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space available to the fish. This is because other environmental variables--the availability of food or of hiding places away from predators, for example--become limiting factors on the fish population before the population reaches a size that is limited by the availability of physical space. However, if the volume of physical space is reduced because of reduction in river flow, there will be a point reached where the available habitat volume is effectively filled by the fish, food availability becomes inadequate or other structural or functional aspects of the habitat become limiting factors and further reductions in volume therefore will have a direct, negative effect on fish population size. But how might flow recommendations to maintain or enhance important fish populations be developed despite the prevailing scarcity of empirical relations between discharge and the populations themselves? To begin, two critical concerns must be addressed. First, an understanding of the important factors limiting the fish population must be developed. If the important factors are related to flow, either directly or indirectly, then the factor vs. flow relation would provide a good basis for developing a flow recommendation. If the important limiting factors are not related to flow, there remains a need to identify flow values required to sustain some level of population because, as discussed earlier, as flow decreases there will be a point reached where further flow reduction will reduce the carrying capacity of the river and thereby reduce the maximum attainable population size. An extreme example of this might be a river flowing at 10 ft3/sec but otherwise having suitable conditions for Colorado squawfish. Such a river will not sustain a population of 100, 10-pound adult squawfish per mile, although this might be an established population goal considered important to recovery. 3