Unwelcome visitors: Invasive fish species spell trouble for Colorado waterways

Division of Wildlife workers near Silverton
Photo courtesy of Colorado Parks and Wildlife

The next time you consider ways of getting rid of a pet fish, dead or alive, know that dumping it into the nearest lake or flushing it down the toilet are not valid options.

The recent removal of koi goldfish from Thunderbird Lake in East Boulder is just a small example of the types of problems presented by the introduction of non-native aquatic nuisance species in Colorado waterways.

The koi goldfish were reportedly introduced into the lake illegally sometime over the past year and were removed on Monday, Nov. 19 due to the harm they pose to the aquatic ecosystem.

“We do have people who bring certain fish that they prefer to recreationally fish to waters and sometimes that upsets the balance,” says Jennifer Churchill, public information officer for Colorado Parks and Wildlife northwest region. “So we really ask people not to do that or not to dump any pets they don’t want any more or any aquatic species into our waterways –– it can really be very detrimental to fisheries and the health of the water in general.”

These non-native species can ruin a fishery through the introduction of disease, or the fish may become predators of the species already stocked in the fisheries by Colorado Parks and Wildlife.

“We have yet to test the koi for fish diseases, but koi are among the most highly susceptible and known to carriers of VHS [Viral Hemorrhagic Septicemia], bacterial kidney disease, and a variety of viruses,” says Ben Swigle, Colorado Parks and Wildlife aquatic biologist for the Fort Collins and Boulder areas. “We do not want potential further contamination resulting from additional movements of these illegally introduced fish.”

The threat of disease and the disruption of healthy aquatic ecosystems can also have an effect on residents and anglers in areas in which these fish are introduced.

“I can’t speak to what may be direct threats to residents, but in general, the spread of fish disease can certainly impact recreational fishing opportunities, threat en closure to public areas and affect the benefits of healthy aquatic ecosystems benefits to the community at large,” says Matt Claussen, urban resource manager for the Boulder Parks and Recreation Department.

Colorado Parks and Wildlife officials say they recognize the illegal introduction of non-native fish into Colorado fisheries as a “significant and rapidly growing threat,” according to a press release. Some other recent examples of this illegal practice include the introduction of Northern pike and smallmouth bass in the Yampa River in northwest Colorado, which has compromised the populations of native species there through predation, according to Swigle.

The introduction of non-native species is not always intentional. Invasive species such as the New Zealand mud snail and crayfish can be transferred between eco systems by boats and boating equipment.

“As people enjoy boating and go to different waterways they can sometimes transfer those [species],” Churchill says. “They’re so tiny. Even the remnants of these little snails, veligers are what they’re called, can attach to the prop or different little hiding crevices on a boat.”

New Zealand mud snails are particularly hazardous because of their ability to reproduce in large quantities and clog up water treatment facilities. Boats used in waterways that contain New Zealand mud snails can transfer the species if the boat isn’t properly cleaned and inspected before entering another waterway or fishery.

“These species can actually really devastate a waterway and especially the equipment and operations of a waterway, especially if it’s used for a reservoir purpose,” Churchill says.

For this reason, Colorado Parks and Wildlife has implemented mandatory boat inspections throughout the state.

“In the last two or three years, Colorado Parks and Wildlife is doing very stringent inspections of those boats,” Churchill says. “So there’s a lot of spots in the state where you actually have to have your boat inspected before you’re allowed to go on the water.”

A list of inspection sites, including the Boulder Reservoir, can be found on Colorado Parks and Wildlife website.

In addition to boat inspections, Colorado Parks and Wildlife biologists monitor most lakes and streams every five to 10 years though fishery surveys. High priority waters such as Chatfield, Pueblo and Horsetooth reservoirs and the Gunnison River are surveyed annually, according to the Colorado Parks and Wildlife website.

“We use a combo of electrofishing and netting to gauge the health of fish [populations] throughout the state,” Swigle says. “The surveys ultimately determine future stocking rates, angler regulations, and habitat manipulation measures we can use to improve the fishery for both angler use and the general health of the ecosystem and fish community.”

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