Colorado’s Least Wanted: Profiles of Some of the Most Destructive Aquatic Nuisance Species Threatening Colorado

Aquatic nuisance species (ANS), also known as aquatic invasive species, are one of the most significant and rapidly growing threats to Colorado’s natural resources.
IMAGE FOR ELIZABETH updated mussels
Image/design by Jerry Neal/CPW. Content suggestions/edits by Mindy Blazer/CPW.

The historical Wild West is famous for its outlaws and ruthless invaders. Had you visited most any Colorado town in the 1870s, you would have likely seen wanted posters with photos and descriptions of the most infamous villains and the crimes for which they were sought “dead or alive.” Thankfully, we live in a more civilized era today, but there are still dangerous and menacing invaders out there that are threatening Colorado and the West.

Aquatic nuisance species (ANS), also known as aquatic invasive species, are one of the most significant and rapidly growing threats to Colorado’s natural resources. In nearly every direction,  non-native and exotic invasive animals, plants and pathogens exist that could devastate our fisheries, negatively impact boating and outdoor recreation and even affect water delivery systems.

Some ANS, like those featured on this parody “Wanted” poster, are already present in Colorado and every precaution must be taken to ensure that these intruders remain confined to their current locations. However, there are other equally destructive, or potentially even more destructive, species just beyond our borders that could be accidentally transported into Colorado at any time. 

Combating invasive species, while protecting native species and their habitats, is an important component of wildlife conservation. Therefore, Colorado Parks and Wildlife’s (CPW) Invasive Species program is taking proactive measures to contain existing ANS and prevent others from being brought into our state.

Because recreational boating and angling are the primary vectors for spreading ANS, CPW has implemented a statewide watercraft inspection program at more than 70 locations across the state. Since the program’s inception in 2008, boat inspectors have intercepted nearly 100 vessels transporting invasive zebra/quagga mussels. By quarantining and decontaminating these boats prior to launching, watercraft inspectors have protected dozens of waters from possible infestation.

In addition to managing watercraft inspections, CPW’s invasive species crews monitor 150 lakes, reservoirs and streams throughout the state for 12 animal and eight invasive plant species, and CPW biologists are constantly on the lookout for pathogens and diseases that could negatively impact our native aquatic resources.

Due to their destructive nature and ability to reproduce at alarming rates, the following are four ANS that biologists believe pose a serious threat to Colorado’s ecosystems, branding them  Colorado’s “Least Wanted” invasive species:

1. Zebra and Quagga Mussels

zebra mussel-Credit 100th Meridian
A zebra mussel. Photo by 100th Meridian.

Status: Native to Eastern Europe, zebra and quagga mussels were introduced to the Great Lakes Region in the late 1980s by European cargo ships. Throughout the last two decades, the invasive thumbnail-size mussels have spread throughout the United States, causing devastation in countless rivers, lakes and reservoirs.

Adult zebra/quagga mussels have never been found in any Colorado lake or reservoir. However, ANS sampling crews first discovered the larvae of the invasive mussels, called veligers, at Lake Pueblo in 2007, along with several other locations in 2008. Despite the past detection of larvae, CPW biologists believe that the boat-inspection program has stopped the inoculation of our waters and is working to keep invasive mussels and other ANS from establishing self-sustaining populations anywhere in Colorado.

Lake Pueblo remains the only “positive” water for quagga mussel veligers in Colorado, and there are no waters that have tested positive for zebra mussels. However, if ANS crews do not detect veligers at Lake Pueblo in 2016, CPW will de-list the water to “negative” status in 2017, following five years of negative test results—per regional standards.

While other states are becoming infested each year (those that do not have watercraft inspection programs), Colorado is de-listing waters to “negative” status for mussels because the ANS Program works so well to protect our waters from invasion.

Description/Identifying Marks: Zebra and quagga mussels are freshwater, bivalve (two-shelled) mollusks that typically exhibit an alternating light and dark pattern on their shells. Adults range in size from ½ inch to 2 inches in length. The mussels can attach to nearly any hard object with small fibers called byssal threads. They also attach to each other, forming large, dense colonies.

Violations/Crimes: When it comes to invasive species, few are as destructive and costly as zebra and quagga mussels. These mollusks outcompete native species for food and habitat, foul boats and engines, damage and clog water infrastructure of hydroelectric, agricultural and municipal water facilities. Zebra and quagga mussels reproduce at alarming rates—one female mussel can produce up to one million eggs annually. As highly efficient filter feeders, zebra and quagga mussels attack the base of the food chain by removing large quantities of the planktonic food sources necessary to native fish and other aquatic organisms. In the Great Lakes, mussels have caused damages in the billions of dollars. If allowed to spread further in Colorado, zebra and quagga mussels could bring forth devastating social, environmental and economic impacts.

zebra mussels cover a boat propeller. Photo by

Means of Spread: The primary vector for spreading zebra and quagga mussels is overland on recreational boats, trailers and other watercraft devices. Adult mussels attach to or “hitchhike” on boats and trailers, and can, depending on temperature and humidity, survive up to 30 days out of the water until they are deposited in a new location. Microscopic mussel larvae, called veligers, can be transported in water stored in livewells, bait buckets, engine cooling systems and bilge/ballast tanks. Boaters and anglers must follow the Clean, Drain and Dry protocol before launching or leaving any reservoir to prevent spreading this dangerous species. Once introduced, there is no way to get rid of zebra and quagga mussels, and only expensive, regular maintenance mitigates damages.

2. New Zealand Mudsnail

A New Zealand mudsnail. Photo by Minnesota Sea Grant.

Status: Native to New Zealand, the mudsnail was first detected in the United States in 1987 in Idaho’s Snake River and has since spread throughout the West. The mudsnail is currently found in several Colorado locations, most recently in Chatfield State Park in 2015.  Previously, there were detections from 2010-2013 in Fountain Creek in Colorado Springs, Spinney Mountain State Park, Eleven Mile State Park, Delaney Buttes State Wildlife Area, College Lake at CSU in Fort Collins and Dry Creek within the city of Boulder.  The invasive snail was first found in Colorado in 2004 in Boulder Creek, the South Platte River below Eleven Mile dam and the Green River in Dinosaur National Monument.  There were no detections from 2005-2009.

Description/ Identifying Marks: The New Zealand mudsnail averages 1/8 of an inch in length and has a brown or black cone-shaped shell with five whorls. One way to identify this species is to hold the point of the shell upward. Unlike native snails, when the point of the shell is facing upward, the New Zealand mud snail’s shell opening is on the right.

Violations/Crimes: The New Zealand mudsnail competes with native invertebrate species and can eliminate forage important to trout and other native fishes. Mudsnails reproduce asexually (it takes just one to form a population) and spread rapidly, reaching densities of 100,000 to 700,000 per square meter. The New Zealand mudsnail has no natural predators in the United States and once introduced, is extremely difficult to contain them.

Means of Spread: Mudsnails typically hide in mud and spread by attaching to boats, waders and other recreational fishing gear. Anglers should clean their waders and equipment after every use. Fisherman also should avoid using felt-soled wading boots, as felt greatly increases the risk of spreading mudsnails and other ANS.

3. Eurasian Watermilfoil

watermillfoil on boat-Credit Michigan Sea Grant
EWM entangles a boat propeller. Photo by Minnesota Sea Grant.

Status: Native of Europe, Asia, and Northern Africa, Eurasian watermilfoil (EWM) was first documented in the eastern United States in the 1940s. EWM was first discovered in Colorado in 1999 in the Rio Grande, and has since been found in many Front Range locations from Fort Collins to Walsenburg.

Description/ Identifying Marks: EWM is a submersed, aquatic, perennial weed that roots to the bottom of water bodies. EWM leaves are finely divided with 12-20 leaflets and occur in whorls of 3 to 4 along the stem, giving milfoil a unique feather-like appearance. EWM stems are pink or olive in color, and they usually grow 3 to 10 feet in length, but can exceed 30 feet. New plants emerge from each stem-joint forming thick mats.

Violations/Crimes: In many parts of the U.S., EWM is a problematic and destructive invasive aquatic weed. With a rapid growth rate—averaging 1 foot per week—the highly aggressive species forms dense mats, which cover the surface of lakes and reservoirs. EWM mats impede all forms of water-based recreation, such as swimming, fishing and boating. Dense weed beds can also have adverse effects on native aquatic vegetation important for waterfowl and other native species. EWM disrupts the forage mechanism of game fish by providing ample hiding spots for smaller prey fish causing a negative bottom-up effect on the food web. This nasty aquatic weed can colonize a variety of habitats including lakes, reservoirs, rivers, ditches and canals. EWM is notorious for slowing or stopping the flow of water in natural and man-made systems.

Means of Spread: EWM is spread from lake to lake on recreational boats and trailers. Boaters and anglers should inspect and remove all plant materials before leaving any body of water. A small plant fragment attached to boat, trailer or waders can take root in a new location and form an entire colony. Once EWM becomes established in a waterway, it is nearly impossible to remove.

WARNINGS: While prohibited for sale in Colorado, some internet sites sell EWM and other aquatic weeds for ornamental or aquarium use. Never release any ornamental aquatic plants/weeds into the wild!

4. Rusty Crayfish

rusty front high res provided by Minnesota Sea Grant
The aggressive and destructive rusty crayfish. Photo by Minnesota Sea Grant.

Status: Native to the Ohio River Basin, the rusty crayfish has spread throughout the northeast United States. Rusty crayfish are used in the bait industry in other states, so the risk for introduction to Colorado is extremely high. Rusty crayfish inhabit lakes, ponds and both pool and fast-water areas of streams, making many areas in Colorado potentially suitable habitat.

The rusty crayfish was first discovered in Colorado in 2009 in a main-stem impoundment of the Yampa River and at two other river locations between Stagecoach Reservoir and Steamboat Springs. CPW conducted extensive surveys statewide and also detected a population in Sanchez Reservoir State Wildlife Area in 2010 and Stagecoach State Park in 2011.

Description/ Identifying Marks: The rusty crayfish has two rust-colored marks on its mid-back area. Adults can reach a maximum length of 4 inches.

Violations/Crimes: The rusty crayfish is an aggressive and opportunistic feeder with a voracious appetite for aquatic plants, native crayfish, juvenile fish and fish eggs. Large rusty crayfish populations can harm fish, resulting in reduced angling opportunities. In heavily infested areas, rusty crayfish also affect recreational swimming. The fear of stepping on and being pinched by the aggressive, large-clawed “rusties” is very real.

Means of Spread: Rusty crayfish can be spread by anglers, aquarium hobbyists and commercial harvesters. Teachers and students who use crayfish for classroom studies sometimes release them into local waters.

WARNINGS: It is illegal to use rusty crayfish as bait anywhere in the state of Colorado. Anglers should always dispose of any unused live bait into the trash to prevent introducing any unwanted species. In addition, state law prohibits releasing classroom pets or surplus laboratory specimens into the wild. Never “set free” any crayfish, plants or other animals into local waters.

An Ounce of Prevention . . .

A boater cleans his vessel. Photo by Elizabeth Brown/CPW.

Benjamin Franklin once said, “An ounce of prevention is worth a pound of cure.” While this is certainly universal and valuable advice, nowhere does this statement ring truer than in the war against invasive species. Once introduced, most invasive species cannot be eradicated. Therefore, preventing their introduction is critical.

CPW thanks boaters, anglers and other recreationalists for their continued support in helping to protect Colorado’s waters by keeping their watercrafts, fishing and recreational equipment Clean, Drained and Dry.

For further information about CPW’s Invasive Species Program and to learn more about the invasive species threatening Colorado, click HERE.

Written by Jerry Neal. Neal is the editor for Colorado Outdoors Online and is an information specialist for Colorado Parks and Wildlife. 

3 Responses

  1. This is an excellent story. This should be mandatory reading for all boaters and fisherman. I’m from Michigan and can tell you first hand that you don’t want zebra or quagga mussels in Colorado. Many of our beaches are now unusable because of the millions of shells that litter our banks. Thank you DOW for working to protect our waters.

    1. Adam, Thanks for reading. Invasive mussels are truly a serious problem. Most people don’t fully understand the threat until they have lived or recreated in an area that has been affected. It’s amazing the amount of damage they can cause!

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