In 2003, scientists targeted invasive fish in the Grand Canyon. By 2006, Rainbow Trout fell from 90% to less than 10% of the catch

For four years, scientists carried out an ambitious fish-removal experiment in the Grand Canyon in an effort to protect one of the Colorado River’s most important native species.

Between 2003 and 2006, researchers mechanically removed non-native fish from a section of the Colorado River to reduce predation and competition affecting the federally protected humpback chub.

The intervention dramatically changed the composition of the fish catch. Rainbow trout, which accounted for approximately 90% of the catch at the beginning of the experiment, represented less than 10% by August 2006.


The results, however, were not as straightforward as the dramatic decline suggested.

A 2011 study by Michael D. Coggins Jr., Michael D. Yard and William E. Pine III, published in Transactions of the American Fisheries Society and available through the U.S. Geological Survey, examined the outcome of the experiment and the broader ecological factors affecting the Colorado River.

Why scientists targeted non-native fish

The experiment focused on the humpback chub, a native Colorado River fish adapted to the river’s historically warm, sediment-rich and fast-flowing conditions.

The construction of Glen Canyon Dam fundamentally altered that environment. Water released downstream became colder and clearer, creating conditions that favored introduced trout.

Rainbow and brown trout can prey on native fish, including humpback chub, while also competing with native species for food and habitat. The National Park Service continues to identify invasive fish as an important challenge for native fish conservation in the Grand Canyon.

The experiment was therefore designed to test a relatively direct question: could reducing the number of non-native fish improve the survival and recruitment of native species?

More than 36,500 fish were caught

During the 2003–2006 experiment, crews captured more than 36,500 fish representing 15 species within the mechanical-removal reach.

Approximately 64% of the catch consisted of non-native fish.

Rainbow trout were by far the most numerous introduced species, with 19,020 individuals captured. Other non-native species included fathead minnows, common carp and brown trout.

Native species accounted for the remaining 36% of the catch. These included more than 7,300 flannelmouth suckers, about 2,600 humpback chub, 2,200 bluehead suckers and more than 1,000 speckled dace.

The composition of the catch changed substantially over the course of the experiment.

Rainbow trout declined from approximately 90% of the catch in January 2003 to below 10% by August 2006. The overall proportion of non-native fish also fell sharply, dropping from more than 95% at the beginning of the experiment to below 50% after July 2005.

Native fish appeared to benefit

The researchers observed an increase in juvenile native fish within the removal zone, a result consistent with the possibility that reducing non-native fish improved native fish survival and recruitment.

But the study did not present the intervention as an uncomplicated conservation success.

At roughly the same time, the Grand Canyon was experiencing drought conditions that increased river temperatures. Rainbow trout populations were also declining across the broader river system.

Those simultaneous changes created a major challenge for the researchers: determining how much of the improvement in native fish recruitment could be attributed directly to the fish-removal program.

The uncertainty was captured in the study’s title: Nonnative fish control in the Colorado River in Grand Canyon, Arizona: An effective program or serendipitous timing?

In other words, the experiment clearly altered the fish community, but separating the effects of deliberate intervention from naturally occurring environmental changes was considerably more difficult.

The conservation effort continued after 2006

The experiment became part of a broader effort to protect native fish in the Grand Canyon.

The National Park Service continues to manage invasive trout in portions of the Colorado River system using techniques including electrofishing and trapping. Conservation programs also include relocating humpback chub to suitable tributaries and monitoring native fish populations.

The humpback chub has since been downlisted from endangered to threatened status, although it remains a conservation priority. Grand Canyon National Park contains the largest remaining population of the species.

The four-year experiment demonstrated how quickly removing a dominant non-native species can reshape a river ecosystem. It also illustrated a fundamental difficulty in conservation biology: ecosystems rarely respond to a single intervention in isolation.

In the Grand Canyon, the sharp decline in rainbow trout coincided with drought, warmer water and broader changes in trout abundance. The experiment therefore became not only a test of invasive-fish removal, but also a case study in how difficult it can be to distinguish the effects of human intervention from the influence of nature and timing.