CLEAR CREEK COUNTY, Colo. (CBS4) – A small stream turned a hue of purple right now as state fish biologists begin a large scale fish management program, leaving hundreds of fish dead, while making way for another threatened species of native fish to be moved there.

Colorado Parks and Wildlife set up a camp with more than 20 people working around the clock along the banks of the Herman Gulch in Clear Creek County. They are working to kill all the fish that live in the waterway currently, and then restock that waterway with the greenback cutthroat trout, Colorado’s state fish.

Presumed to be extinct by 1937, several wild populations of what were thought to be greenback cutthroat trout were discovered in the South Platte and Arkansas river basins starting in the late 1950s. According to the CPW, those discoveries launched an aggressive conservation campaign that replicated those populations across the landscape so that they could be down-listed from endangered to threatened under the Endangered Species Act.

Momentum for preserving the native jewels continued to build, and in 1996 the greenback was designated as Colorado’s state fish. Efforts to establish new populations were proceeding along a track that suggested the recovery plan benchmarks might soon be met, and the subspecies could be delisted entirely.

Recent genetic work on museum and extant populations however suggests that in fact the true native cutthroat of the South Platte basin (and heir to the name greenback cutthroat trout) can be found in only a single stream outside their native range. Aggressive recovery efforts have been implemented to replicate this population, with the first reintroduction in the wild occurring in August 2014.

Currently, biologists estimate there are less than 5,000 wild greenback cutthroat in the state, but once this project is complete, they hope to double or triple that number.

“We choose this creek in particular because once we clear out the invasive fish species that live in these waters it will be impossible they will be able to get back into the creek to compete with the greenback cutthroat once we stock them here,” Ken Kehmeier, senior aquatic biologist, South Platte River basin, Colorado Parks and Wildlife, said.

Biologists are using a substance called rotenone to kill the fish that currently call the creek home. They add the liquid upstream of a temporary water treatment and testing center at the bottom of the stream. Once the substance does its job they then dilute and consternate the deadly substance. The process turns the water a purple color for a few hundred yards downstream of the treatment center, but water samples taken downstream from that location show the water quality is back to safe levels as it enters Clear Creek.

Right now, biologists are raising thousands of greenback cutthroats in fish hatcheries in Lake and Chaffee counties.

Rotenone FactSheet

What is Rotenone?

Rotenone is a naturally occurring substance derived from the roots of tropical plants in the bean and pea family that are found primarily in Malaysia, South America, and East Africa. It is derived from ground up plant roots to make a powder formulation or extracted from the roots to make a liquid or crystalline formulation. People have utilized rotenone for centuries to capture fish for food in areas where these plants are naturally found, and it has been used in fisheries management as a piscicide (pesticide that kills fish) in North America since the 1930s. Rotenone affects gill breathing organisms by inhibiting respiration by blocking biochemical pathways of cell metabolism.

When is it appropriate to use rotenone as a fish removal tool?

Fish removal has remained a necessary tool in fisheries management throughout history. Harmful fish species (invasive species, those not native to the area, or those that have expanded beyond their native range with the aid of humans or due to anthropogenic change to the environment), including exotic species (those from a foreign land) can adversely affect wildlife. Harmful aquatic species have contributed to the decline of approximately two-thirds of the threatened or endangered fishes in the U.S. through competition for resources, predation, and hybridization.

How and when is rotenone applied?

Rotenone liquid is typically packaged in 1-, 5-, 30- and 50-gallon containers and powder is typically in 50- and 200-pound containers. Applications are generally made with boats in lakes, reservoirs and ponds, with direct metering into moving water such as streams, and with handheld equipment such as backpack sprayers in difficult to reach areas. Rotenone may be applied at any time of year, but most applications typically occur during warm months when the compound is more effective and degrades more rapidly. Rotenone is usually applied during low water conditions to limit amount of area treated and piscicide needed. On-site bioassays are performed to identify the lowest effective concentrations for use during the treatment.

What are the risks of contaminating groundwater?

Rotenone is highly insoluble in water and strongly absorbs to soil particles in bottom sediments and to suspended particles in the water column, limiting its mobility and availability to bioaccumulate in organisms. These factors also make rotenone unlikely to leach through soils and reach groundwater .

Matt Kroschel covers news throughout Colorado working from the CBS4 Mountain Newsroom. Send story ideas to mrkroschel@cbs.com and connect with him on Twitter @Matt_Kroschel.