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females (year classes unknown) were selected from this stock to produce the 1990 progeny, which exhibited reduced h and numbers of haplotypes (Table 4), and substantially lower survivorship (11.1°/0) to swim-up than any previous year. Given that specific origins of individuals used to produce the 1990 year class are not identifiable, it is possible that Fes were interbred with close relatives (parents, siblings, etc.), reducing genetic variability and even viability of resulting progeny. Maintaining populations and genetic variability in Lake Mohave Ecology of species' decline.- Razorback sucker populations seem to have "exploded" in newly constructed reservoirs in the lower Colorado River basin; however, recruitment was short-lived. In three instances, strong initial year class(es) failed to successfully reproduce, and populations persisted +40 years, then essentially disappeared (Hinckley, 1983). Thus, if the estimated 55,000 fish in Lake Mohave hatched when Davis Dam was closed in 194 (McCarthy and Hinckley, 1987), all should soon die. Recruitment failure seems prevalent throughout the species' range (Lanigan and Tyus, 1989; Hinckley et al., 1991; USFWS, 1991); extinction therefore appears inevitable. Three hypotheses for recruitment failure have been examined: 1) transport of larvae from Lake Mohave by operations for water delivery and hydropower, 2) insufficient resources, and 3) predation by non-native fishes on eggs and larvae. The first alternative seems unlikely since larval fish could not be collected below Davis Dam (Hinckley et al., 1991). Resource availability may prove to be important, as zooplankton densities sometimes fall below levels required for larval persistence under experimental conditions (Papoulias and Hinckley, 1990, 1992). The third hypothesis also has considerable support (Marsh and Langhorst, 1988; Hinckley et aL, 199I). When stocked along-with predatory fishes, larval razorback suckers 23