Non-native species compete with natives and generally disrupt the local ecological balance. In the American Southwest, non-native nuisance species in the aquatic environment consist primarily of fish, although the zebra mussel is of concern in some areas. The number of non-native fish species in the Southwest is considerable. For example, at least 16 species of non-native fish can be found in the Grand Canyon stretch of the Colorado River, which is twice the number of native species present when Glen Canyon Dam was constructed. Wherever non-native fish occur, they may compete with native fish, including endangered fish species.