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i 7 enrichment and waste removal, and results in decreased hatching success (Reiser and Bjornn, 1979). Clean gravel also appears to be important to the endangered fishes of the upper Colorado River. Tyus and Karp (1989) reported that Colorado squaw fish spawning sites on the Yampa River \ were free of fine sediment. In addition to the effect of sediment on reproduction, as gravels and cobbles become more embedded with fine materials, the biotic productivity decreases, therefore affecting the entire food chain (Stanford, 1994; Richards and Bacon, 1994). -::1 I I J o-q ~~:J oJ "' .J J I J ] J .J J J ] J Nonnative Fishes Nonnative fishes provide an additional pressure on the Colorado squawfish. Forty-two nonnative fish species have been introduced into the upper basin, only 7 of which are limited to reservoirs. Of the 13 native fish species, 6 are threatened or endangered (Tyus et a1., 1981). Native fishes serve as prey for nonnative fishes (Tyus, 1991), and nonnatives compete for food and space. Some nonnative species such as the green sunfish (Lepomis cyanellus), red shiner (Cyprinella lutrensis), and fathead minnow (Pimephales promelas) have been shown to be more aggressive than young Colorado squaw fish, and therefore competitively superior in a resource-limited environment (Karp and Tyus, 1990). Now even the adult Colorado squawfish, historically the top piscivore in the system, must compete with nonnative fish such as the channel catfish (Ictalurus punctatus) for forage. Although nonnatives are believed to be detrimental to the survival of the native fish, it is also important to note that nonnative fathead minnow and red shiner are a food item for young Colorado squawfish (Muth and Snyder, 1995).