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two million years. The largest dam was 420 m high forming a lake extending <br />the length of the Grand Canyon and lasting several thousand years. <br />Formation of these dams would have formed temporary barriers to fish <br />migration that lasted several hundred to thousands of years and provided <br />opportunities for native fishes to take advantage of productive lake and inflow <br />F:.environments periodically. It is likely that native fishes, such as the razorback <br />sucker, flourished in these habitats as evidenced by initial exploitation of <br />artificial Lake Mohave, Lake Mead, and the Salton Sea by razorback sucker <br />(Minckley 199 1). <br />The Fishes and their Niches <br />Of the 13 native fishes of the Green River, four are federally <br />endangered and two have been proposed for listing. The combined <br />circumstance of an isolated river system, highly variable physical conditions <br />and a depauperate base stock of cypriniform fishes led to peculiar adaptations <br />to the Colorado River environment. The availability of open niches provided <br />opportunity for the evolutionary process to mold these fishes into bizarre <br />forms to fit their unique niches. <br />The endangered Colorado squawfish (Ptychocheilus Lucius has the <br />distinction of being the largest minnow in North America and occupied the <br />top predator niche among the Colorado River native fish community. This <br />giant predacious minnow historically approached a length of 1.5 meters (five <br />feet) and weights between 25-35 kg (60-80 pounds). One key identifying <br />characteristic of the minnow family is absence of mouth teeth. Once lost, <br />primitive characteristics are seldom regained through evolution. While <br />evolving into an apparently vacant top predatory niche the Colorado <br />squawfish developed bony ridges in its mouth. This provided adaptive <br />advantage in seizing and holding prey. <br />7