In the depths of the Grand Canyon, scientists were trying to give native fish a better chance of survival, by taking other fish out of the Colorado River. A four-year fish-removal experiment conducted between 2003 and 2006 dramatically changed the composition of fish being caught in one section of the river. According to 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, rainbow trout went from making up about 90% of the catch at the beginning of the experiment to less than 10% by August 2006. But the story was more complicated than simply removing an invasive species and watching native fish return.
Why were the fish being removed?
The target was the federally protected humpback chub, a native fish of the Colorado River that had faced major ecological pressures. The humpback chub is specially adapted to the warm, sediment-rich and fast-flowing waters that historically characterized the river. After the construction of Glen Canyon Dam, the river’s conditions changed substantially, including becoming colder and clearer in many areas. Those changes helped create conditions in which introduced trout could thrive.
Rainbow and brown trout can prey on native fish, including humpback chub, while also competing with them for resources. The National Park Service identifies invasive fish as one of the continuing challenges facing native fish populations in the Grand Canyon. So researchers set out to test whether reducing the number of non-native fish could improve conditions for native species.
More than 36,500 fish were caught
During the 2003–2006 experiment, crews captured more than 36,500 fish belonging to 15 species in the mechanical-removal reach. About 64% of the total catch was made up of non-native fish. Rainbow trout alone accounted for 19,020 fish, followed by fathead minnows, common carp and brown trout.Native fish made up the remaining 36%, including more than 7,300 flannelmouth suckers, 2,600 humpback chub, 2,200 bluehead suckers and more than 1,000 speckled dace. The change in the catch was striking. Rainbow trout’s contribution fell steadily from approximately 90% in January 2003 to below 10% in August 2006. The overall share of non-native fish also fell sharply, from more than 95% at the beginning of the experiment to below 50% after July 2005.
Native fish appeared to benefit but scientists couldn’t take all the credit
The researchers also observed an increase in juvenile native fish within the removal area, suggesting that reducing non-native fish may have improved native fish survival and recruitment. Yet the scientists were careful about claiming a straightforward cause-and-effect victory.During the same period, drought increased river temperatures, while rainbow trout abundance was declining across the wider river system. Those simultaneous changes made it difficult to determine exactly how much of the improvement in native fish recruitment came from the removal program itself and how much resulted from changing environmental conditions. That uncertainty is reflected in the study’s title: Nonnative fish control in the Colorado River in Grand Canyon, Arizona: An effective program or serendipitous timing?
The battle did not end in 2006
The experiment became part of a much larger effort to protect the Grand Canyon’s native fish. Today, the National Park Service continues to control invasive trout in parts of the Colorado River system, including through electrofishing and trapping. Its conservation work also includes moving humpback chub into suitable tributaries and monitoring native fish populations.The humpback chub itself has since been downlisted from endangered to threatened status, but it remains a conservation priority. Grand Canyon National Park contains the largest remaining population of the species. The 2003–2006 experiment therefore offers a revealing lesson in conservation: removing a non-native predator can rapidly reshape an ecosystem, but nature rarely provides a single explanation for what happens next. In the Grand Canyon, the falling numbers of rainbow trout coincided with other environmental changes, leaving scientists to disentangle how much was deliberate intervention and how much was timing.