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general, a good approximate of NJN is 0.5 for a variety of mating systems. However, he also notes there is little reliable demographic'data necessary for a sound estimate of Ne/N and several studies have suggested that typical values of Ne N may be lower than 0.05 or less. Population Demographics 1 1 1 1 1 1 1 1 Empirical Information Colorado Nuaw fish. Colorado squawfish have been the most extensively studied endemic fish in the UCRB (USFWS 1991; Tyus 1991). Current studies suggest that three sub-populations are naturally reproducing in the Upper Colorado River basin (Table 4): the middle Green River population which includes the Yampa River to Desolation Canyon of the Green River, the lower Green River which includes Desolation Canyon to the confluence of the Green and Colorado rivers; and the Colorado River from its confluence with the Green River upstream to Palisades, Colorado. Some exchange of individuals occurs between these areas, and researchers speculate the population structure may reflect metapopulation or source/sink population dynamics (Lentsch et al. 1997). Population demographics have been described for several life stages. Most studies suggest an unequal sex ratio during spawning with males outnumbering females at about 6:1 (Vanicek 1967; Seethaler 1978; Tyus 1990; Hawkins 1991). Fecundity has been reported for spawning induced wild individuals in a hatchery at approximately 10,000 eggs/kg/female (Hamman 1981). Larval fish passively drift downstream from spawning areas and are deposited in alluvial reaches as much as 200 km downstream. Survivorship during this period is thought to be low and generally limiting to recruitment. Valdez and Cowdell (1996) report overwinter survivorship at approximately 40%. When including the first full year, survivorship probably decreases and may be closer to 20%. Very little is known about juvenile life-stages, because fish of this size are more rare and more difficult to capture with conventional sampling techniques. No information exists to document survivorship of juvenile life-stages. Most researchers assume that survivorship increases with size and the main recruitment bottleneck occurs in the first year (Kaeding and Osmundson 1988; Thompson et al. 1991; Valdez and Cowdell 1996; Bestgen 1997; Converse and Lentsch 1997). Survivorship estimates of adults consistently have been reported at about 75-86% (Gilpin 1993; Osmundson and Burnham 1996). Several researchers have made population estimates. Tyus (1991) suggested that data shows densities are an order of magnitude higher in the Green River system than in the upper Colorado River. He reported estimates from several methods and calculated an average of approximately 8,000 mature individuals for the mainstem Green River. However, he cautions about potential inaccuracy of this estimate because of assumptions of each method. Osmundson and Burnham (1997) used standardized monitoring and additional mark/recapture data. They suggest approximately 250 mature individuals occupy the upper reach of the upper Colorado River and 350 juvenile and mature individuals occupy the lower reach for a total of about 600. Crowl and Bouwes (1998) report estimates of 1400 for the middle Green River and 1000 for the lower Green Table 4. Summary of empirical demographic information for the endangered Colorado River fish 9 F1