Threats to Native Fish
Human-caused changes to the Colorado River in Grand Canyon have caused serious declines in the park's native fish populations. The introduction of non-native fish and the construction of major dams on the Colorado River both have vastly changed the aquatic habitat for Grand Canyon fish.
The Colorado River's natural ecosystem was first altered when non-native fish such as channel catfish were introduced into the Colorado River system in the 1890s. The construction of Hoover Dam in 1933, which inundated the lower portion of Grand Canyon and blocked the migration of Colorado pikeminnow, added to the decline of the canyon's native fish. Glen Canyon Dam, which was completed in 1963, caused the single largest change to the aquatic ecosystem in Grand Canyon.
Glen Canyon Dam, located just upstream of Grand Canyon, blocks more than 90% of the sediment that used to flow through Grand Canyon, and eliminated large annual floods, creating massive changes to the physical and aquatic environments downstream. Because they were adapted to a river that was seasonably warm in the summer, native fish do not spawn or grow well in much of the Colorado River, which now flows cold year-round since the dam releases water at 47º F.
Today, non-native cold-water fish, especially rainbow and brown trout, thrive in the post-Glen Canyon Dam Colorado River, especially in the upper reaches of Grand Canyon. Non-native fish, which are now much more abundant than native species in a large part of the canyon, both prey on and compete with native fish for food and habitat resources.
Trout, most notably brown trout which are native to Europe and Asia, are voracious predators of fish. Because of the high turbidity of the Colorado River prior to the construction of Glen Canyon Dam, native fish including humpback chub are not adapted to "sight predators" such as trout. In addition, colder waters may decrease the ability of native fish to escape predation by slowing their swimming abilities.
A total of 13 non-native species are documented in the park, include common carp, channel catfish, and fathead minnows.
Non-native parasites, such as the Asian tapeworm, and environmental threats to tributaries, some of which host the healthiest native fish populations in the canyon, further imperil Grand Canyon's native fish.