Why some native fish in the upper Colorado River basin are endangered (Continued)

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.

Trout tend to inhabit colder stretches of river than the four endangered fish and generally are not considered a threat. But many of the non-native species, such as northern pike, channel catfish, red shiner and fathead minnow, are efficient predators that prey on the eggs and young of endangered fish.

Non-native fish 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.
• At least four studies have shown predation by channel catfish, green sunfish and carp on razorback sucker eggs and larvae.
• In a study of predation on Colorado pikeminnow and various non-native fish, the largemouth bass showed a strong preference for native fish, eating twice as many young pikeminnow as red shiners and as much as 20 times the number of pikeminnow as green sunfish. (Largemouth bass, red shiners and green sunfish are not native to the Colorado River basin.)
• Young Colorado pikeminnow have been found in the stomachs of channel catfish collected in the Dolores River, even though pikeminnow are very rare in the area. Also, researchers as well as early settlers reported finding pikeminnow that had died trying to swallow channel catfish. Apparently, the barbs of the catfish make it difficult or impossible for pikeminnow to swallow and digest this non-native fish.
• Utah State University researchers found that in the Duchesne River, 1- to 3-year-old Colorado pikeminnow constituted 5 percent of the diets of northern pike - a disproportionately high percentage, given that pikeminnow constitute a much smaller portion of the available food base in the river. Based on their findings, the researchers concluded that a single northern pike could consume more than 100 year-old pikeminnow per year.

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