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searching for and establishing new home range and colonizing upstream reaches, or seeking better foraging areas. Larger sub-adult (420-450 mm) and young adult (450-550 mm) Colorado pikeminnow have been shown to exhibit greater upstream dispersal distances (mean of 20.9 miles upstream) than larger (greater than 550 mm; mean 4.7 miles) pikeminnow in the Upper Colorado River (Osmundson et al. 1998). Osmundson et al. (1998) offered several hypotheses for the upstream movements by smaller pikeminnow: 1) a response to food resource gradients, (i. e., movements may be motivated by hunger), 2) an innate physiological mechanism to move upstream, and 3) the urge to spawn. Colorado pikeminnow smaller.than 550 mm may not have established a home range like that of their larger counterparts. Their inclination to move long distances upstream as they mature may make them candidates to naturally colonize upstream reaches. Another possible reason is that pikeminnow found in the fish trap may have been progeny produced years earlier in the Gunnison River upstream of Redlands Dam that had drifted downstream over the dam, and now were moving upstream to their natal spawning areas (Tyus 1985; 1986; 1990; see Burdick 1995). Another possibility could be that pikeminnow that reside in the Lower Gunnison River are overall smaller than those found in the Colorado River. However, available evidence does not support this hypothesis. The lengths of pikeminnow collected with trammel nets and electrofishing over an 8-year period (1992-1999) in the Lower Gunnison River and a 10-year period in the Upper Colorado River (Westwater Wash to Price-Stubb Dam) were compared. This analysis indicated that the mean length of pikeminnow captured from the 2.3-mile reach of the Gunnison River between Redlands Diversion Dam and the Colorado River confluence (mean=641, n=106, S.E.=10.9), was actually larger than those collected in the 64-mile reach of the Colorado River from Westwater Wash to Price-Stubb Dam (mean=601 mm n=994, S.E.=2.83; t=3.40; P < 0.001). Larger pikeminnow may have not needed or wanted to move upstream past the Redlands Diversion Dam. There are a couple of possibilities for this. First, these larger fish may be pikeminnow that have returned to the Lower Gunnison River year after year following spawning to feed on the numerous native fishes that occur there. If adequate prey are available in the Lower Gunnison River and plunge pool of the dam, then, they may have no reason to continue moving further upstream in the Gunnison River. Second, these larger pikeminnow may have become accustomed to or programmed to stopping at the base of some of these diversion dams, like Redlands, because they have precluded their upstream movements continuously for about 80 years. Not only have low-head diversion dams interfered with pikeminnow movements, but they could have disrupted and changed their reproductive behavior. Last, larger Colorado pikeminnow may have an aversion to the fish passageway. 25