Numerous native fish species of the Colorado River (such as Razorback Sucker, Roundtail Chub, Humpback Chub, Bonytail Chub, and Pikeminnow) have suffered significant declines over recent decades. During that same time period, non-native fish species (such as Rainbow Trout, Brown Trout, Striped Bass, Bluegill, and Crappies) have thrived. A team of scientists from Northern Arizona University led by Dr. Alice Gibb has been seeking to explain these trends and establish a set of recommendations on how to better protect the Colorado River's native fish species.
The researchers started their investigations in the laboratory environment. They first compared the escape responses of native and non-native fish species—they wanted to find out how successful each species was at escaping predators. Their observations revealed that native fish were at a clear disadvantage when compared to non-native fish. The larvae of native fish were less developed than non-native fish when they hatched. As a result, the native fish lacked the agility and morphological development needed to avoid predation. Non-native fish on the other hand were more mature when they hatched. Not only could they escape predators more efficiently than native fish, they also were capable of preying on native fish larvae.
Dr. Gibb and her colleagues then looked to the Colorado River: what conditions in the river made native fishes so vulnerable? To answer this question, they first considered the effects of the damming of the Colorado River. Before the Colorado River was dammed, there were fast moving currents. Turbulent water churned up sediment and plankton, making it easier for the larvae of native species to hide from predators and find food. Water temperatures were higher. After the river was dammed, fast moving water was replaced with lakes: water temperature fell and plankton settled. The larvae of native fish were left exposed as sediments were no longer churned up in the still lake water.
The research team suggests that to reverse the decline of native species in the Colorado River, management efforts must focus on removing non-native predators and reintroducing high-flow, turblent, sediment rich waters into the Colorado.
Find out more: River Damming Leads to Dramatic Decline in Native Fish Numbers (Science Daily)
Top: Photo © Alice Gibb / Northern Arizona University. Roundtail chub, Gila elegans, in the Gibb Laboratory at Northern Arizona University.
Bottom: Photo © David Ward / Arizona Game and Fish. Confluence of Little Colorado River and Colorado River in the Grand Canyon.