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S ~~ <br />northern squawfish Ptychocheilus oregonensis (Beamsderfer and Congleton 1982, <br />Patten and Rodman 1969). <br />It has been difficult to identify spawning grounds for this species in <br />other locations, presumably because of the rarity of the fish. The presence of <br />small larvae in the Upper Main Stem Colorado and the San Juan rivers indicates <br />successful reproduction is occuring, but at very reduced scale in comparison <br />with the Green. Similiarly, no conclusive migratory patterns have been <br />detected in the Upper Main Stem Colorado River despite years of study, <br />although some movements have been detected (Archer gt al 1986). The preceding <br />~ t °I.ov1,~,u.~ w2yCvlow w~-:~ ~ y~°`rw-2 <br />suggests that n, al spawning behavior so well documented in the Green River <br />Basin, may have been disrupted in the Colorado River by years of flow and <br />habitat alterations. The San Juan River has been studied even less, and no <br />other hypotheses can be developed for that system. <br />Larvae and Postlarvae <br />As water levels recede in the late summer, larval Colorado squawfish emerge <br />from the cobbles and are transported downstream to concentrate in shallow <br />alongshore ephemeral embayments (backwaters) (Haynes et al 1984, Tyus et aI <br />1982,1987). Young fish captured in the drift are predominantly protolarvae, <br />and fish collected from shoreline backwaters are metalarvae (Haynes and Muth <br />1984), suggesting that the fish seek warmer and more productive habitats as <br />they mature (Tyus et a1 1987). Young of the year (postlarval) Colorado <br />squawfish are rare in Yampa and Gray canyons a few weeks after spawning, but <br />they are concentrated 100-250 km downstream (Tyus et al 1982b,1987). This <br />9 <br />