The smallmouth bass, an invasive species from the eastern and central United States, is threatening native wildlife in the Colorado River, including the humpback chub.
To slow their spread, the U.S. Bureau of Reclamation has begun releasing cold water from Lake Powell through the Glen Canyon Dam, impacting the water temperature in the Grand Canyon and creating a less hospitable environment for this invasive species, as reported by Newsweek on August 27.
Smallmouth bass threaten many native fish species in the Colorado River. (Photo: iStock).
Smallmouth bass have been spreading throughout the upper Colorado River basin for decades, impacting numerous native fish species, but were only recently detected downstream of the Glen Canyon Dam in 2022. The smallmouth bass is an efficient predator, competing vigorously for food sources and specifically targeting juvenile native fish. The species at risk includes the humpback chub, as 90% of this species resides in the Grand Canyon.
“This invasive predatory fish has recently been found spawning in areas where it has not previously appeared in large numbers, threatening the recovery of the humpback chub”, stated the National Park Service.
After discovering smallmouth bass downstream of the Glen Canyon Dam, authorities feared they would begin to spawn and rapidly spread uncontrollably through this section of the river. To mitigate this risk, they decided to lower the temperature of the Colorado River by releasing water from Lake Powell, reducing the temperature below 15.5 degrees Celsius, which is necessary for smallmouth bass to spawn. Maintaining temperatures below 15.5 degrees year-round would significantly limit or prevent smallmouth bass from spawning, and they would struggle to survive and grow during their early life stages. This plan was implemented in early July and appears to be effective, according to authorities. According to the Bureau of Reclamation, efforts to electrocute, net, and dive have not detected newly hatched smallmouth bass.
This success also means that the National Park Service does not need to introduce rotenone, a fish-killing chemical, into the river as was done last August. Despite this success, many experts remain concerned that smallmouth bass will continue to move downstream through the hydroelectric turbines of the Glen Canyon Dam. Additionally, releasing cold water from Lake Powell has also lowered the water levels of the reservoir, jeopardizing electricity supplies for millions of nearby residents who rely on hydroelectric power.
In fact, the Bureau of Reclamation and the National Park Service are considering several barriers to prevent fish movement and other measures to stop the spread of smallmouth bass downstream in the Colorado River.
