By Andrew Cohen and Peter Moyle

One of the world’s most invasive freshwater mussels has arrived in North America. The Golden Mussel (Limnoperna fortunei), discovered in the California Delta in October, is a voracious plankton feeder and may further reduce the food supply for Delta Smelt and other plankton-feeding fishes in low salinity environments. It will also foul pipes, canals, fish screens, gates and other components of California’s water supply systems. Its effect on fishes will be similar to that of the Overbite Clam (Potamocorbula amurensis), which has been a cause — arguably a major cause — of pelagic fish declines in the Delta since the early 1980s. Golden Mussels will likely colonize the fresher parts of the Delta and spread upstream as far as boats travel, thereby occupying a part of the watershed where nonnative Overbite Clams, limited by salinity, drop out. 

Golden Mussels are native to southeastern Asia, from southern China to Vietnam. They spread northward with human assistance to northern China and Korea, and westward across salt water to Taiwan and Japan. The mussels then traveled in ships’ ballast water to South America. Discovered in 1995 near Buenos Aires, they have spread northward in Argentina and into Uruguay, Paraguay and Brazil, mainly through navigable waterways as fouling attached to boat hulls. They have also been carried overland to a few other watersheds.

Golden Mussels, like Zebra Mussels (Dreissena polymorpha) and Quagga Mussels (Dreissena bugensis), have altered aquatic food webs and energy flows and fouled vessels, maritime structures, beaches, water supply systems and power plant cooling systems, often with substantial economic costs.

However, Golden Mussels pose a greater threat to California than Quagga or Zebra mussels, which were first reported in the state in 2007 and 2008. First, Golden Mussels can live and reproduce in water with lower calcium levels. Waters throughout the Sierra Nevada, the upper Sacramento River watershed, the Trinity River watershed and the mainstem Sacramento River, which include many of California’s most critical reservoirs, intake structures, pumping facilities, pipelines and canals, have been protected until now against invasion by these biofouling mussels. Low calcium concentrations have also protected the Smith and Klamath rivers and Lake Sonoma. With the arrival of low-calcium-tolerant Golden Mussels, these waters are no longer protected. 

In addition, Golden Mussels are already in the Delta — the heart of the California water supply system. From there, they have already spread westward into one canal and 100 km southward through either (or both) the federal and state canals. In contrast, Zebra Mussels in California have so far been restricted to the small and isolated San Justo Reservoir in San Benito County, and Quagga Mussels to southern California reservoirs and water systems, without ever making it into the Delta watershed.

In 2007, we served on the Science Advisory Panel for California’s response to the discovery of Quagga Mussels in Lake Mead. Based on their limited known distribution at the time, we recommended making a multipronged effort  to eradicate Quagga Mussels in the lake, primarily by first, drawing the water down as far as feasible, which would kill off much of the infestation; and then enclosing the remaining infested sites behind plastic curtains and treating the isolated areas with biocide. It was an ambitious proposal with no guarantee of success, but we thought it warranted given what was at risk.

We similarly feel that if there is an approach with a reasonable chance of eradicating Golden Mussels from California (and thus from North America), it should be done immediately and aggressively. An all-out response would be warranted, due to the scale of potential harm. However, eradicating an invasive biofouling mussel from a water body isn’t easy.

Biofouling mussels have been eradicated from only six water bodies where they were clearly established; more than four times as many efforts have failed. In five of the successes entire waterbodies (6-32 acres in size) were treated with biocides, and in the sixth a Zebra Mussel population restricted by low oxygen to the upper portion of a reservoir in Nebraska was exposed to the air by lowering the water level for six months, including a few months of freezing temperatures. Neither approach is possible in California’s Delta. Tidal and river currents would make using barrier curtains to isolate areas for biocide treatment extremely difficult, if not impossible.

As of this date, Golden Mussels have been reported from 30 sites in the southern and San Joaquin portions of the Delta, with hundreds of mussels observed at some sites, and additional sites still being reported. The largest mussels are probably 2-3 years old, with the size range indicating that at least two annual cohorts are present at several sites. It’s possible that the mussels present today are already a few generations removed from the initial introduction. It is not clear to us that there is an approach with a reasonable chance of eradicating them from the Delta’s highly interconnected, tidal waters. 

The simplest explanation for the discoveries to date is that a cargo ship’s ballast water containing Golden Mussel larvae from an Asian or South American port was discharged at the Port of Stockton. The released larvae and those from subsequent generations would have mainly been carried downstream, though some would have traveled upstream on tidal currents. Others were pumped into the Contra Costa Canal, into Bethany Reservoir, and 100 km southward into O’Neill Forebay via Bethany Reservoir and the California Aqueduct, and/or via the Delta-Mendota Canal. These likely pathways explain the observed distribution.

In the coming months, additional records could modify and complicate this story. For example, mussels attached to boat hulls may have already traveled to other watersheds, though it’s not likely. With Zebra and Quagga Mussels, years passed before isolated waters were infested through overland transport (e.g. attached to boats), and there are few cases of overland introduction of Golden Mussels in South America. However, we know that intervening waters and probably some nearby waters in the Delta and the water system have been exposed to Golden Mussel larvae, and we can expect to find settled mussels at additional sites.

A potential complication is interactions with other invasive mussels. Quagga Mussels have largely replaced Zebra Mussels in several water bodies where they co-occur. Until now, however, Golden Mussels (present in Asia and South America) had never lived on the same continent as Zebra or Quagga Mussels (present in Europe and North America). From O’Neill Forebay, Golden Mussels will likely be carried southward in the California Aqueduct to the Pyramid Lake-Castaic Lake system and Lake Piru, where they will encounter Quagga Mussels; and carried westward via San Luis Reservoir and the Pacheco and Hollister Conduits to San Justo Reservoir, where Zebra Mussels await. Invasive mussel experts should make predictions now about what will happen when these mussel match-ups occur, and researchers should initiate surveys to establish baseline conditions.

Finally, an obvious lesson from this incident is that 40 years after ballast water made headlines by introducing Zebra Mussels into North America, ineffective federal regulation still leaves the nation vulnerable to major invasions arriving in ballast water. The US Environmental Protection Agency has refused to establish ballast water discharge requirements that comply with the Clean Water Act, despite Court orders issued over a decade ago. Two years ago, 34 members of Congress asked EPA to comply with the Clean Water Act’s requirements, and 180 environmental, commercial and sport-fishing and public health organizations, water agencies, and Native American tribes asked the President to order EPA to do so. Yet in October (eight days before Golden Mussels were discovered in the Delta), EPA re-issued the same inadequate discharge standards that the Court held were unlawful in 2015.

Even EPA’s inadequate discharge standards, adopted in 2008 and 2013, would have prevented this invasion if EPA had enforced them effectively. However, until stronger ballast water regulations are adopted, and those regulations are effectively enforced, US waters will remain vulnerable to introductions of organisms that could prove to be even more destructive than Zebra, Quagga or Golden Mussels, and to novel strains of human pathogens carried in ballast from other parts of the world. This last risk is especially significant because the Delta is a source of drinking water for over 25 million Californians.

Andrew Cohen is Director of the Center for Research on Aquatic Bioinvasions (Richmond CA). Peter Moyle is an associate Director of the Center for Watershed Sciences, University California, Davis.

Further reading

Lockwood, J.L., M.F. Hoopes, and M.P. Marchetti  (2007) Invasion Ecology.  Blackwell Publishing. 304 pp.   This is an excellent review of how and why invasive species are such a major problem worldwide, as well as in California. It is a readable gateway to the huge ecological literature on invasions, showing where solutions dwell.

California Department of Fish and Wildlife (2008). California Aquatic Invasive Species Management Plan. www.dfg.ca.gov/invasives/plan. This report shows that the state, in theory, knows what to do when faced with new aquatic invaders. There is even a short section on Golden Mussels.