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At least four studies have shown predation by channel catfish,green sunfish and carp on razorback sucker eggs and larvae. I I I I I I I I I I I I I I I I I I I Why these fish are now endangered Summary The fish are now endangered because of human impact on their habitat over the past 100 years. The two types of habitat alterations that appear to have had the greatest impact have been water development and introductions of non-native fish. Hundreds of dams have been constructed; river flows have been cut by a third; and more than 40 species of non-native fish have been introduced in the upper Colorado River Basin. Water development Human population growth since the turn of the century has created a significant demand for water and hydroelectric power in western states. To'meet that demand, hundreds of water projects, including dams, canals and irrigation projects, have been constructed on the Colorado River and its tributaries. Water projects have restricted the fish to about 25 percent of their former range and have blocked some of the spawning migration routes of the Colorado squawfish and other species. The river has been "channelized" by dikes and other barriers that block the fishes' access to slower moving backwaters and riverside wetlands. Also, tailwaters six to 20 miles downstream from dams can be as much as 15 degrees colder than the rare fishes' preferred habitat. Downstream of Lake Powell, dams have segmented the once free-flowing, silty and wann waters of the Colorado River into a series of lakes connected by cold, clear waters. Introductions of non-native ,fish Introductions of non-native fish into rivers, lakes and reservoirs also have taken a toll on the native fish. Historically, there were only 14 fish species in the upper Colorado River Basin. Then in the late 1800s, private citizens and state and federal wildlife agencies began stocking , non-native fish into lakes, reservoirs and streams in the Colorado River drainage. Now the four endangered fish have to compete with more than 40 non-native species. Many of the non-natives are efficient predators that prey on the eggs and young of en,dangered fish. They also compete with native fish for food and space, often with greater success. Several studies have documented these effects. For example: In some areas of the upper Colorado River basin, the percentage of native fish is quite low. At the confluence of the Colorado and Green rivers in Utah, biologists have found that 95 percent of the fish species are non-native; only 5 percent of the fish that now exist in this part of the river basin are native.