From 1886 to 1968, more than 20 million fish were stocked in waters that became Rocky Mountain National Park; In 2026, crews treated 14.3 miles to remove non-native fish and restore threatened green trout.

For more than 80 years, fish were deliberately introduced into the waters that would become Rocky Mountain National Park. According to the National Park Service (NPS), between 1886 and 1968, more than 20 million fish were stocked in waters that are now part of Rocky Mountain National Park, most of them non-native trout. What was once intended to support recreational fishing eventually created challenges for native fish and aquatic ecosystems. In 2026, park managers are working to reverse some of those effects. As part of the Poudre Headwaters Project, crews treated waterways connected to the 14.3-mile Grand Ditch to remove non-native fish and prepare habitat for the recovery of the federally threatened cutthroat trout.

A long history of storage

Fish stocking began in the area in 1886, before Rocky Mountain National Park was established. Colorado stocked fish in mountain waters to create and expand recreational fishing opportunities. The program grew over the next few decades. Hatcheries were established in Estes Park and Grand Lake, which produced large quantities of fish for release into local waters. Some of the introduced species were not found naturally in the region. Fish were even stocked in certain waters that had historically had no fish. By 1968, more than 20 million fish had been introduced to the waters that are now part of the national park. Non-native fish had become established in much of the park’s aquatic environment.

Why did the focus change?

The park’s fish management philosophy eventually shifted from increasing fishing opportunities to protecting native ecosystems. The National Park Service issued formal guidance on fish management in 1936, encouraging the use of native fish and discouraging the introduction of non-native species. Stocking in historically fishless waters was also prohibited. By 1968, the fish population had ceased.However, the end of the population did not eliminate the non-native trout that were already established. Species such as brook trout remained in the streams and were able to compete with native fish for food and habitat. That left park managers with a more complicated task: removing introduced fish from selected waterways so native species could recover.

Restoring green trout

Cutthroat trout is one of the native species targeted by the restoration program. Historically, these trout occupied Colorado’s mountain waters, but their populations have been affected by competition and interactions with introduced fish. Brook trout can compete with green cutthroat trout for resources, making removal of non-native fish an important part of restoring native populations. The goal is not simply to reduce the number of fish in a stream. Instead, managers want to restore waterways where native trout can survive, reproduce and eventually expand their populations.

The 2026 project

The latest work is part of the Poudre Headwaters Project, a large-scale effort to restore native trout habitat in the headwaters of the Cache la Poudre and Colorado River watersheds. The project includes the Grand Ditch, a historic water diversion system that spans approximately 14.3 miles. Another phase of the project was carried out in August 2026 in the Kawuneeche Valley. Some areas were temporarily closed from Aug. 24 to Aug. 30 while crews performed the work. The treatment focused on non-native fish in the Grand Ditch and connected waterways within Rocky Mountain National Park.

Removal of non-native fish

Completely removing fish from remote mountain streams can be difficult using conventional methods. Electrofishing, netting, and angling can remove fish in some situations, but may not be enough when managers need to remove non-native populations from an entire connected drainage. For the Poudre Headwaters Project, crews use rotenone, a fish toxicant, in specific waterways. The treatment is controlled to prevent the chemical from affecting areas outside the project area. Potassium permanganate is subsequently used to neutralize rotenone, while the chemical also breaks down naturally over time. This allows crews to treat waterways that would otherwise be difficult to completely clear.

A different future

The work represents a major reversal of a practice that lasted more than eight decades. Fish were once introduced to make mountain waters more attractive to fishermen. Today, managers are deliberately removing some of those same introduced species to give native trout a better chance to return. Restoration will not undo storage history overnight. But by removing non-native fish and restoring suitable habitat, the Poudre Headwaters Project is working toward a landscape where green cutthroat trout can reoccupy parts of their native range. The effort shows how conservation priorities can change over time: What was once considered beneficial for recreation may later require active restoration to protect the ecosystem.