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the translocation, and there is a 1: 1 sex ratio, and all fish have an equal probability of contributing offspring to the next generation, this would result in a founder population with a maximum Ne of 600. A more realistic scenario is that a large proportion of fish will not survive the translocation effort (because of stress, out-migration, floods, etc.), the sex ratio will not be 1: 1, and there will be differential reproduction (because of multiple year classes, unequal family sizes, etc.). As a result, the founding effective population size should be expected to be far less than 600 (i.e., below the minimum viable population standards of Ne = 500; Franklin 1980). Third, should the translocation continue to be successful, and a large number of offspring occur from an insufficient number of founders, the action may have potential to decrease the Ne of the main population of humpback chub below Chute Falls. This could happen if a large enough number of offspring from the founder population (with low Ne) survive, and interbreed with fish below Chute Falls (with a higher Ne). This potential problem is considered by some as highly unlikely because, if the fish above had reduced fitness from inbreeding, they would be less likely to survive, mate, and reproduce than the fish in the main LCR population. As a result, the contribution of such hypothetical fish is likely to be much less than their numbers would predict (P. Hedrick, ASU, pers. com.). Nevertheless, offspring produced from above Chute Falls may have an increased chance for survivorship, since they are less proximate to the mainstem Colorado River and less likely to be transported by flood events into the mainstem Colorado River. In addition, offspring from the translocated fish may demonstrate increased survivorship because of the unexploited habitat and the apparently much higher food resources above Chute Falls. Fourth, there are potentials for selection to act upon the translocated fish (i.e., move away from the main genotype), and thus there is potential for migrants leaving this isolated group to impact the genotype of the main LCR population, even with minimal movement (see Ford 2002). There are documented cases of subtle decreases occurring in fitness when gene flow occurs between subpopulations experiencing different or conflicting selective forces (review in Storfer 1999). If fish are successful at reproducing and remaining above Chute Falls,. this might be expected to impose selection for a resident genotype. Douglas and Marsh (1996) hypothesized that there may be a resident genotype developing in the LCR since closure of Glen Canyon Dam. Because humpback chub migrate between the mainstem Colorado and the LCR during their life history (Valdez and Rye11995, Gorman and Stone 1999), this has led some to speculate that before reproduction occurs, the translocated fish will move toward the mainstem. However, Gorman and Stone (1999) found that smaller adults (< 300 mm) tend to remain as residents in the LCR. In addition, the unusually high growth rates already observed in the fish translocated in 2003 (Stone and Sponholtz 2003) suggest that within year or two some fish may commence spawning activities. If Chute Falls is a barrier against upstream movement of fish (and no data has convincingly shown otherwise), fish that survive generations to reproduce above Chute Falls will undergo selection for being non-migratory. 37