California Department of Fish and Wildlife

Quagga and Zebra Mussels
Incident Description

Quagga mussels were discovered in Lake Mead in Nevada on Jan. 6, 2007, and later throughout Lake Mead’s lower basin. It was the first discovery of either of these mussels west of the Continental Divide. Subsequent surveys found smaller numbers of Quagga mussels in Lakes Mohave and Havasu in the Colorado River, and in the Colorado River Aqueduct System which serves Southern California. Surveys in August found Quagga in Lake Dixon and San Vicente Reservoir in San Diego County. All reservoirs, lakes and watersheds receiving raw Colorado River water have been exposed to Quagga mussels. The first confirmed find of Zebra mussels in California occurred at San Justo Reservoir Jan. 10, 2008.

Zebra mussels arrived in North America from Europe in the 1980s followed shortly thereafter by their close relative the Quagga mussel. As prodigious water filterers, they remove substantial amounts of phytoplankton, zooplankton and suspended particulate from the water, which reduces the food sources for zooplankton and small fish, altering the food web. With the filtering out of suspended particulates and phytoplankton, water clarity increases allowing sunlight to penetrate the water deeper triggering increased vegetation growth that can affect oxygen levels resulting in fish die offs.

Quagga/Zebra mussels accumulate organic pollutants within their tissues to levels more than 300,000 times greater than typical concentrations in the environment. The mussels’ wastes significantly lower the oxygen levels, lowering the pH to an acidic level and generating toxic byproducts. The mussels have also been associated with outbreaks of botulism poisoning in wild birds.

Zebra mussels heavily colonize hard substrates while Quaggas colonize both hard and soft substrates. It appears as though Quaggas colonize deeper than Zebra mussels, infesting a wider range of habitats. In locations where both mussels exist, the Quagga mussel appears to compete with the Zebra mussel, eventually replacing it. Quagga/Zebra mussels clog water intake structures, such as pipelines and screens, reducing pumping capabilities for power and water treatment facilities. Recreation-based industries and activities are also affected by the mussels which take up residence on docks, breakwalls, buoys, boats and beaches. For boaters, Quagga/Zebra mussels increase drag, clog engines causing overheating and can affect steerage.