by Amy Stengel, Assistant Attorney General, Natural Resources & Environment, Colorado Attorney General’s Office

Figure 1. Zebra mussel, Dreissena polymorpha (top) and quagga mussel,Dreissena rostriformis bugensis (bottom). Source: USGS.

An infestation of aquatic nuisance species including zebra and quagga mussels, the New Zealand mudsnail and the rusty crawfish has occurred in recent years in Colorado’s lakes, streams and reservoirs. Aquatic nuisance species are invasive or non-native species that are introduced into an ecosystem where they typically result in harm to the natural environment and may cause damage to man-made structures. In Colorado, aquatic nuisance species have been introduced by human activity including transport in fishing gear, boats and trailers and use as live bait. While the effects of invasive aquatic species have not been as dramatic and damaging as in other regions such as the Upper Mid-West, the potential impact on habitat, fisheries and wildlife as well as costs associated with damage to water infrastructure are significant.
Zebra and quagga mussels were first discovered in Lake Pueblo in 2007 and subsequently appeared in a number of other reservoirs in the state by 2008. Originally transported to the United States from Eurasia on contaminated ships, aquatic nuisance mollusks spread to the Northeast and Great Lakes and were then introduced into the West after reaching the Mississippi River and its tributaries. Once immersed in water the mussels are capable of reproducing at rapid rates and begin to attach themselves to hard surfaces such as water diversion structures, boats and other equipment, thereby impeding water treatment and supply systems. They also out-compete native aquatic species for food and nutrients resulting in a significant disturbance to the entire ecosystem.
In response to the infestation of non-native mollusks, the Colorado Division of Wildlife implemented a program to educate the public about the multitude of threats caused by zebra and quagga mussels and began requiring routine inspections and decontamination measures to prevent further spread. One advantage to controlling the spread of mollusks in Colorado compared to other states is that it is believed that cool water temperatures at altitudes above 7000 feet may shorten the breeding season for mussels. The Bureau of Reclamation has also begun various test programs for eradicating mussels including introduction of predatory fish, poisons and underwater UV-ray devices intended to prevent mussels from attaching to hard surfaces. For now efforts at control within the state appear to be working as there have not been any new colonies detected in reservoirs or lakes. In 2009, the only evidence of invasive mussels was at Lake Pueblo where the original populations have not been fully contained.

Figure 2. New Zealand mud snail (Potamopyrgus antipodarum). Source: USGS.

Another aquatic nuisance known for causing extensive damage to riparian ecosystems is the New Zealand mud snail which was first discovered in Boulder County in 2004. In contrast to mussels the mud snail does not attach to hard surfaces so it is not responsible for the costly impacts to water infrastructure. However, the mud snail’s real danger is its impact on fisheries and aquatic communities because of its ability to out-compete native species and reduce the availability of nutrients for native fish. New Zealand mud snails are transported in mud or plants attached to fishing gear and boats. In July, the Division of Wildlife discovered mud snails in South Delaney Butte Reservoir. Additional populations were confirmed in prior years in the South Platte and Green Rivers. The New Zealand mud snail was first detected in the United States in the late 1980s along the Snake River in Idaho and has since then spread rapidly throughout the rivers, lakes and reservoirs of the West. The Division of Wildlife recommends cleaning and drying waders, boots and boat exteriors as a means to prevent infestation to new areas.
Fishing is also responsible for the introduction of the Rusty crawfish to the Yampa River. Native to the Ohio River basin, the crawfish are large, aggressive and disruptive of native food webs. The crawfish consumes smaller fish and their eggs and is not selective about which plants it will eat so it negatively impacts the entire aquatic community. Also capable of disrupting native aquatic ecosystems are the Eurasian milfoil, a submerged aquatic plant capable of clogging water bodies and impairing supply systems, and the Purple Loosestrife, a weed which grows in wetlands and crowds out native plants and wildlife.
Currently, Colorado has not had the same severity of problems associated with invasive aquatic species as some other states but state wildlife officials and water managers have taken a cautious approach to prevent further contamination of the state’s unique riparian habitats. It is possible that cool temperatures at altitude have naturally prevented infestation of mountain ecosystems by slowing reproduction of aquatic nuisance species but a few years of warm, dry summers has the potential to reverse this pattern. In the mean time, Colorado plans to continue an aggressive control program aimed at protecting native habitat and fisheries and minimizing disruption of water supply and treatment systems at risk of being clogged by invasive aquatic plant and animal species.
Note: Real time maps of quagga and zebra mussel distribution throughout the United States can be viewed on the USGS website at:

Figure 3. Rusty crawfish (Orconectes rusticus). Source: Maryland Department of Natural Resources.

Colorado Division of Wildlife, 2010, Colorado Division of Wildlife, Wildlife Species, Invasive Species,
U.S. Fish and Wildlife Service, 2010. U.S Fish and Wildlife Service Invasive Species program
Summit County girds against unwanted species, Julie Suitor, Summit Daily News, July 10, 2010
Jackson County New Zealand mud snails found in South Delaney Butte Reservoir, Bob Berwyn, Summit County Voice, August 8, 2010
Experts testing tactics to keep harmful mussels from muscling their way in, Bruce Finley, The Denver Post, July 21, 2010
Mussel Watch: state finds less, expects more, Chris Woodka, The Pueblo Chieftain, March 31, 2010

Colorado Riparian Association