monitors for these species and acts to prevent their spread to non-infested waters. DWR partners with agencies such as the California Department of Fish and Wildlife (CDFW) and California State Parks’ Division of Boating and Waterways (DBW) to inform boaters to always remember to clean, drain, and dry their boats before entering and leaving lakes, rivers, and other waterways. DWR also works with partner agencies to routinely sample for adult and juvenile mussels at 18 SWP locations.
To prevent the spread of invasive mussels, watercraft must pass an entrance inspection before launching in most SWP reservoirs. If a watercraft fails an entrance inspection or has been in an infested waterbody, it must observe a seven to eight-day dry-out period before it can be launched in SWP waters. Dry time requirements vary for lakes managed by other entities, so it is best to check before you go. SWP reservoirs with inspections include:
- Lake Del Valle (Alameda County)
- San Luis Reservoir (Merced County)
- O’Neill Forebay (Merced County)
- Los Banos Creek Reservoir (Merced County)
- Pyramid Lake (Los Angeles County)
- Castaic Lake and Lagoon (Los Angeles County)
- Silverwood Lake (San Bernardino County)
- Lake Perris (Riverside County)
DWR urges other waterbody managers with invasive species prevention programs to treat watercraft originating from Bethany Reservoir, O’Neill Forebay, San Luis Reservoir, Los Banos Creek Reservoir, Pyramid Lake, and Castaic Lake and Lagoon as high-risk and should be managed accordingly.
To prevent the spread of mussels to new lakes, mandatory exit inspections are in place at these infested and potentially infested SWP lakes:
• San Luis Reservoir (Merced County)
• O’Neill Forebay (Merced County)
• Los Banos Creek Reservoir (Merced County)
• Pyramid Lake (Los Angeles County)
• Castaic Lake and Lagoon (Los Angeles County)
A “clean boat” tag, which normally is attached to boats upon departure, will not be granted to boats leaving San Luis State Recreation Area (SRA) lakes. Until DWR can determine the status of mussels in San Luis Reservoir and Los Banos Creek Reservoir, all boats leaving San Luis Reservoir, O’Neill Forebay and Los Banos Creek Reservoir will not be allowed to launch in another San Luis SRA lake until a seven-day dry-out period has been observed. Boats may be permitted to launch in the same lake they last visited if exit inspection tags are in place.
When recreating on California’s waterways, always remember to:
- Clean off aquatic plants and animal material from your boat, trailer, and equipment before leaving the ramp or parking lot
- Drain ballast, bilge, live-wells, and other water-containing devices. Never drain back into the waterway
- Dry equipment before launching into another body of water.
- Description: Mussels are two-shelled invertebrates. They grow attached to hard surfaces and form colonies.
- Shell Shapes: Their shells are generally D-shaped, triangular, and are smooth or shallowly ridged.
- Color and Patterns: Golden mussels range in color from light yellow to dark brown. Zebra and quagga mussels range in color from light cream to dark brown and usually have stripes.
- Size Range: Adult mussels range from about 3/4 inch to 2 inches.
- Young mussels may only be a few millimeters long and may be hard to detect without magnification.
Additional Resources
In California, it is illegal to import, transport or possess quagga, zebra, or golden mussels. Report any mussels you find to the local marina or park ranger and to the CDFW hotline at (866) 440-9530 or email: invasives@wildlife.ca.gov. For more information on invasive species laws and regulations, visit CDFW's website. Boating and inspection information are available at the Division of Boating and Waterways website.
Check out a history of quagga and zebra mussel sightings in California.
For more about quagga and zebra mussels, go to:
DWR’s Quagga Mussels: Clean, Drain, and Dry Video
Department of Fish and Wildlife website
For golden mussel information and how to report sightings, go to:
Contact
(916) 820-7662 | maggie.macias@water.ca.gov