Removing Non-native Trout to Protect Aquatic Ecosystems

Lower Skymo Lake at North Cascades National Park.

NPS / Hugh Anthony

Once Upon a Fishless Time

One of the primary threats facing aquatic resources in mountain lakes is the widespread stocking of non-native fish. At North Cascades National Park, hundreds of mountain lakes and ponds were home to biological communities which evolved without the presence of fish. The isolation of these high elevation lakes created a unique environment where dragonflies and amphibians flourished as top predators. In the late 1800s, settlers began stocking remote mountain lakes with non-native trout for food and recreation. In 1968, the North Cascades National Park Service Complex was established which was comprised of three NPS administrative units: North Cascades National Park, Ross Lake National Recreation Area, and Lake Chelan National Recreation Area. The conflict over fish stocking became a hot topic as fish stocking is prohibited in most NPS waters, particularly in wilderness areas, to protect native ecosystems.

 

Westslope cutthroat trout are native to the North Cascades.

NPS / Hugh Anthony

The introduction of non-native fish, such as rainbow trout and brook trout, can cause a dramatic shift in trophic relationships within a lake. Native amphibians such as the long-toed salamander, western toad, cascades frog, Columbia spotted frog, and northwestern salamander struggle to compete with newly established trout populations as introduced fish often prey on amphibian larvae and become the top aquatic vertebrate predator. Early studies comparing lakes with and without introduced trout found that those with trout had notably smaller populations of salamander larvae, with the surviving larvae altering their foraging behaviors to avoid predation. These behavioral shifts disrupt their natural development, making it more difficult for larvae to grow and survive to reproductive age.
Trout can also have lasting effects beyond the lakes they were stocked in. When they disperse from lakes into nearby steams, they not only prey on native fish but also compete with them for food and habitat. Introduced trout have also been known to hybridize with native species such as the federally threatened bull trout (Salevelinus confluentus), and westslope cutthroat trout (Onchorhynchus clarkii lewisi), reducing their reproductive fitness and impacting the genetic integrity of unique and distinct native fish populations.

 

Taking Action

Over the years, conflict arose over how to manage fish stocking in North Cascades National Park. To address this issue, NPS developed the Mountain Lakes Fishery Management Plan / Environmental Impact Statement. The purpose of this plan was to showcase different alternative management options for the mountain lakes fishery in North Cascades National Park. After reviewing public input and evaluating various alternatives, Alternative B was identified as the preferred approach for future fishery management. This alternative focused on conserving lake ecosystems by reducing populations of reproducing fish. Under this plan, management actions would cover 91 study area lakes throughout the North Cascades Complex, with 42 designated for nonreproducing populations. It also includes a monitoring plan to allow for adaptive management of the mountain lakes fishery. Based on ongoing monitoring data, management actions may be changed for lake-specific circumstances to provide the best path forward for recreational opportunities and native species recovery.

 

Long-toed salamander emerges from the lake substrate.

NPS Photo

More recently, Mount Rainer National Park finalized a Fish Management Plan Environmental Assessment that prioritized the removal of non-native fish from selected lakes. The goal was to reduce the negative impact of non-native species on native fish and wildlife and to restore lake ecosystems to their natural balance before trout were introduced. These assessments were based on previous research showing that the removal of non-native fish from mountain lakes has led to the recovery of native species such as native amphibians and invertebrate assemblages. Case studies demonstrate that fish eradication resulted in successful recolonization of native organisms, often without the need for additional human intervention.

 

To accomplish fish eradication, the parks will be using a variety of methods. For lakes that are smaller (less than 5 acres and 20 feet deep), mechanical methods such as gillnetting, trapping, and electrofishing, will be used to remove fish. For larger, deeper lakes, the use of the piscicide rotenone will be used. Rotenone is a naturally occurring substance that breaks down quickly into harmless compounds. At the concentration used to target trout, it poses no threat to plants, birds, adult amphibians, or mammals, including humans. Although it can affect gill-breathing invertebrates, most populations recover within three years. Long term surveys and monitoring have been a key part in tracking the effectiveness of these methods and the potential environmental effect it has.

 

Figure 1. The result of fish removal efforts in Upper Triplet Lake and Lower Blum Lake.

 

Figure 1 shows the results of fish removal efforts using gillnets at Upper Triplet Lake and piscicide at Lower Blum Lake. Amphibian populations, which had been monitored from 2009 to 2023, showed notable recovery following the treatments. While year to year variations in population levels occurred, the overall trend indicates that the removal of non-native fish has created a more favorable environment for native amphibian species such as long-toed salamander, tailed frog, and western toad to return to these lakes.

Future Efforts

The long-term objective of this project is to restore native biodiversity, enhance the health of mountain lake ecosystems, and contribute to broader fishery management practices. The removal of non-native fish plays an important role in reestablishing the ecological balance and allows native species to return without competition and predation pressures. These efforts are not only vital for maintaining the biodiversity of public lands but also help guide future restoration initiatives and shape sustainable practices for managing fish populations. Moving forward, continued monitoring and adaptive management is critical in ensuring the resilience of these ecosystems