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Introduction

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The Colorado River Basin is home to many endemic species of fishes, ranging from long-lived Razorback Suckers (Xyrauchen texanus) to distinctly-shaped Bonytail Chub (Gila elegans). Yet, extensive water withdrawals and hydroelectric dam development have drastically altered riverine conditions of the Colorado River, including the populations of its unique fish species.  

Historically, the Colorado River section that flowed through the Grand Canyon was characterized by highly turbid, sediment-rich conditions that are susceptible to high annual fluctuations in flow, periodic flooding, and large sediment input. These highly variable conditions  led to the diverse evolutionary characteristics expressed in native fishes, including the distinct hump shape of Humpback Chub (Gila cypha), used to escape predation, or the large bodies of Colorado Pikeminnow (Ptychocheilus lucius), helpful in navigating high-gradient rapids.     

The construction of Glen Canyon Dam in 1966 changed the ecology of the Lower Colorado River downstream in the Grand Canyon. Once dams such as Glen Canyon were constructed in the 20th century, the highly variable and dynamic hydrological conditions of Western rivers subsided. Today, flows and temperature in the Colorado River are more consistent throughout the year. In addition to dam construction, several non-native fish species were introduced to the Colorado River to provide opportunities for recreational fishing. Within Grand Canyon National Park, salmonid introductions included Brown Trout (Salmo trutta), which are native to Europe, and Rainbow Trout (Oncorhynchus mykiss), native to the western coast of North America.

Key Issues Addressed

The two most significant threats to native fishes in the Grand Canyon are habitat modification from dams and established populations of non-native species. Present day flow and temperature conditions in the mainstem Colorado River are favorable to many introduced fish, especially trout, however these conditions are not ideal for many native fish species. As such, non-native trout are predators of juvenile native fishes and have successfully outcompeted them for habitat and prey resources. 

Native fish such as the Humpback Chub evolved to escape predation from top predators like the Colorado Pikeminnow. As natural top predators declined in response to dam construction, non-native species, including Brown and Rainbow Trout, replaced their niche. Salmonids are generalist predators, and the evolutionary adaptations that worked before for native fish, are not effective for introduced trout. Non-native Brown and Rainbow Trout readily feed on native fish, particularly juveniles in the slower, clearer nursery environments of tributaries. Many native species found in the Grand Canyon today are federally-listed as threatened or endangered, including Razorback Sucker and Humpback Chub (Colorado Pikeminnow and Bonytail Chub are extirpated from the Grand Canyon). Species of concern also include native Bluehead Sucker (Catostomus discobolus) and Flannelmouth Sucker (Catostomus latipinnis).   

Habitat loss in the mainstem river has led biologists and managers to focus native fish restoration efforts on unaltered tributaries. Before reintroducing native fish, however, biologists must first control populations of non-native species. This is particularly necessary in tributaries like Bright Angel Creek, which is the primary spawning ground for Brown Trout within Grand Canyon National Park. Accordingly, Bright Angel Creek has been a primary focus for non-native salmonid removal since 2002. 

Project Goals

• Conduct multi-year salmonid removal to benefit native fish in Grand Canyon National Park
• Minimize the presence and expansion of non-native Brown and Rainbow Trout populations in Bright Angel Creek
• Prevent non-native trout from returning following mechanical removal
• Maintain self-sustaining populations of native fish species
• Reintroduce native Humpback Chub to tributaries devoid of non-native trout, including Bright Angel Creek

Project Highlights

• Effective Suppression of Non-Native Salmonids in Bright Angel Creek: Rarely detected in Bright Angel Creek throughout the 1970s, biologists observed a rapid increase in Brown Trout spawning in the 1990s. Mechanical removal and active monitoring efforts intensified by 2012 and continued for six years prior to Humpback Chub translocations. A 5-year status report (2012 to 2017) showed Brown Trout abundance declined by 65%. Today, the source population of Brown Trout is at 91% overall suppression, with both Brown and Rainbow Trout suppressed by 89% through 2018. Following suppression, native fish dominate the creek, comprising 97% of the fish community.
• Weir and Fish Trap: Each year from 2012 to present, a fish weir is seasonally placed near the confluence of Bright Angel Creek and the Colorado River during trout spawning. A one-way design prevents adult Brown Trout from returning upstream to spawn while allowing native species to migrate downstream. Biologists can monitor, capture, and remove non-native species as well as document native species that pass through.
• Backpack Electrofishing: Backpack electrofishing surveys were conducted to remove both juvenile and adult trout, targeting all habitats from the mouth to headwaters. Electrofishing surveys were also used to document the presence of native species in sampling sites. 
• Positive Response of Native Fishes:Overall native fish populations in Bright Angel Creek have increased in abundance by 480% since salmonid removal. Surveys document increasing populations of Speckled Dace (Rhinichthys osculus) and Bluehead Sucker (Catastomus discobolus), as well as large year-classes of Flannelmouth Sucker (Catostomus latipinnis) and a few individual Humpback Chub. A few non-translocated chub also entered the creek from the mainstem.
• Successful Humpback Chub Translocations: Humpback Chub were successfully relocated to Bright Angel Creek in 2018, with another translocation planned for 2020. For more information visit this CCAST Case Study.
• Collaboration with Local Native American Tribes: Park biologists and managers worked closely with local tribes to provide trout for consumption and ceremonial purposes. 

Lessons Learned

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Early efforts during the 2000s to suppress Brown Trout populations in Bright Angel Creek were not effective in controlling spawning. Effective suppression of Brown Trout required intensive mechanical removal as well as post-removal monitoring. Adaptive management enabled project managers and biologists to evaluate their degree of success and make changes. In addition to mechanical removal, a seasonal fish weir is placed in Bright Angel Creek to prevent non-native salmonids from returning to their spawning locations. This weir is imperative to prevent future recolonization of Brown Trout once native Humpback Chub were translocated to Bright Angel Creek.  

Brown and Rainbow Trout thrive in swift, coldwater conditions from dam releases that support popular recreational angling opportunities elsewhere in the Colorado River Basin. The two species are among the most widely introduced vertebrate species on the planet. National Park Service management policies (2006), however, direct managers to promote native species, through removal of non-natives. Therefore, compared to other locations in the Colorado River Basin, Grand Canyon National Park provides a unique management opportunity for non-native species control. Grand Canyon National Park does not actively manage a trout fishery, but is required to suppress or eradicate, if possible, non-native species. Management goals include reducing the impacts of non-native species on native fishes, and project managers develop management plans that prioritize non-native species removal and native fish conservation.

Next Steps

• Future work includes experimental use of rotenone, a chemical treatment often used to remove fish from streams, in other tributaries (Shinumo Creek). Biologists could then compare chemical to mechanical treatments to determine how effective different treatments are to eradicate salmonids.
• Researchers plan to conduct a “Population Viability Analysisâ€� (PVA) for Brown Trout within Grand Canyon National Park will be used to simulate models under distinct scenarios, including environmental conditions and various populations of both native and non-native fishes. A PVA analysis is informative to both biologists and managers, as they can be used to create management actions that account for the most efficient methods for Brown Trout suppression. 
• Park managers are unable to control flows or temperatures in the Colorado River and plan to conduct vulnerability assessments. Future vulnerabilities, however, are important to account for, such as temperature differences from climate change climate change
  Climate change includes both global warming driven by human-induced emissions of greenhouse gases and the resulting large-scale shifts in weather patterns. Though there have been previous periods of climatic change, since the mid-20th century humans have had an unprecedented impact on Earth's climate system and caused change on a global scale.
  
  Learn more about climate change and successful non-native species. 
• Managers will conduct additional surveys for early detection and provide education for other non-native species. For example, increasing populations of warm water species such as Green Sunfish (Lepomis cyanellus) and Smallmouth Bass (Micropterus dolomieu) pose additional threats to native fishes. While Quagga Mussels (Dreissena bugensis) and New Zealand Mud Snails (Potamopyrgus antipodarum) are not widely abundant within Grand Canyon National Park, their presence in other reaches of the Colorado River pose future risks as well. Park managers are working to prevent future transfer of these species through more monitoring, public education, and outreach.

Funding Partners

Resources

• Ogaz, M. â€�.â€� University of California-Davis.
• Grand Canyon National Park: 
• Grand Canyon National Park: 
• National Park Service: 
• National Park Service: 
• Minckley, W. L., and Arizona Game Fish Commission. (1973). “Fishes of Arizona.â€� Phoenix: Arizona Game and Fish Dept., Print.
• Healey et al. (2020). . Canadian Journal of Fisheries and Aquatic Sciences. 
• Whiting et al. (2014). . Freshwater Science 33:872â€�884.
• Runge et al. (2018). . Flagstaff, Arizona.
• , Native Fish Ecology and Conservation Program

Contacts

• Brian Healy, Fisheries Program Manager, Native Fish Ecology and Conservation Program, Grand Canyon National Park and PhD candidate, Utah State University: [email protected]
• Sarah Haas, Deputy Chief, Science and Research Management, Grand Canyon National Park: [email protected]
• Emily Omana Smith, Fisheries Biologist, Grand Canyon National Park: [email protected]
• Robert Schelly, Fisheries Biologist, Grand Canyon National Park: [email protected]
• Rebecca Koller, Biological Science Technician, Grand Canyon National Park 

Case Study Lead Author

Alex Koeberle, CART Research Specialist, University of Arizona, [email protected]

Suggested Citation

Koeberle, A.L. (2020). “Non-Native Trout Removal for Native Fish Conservation in Grand Canyon National Park.â€� CART. Retrieved from /project/non-native-trout-removal.