Invasive Fish Might Make Its Way Downstream Colorado River as They Continue to Spread in Warming Waters

Warming Colorado River waters encourages invasive fish, endangering native species as invasives spread downstream. Climate change and dwindling river reservoirs promote invasive species, notably the smallmouth bass.

Scientists worry about potential harm to the Grand Canyon's native humpback chub. Officials and researchers seek solutions but face regional controversy.

Good For Native Species, Also for Invasives

The Grand Canyon once thrived with native species, a rarity in the Colorado River system, according to Brian Healy, a US Geological Survey biologist. Recently, a shift in temperatures led to the rise of non-native fish like brown and rainbow trout, benefiting the humpback chub.

David Rogowski of the Arizona Game and Fish Department noted the area's warming trend, beneficial to both native and non-native species, raising concerns for the ecosystem's balance.

Invasive Fish in the Colorado River

Smallmouth bass, introduced to the Colorado River system in 1982, thrived in Lake Powell, contributing to a "fantastic fishery." However, with declining reservoir levels due to climate change, warmer waters near the Glen Canyon Dam's hydropower intakes now allow these fish to migrate downstream, posing a threat to native species.

Previously, the water was too cool for smallmouth bass reproduction. Now, it's warm enough for their growth and reproduction, creating an unexpected ecological challenge in the Colorado River system.

National Park Service's Controversial Rotenone Solution

In the past two summers, the National Park Service took proactive steps to remove smallmouth bass and green sunfish from the Colorado River Slough below the Glen Canyon Dam. They used a fish poison called rotenone, but precautions were taken to minimize exposure to other species. While rotenone can be harmful to people and animals in high concentrations, any entering the main channel would quickly dilute to insignificant levels.

Targeting specific breeding areas is not a comprehensive solution, but it's a productive strategy. The Colorado River Slough is the primary known location for invasive fish reproduction, making it a focal point for control efforts.

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Warming Water Temperatures

The Bureau of Reclamation recently initiated a formal evaluation process to explore proposals for modifying flow velocity and temperature at the Glen Canyon Dam. This action aims to disrupt the downstream reproduction of invasive species. Located downstream from Lake Powell, the dam's warming upper layer has created optimal conditions for smallmouth bass reproduction.

This evaluation has sparked controversy, with environmental groups and utilities expressing contrasting opinions. In August 2022, the agency's initial environmental assessment generated nearly 7,000 public comments. Some advocated for a solution involving regular cool water releases supplemented by high-flow cold water "spikes" during the summer, seen as the most viable and cost-effective approach by groups like Trout Unlimited.

However, other environmental organizations called for more drastic measures. Despite the comprehensive evaluation, the Center for Biological Diversity considered it "too little too late." The situation underscores the complex challenges and differing perspectives surrounding the management of invasive species in the Colorado River system.