Pick Our Brain - February 2013

What are the potential impacts of a quagga or zebra mussel invasion of the Delta? Can such an invasion be prevented?

The quagga mussel (Dreissena bugensis) and zebra mussel (Dreissena polymorpha) are two closely related freshwater species that look very much like smaller versions of the more familiar saltwater mussels. Their names come from the striping on their shells with the zebra mussel having more distinctive markings than the quagga mussel. Both of these mussels, native to Eastern Europe, are believed to have entered the Great Lakes in the ballast water of cargo ships. They have since expanded their range in the U.S. through rivers, canals, and by hitchhiking on or in trailered boats. They are now broadly distributed in the Great Lakes and the Mississippi River watershed.

Quagga mussels were first spotted in Lake Mead, a reservoir on the Colorado River in southern Nevada, in 2007 and have since spread through the lower Colorado River to canals serving southern California. They are now found in many of the southern California reservoirs that receive Colorado River water. Zebra mussels have also invaded California even closer to the Delta. They were discovered in San Justo Lake near Hollister in 2008. Both of these invasions probably occurred when pleasure boats carrying adult or larval mussels were launched in these lakes.

Once established, zebra and quagga mussels have proven nearly impossible to control or eradicate. Like many saltwater mussels, clams, and oysters, they produce large numbers of free-swimming microscopic larvae (veligers) that drift with the current until they are ready to attach to a hard surface and change into their adult form. Larvae can survive in the water inside the hull of a boat or in the engine’s cooling system. Live adults have been found attached to the outside of boat hulls being moved from one body of water to another. Water diverted from a lake or river in a canal or pipeline can also spread larval or adult mussels. Although these are the primary ways that mussels can spread, it is also possible that water birds could transport mussel larvae in mud attached to their legs or feathers.

Recognition that most new U.S. invasions are linked to ballast water or pleasure boats has resulted in the establishment of programs to prevent transport of live mussels. Many lakes in California require that boats be inspected before they can be launched. State and local agencies also have education programs for boaters to help stop the transport of live mussels. High pressure cleaning and thorough drying of boats between uses is an effective means of preventing the spread of zebra and quagga mussels.

When temperature and water quality conditions are suitable, zebra and quagga mussels reproduce in huge numbers and cover any hard surface. Analysis of their water quality and temperature preferences shows that they could thrive in the Delta and much of its watershed. Wherever they have become established, they have two major impacts: the physical obstruction of pipelines and other water supply structures, and disruption of the aquatic food web through consumption of microscopic plants and animals. Cleaning mussels from water intakes and pipelines would add greatly to the cost of providing water for Delta water agencies. The impacts to the Delta ecosystem from a mussel invasion could be devastating.

For more information about zebra and quagga mussels in California, click here.


Quagga and Zebra Mussel Sightings Distribution in in California
Mussels in California
(Photo courtesy of USGS)