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in more natural habitats. Very little information exists concerning growth rates of young razorback suckers in natural environments because survival rates are negligible (Minckley et al. 1991, Marsh 1995, Mueller 1995). Growth rates of razorback suckers that have been spawned and reared under hatchery conditions can be rapid. Newly-hatched larvae are generally 7-9 mm TL (e.g., Hamman 1985, Papoulias and Minckley 1990, Tyus and Severson 1990). Larvae fed brine shrimp reached an average length of 23.2 mm after 50 days (Papoulias and Minckley 1990). Larvae (i.e., swim-up fry) produced from ripe females in the Green River and reared on natural foods in ponds at Ouray, Utah, grew to an average total length of 127 mm (range: 49-205 mm) during the 1987 growing season (i.e. April to October) and 156 mm (range: 40-271 mm) in 1988 (unpublished data on file with the USFWS, Vernal, Utah). In Lake Mojave, growth of razorback sucker larvae has been studied in isolated backwaters from which other fishes have been removed. In one study, fish grew as much as 35 cm between January and November, 1992 (N=296; Mueller 1995). In other studies, larvae hatched in late March attained average length of 18.9 cm by December (N=12; Minckley et al 1991). Marsh and Langhorst (1988) evaluated the feeding and fate of wild razorback sucker larvae in Lake Mojave and also found that fish placed in a backwater free of predaceous fishes survived and grew rapidly. Rapid growth also has been documented for juveniles that have been stocked in ponds and streams. Osmundson and Kaeding (1989) reported growth from 55 mm to 307 mm TL in 6 months for fish stocked in a small pond near Clifton, Colorado. Two years and seven months after this stocking, the survivors had a mean length of 422 mm. Juveniles of 40 mm TL that were stocked in two stream locations, where they grew an average of 43.4 and 47.5 mm in two months (Brooks 1986). Growth of fish stocked in two isolated backwaters on Bonita Creek Arizona, averaged 6 and 36 mm respectively, in three months (Brooks 1986). Genetic Diversity With any rare or endangered species, there is concern about loss of genetic diversity in small or isolated populations. A reduction of genetic diversity is a concern for the razorback sucker not only for extant populations, but also for hatchery stocks as well. Reduced genetic diversity and adaptation to captivity could impair prospects for successful reintroduction of the species. In a recent study of genetic diversity in hatchery and Lake Mojave stocks, Dowling et al. (1996) found the large stock in Lake Mojave had a high degree of mitochondria) (mtDNA) diversity (0.97) with all known genotypes of the fish are represented there. This suggests that the Lake Mojave stocks are descended from a large panmictic population. An earlier, preliminary study (Dowling and Minckley 1993) also examined haplotypes, finding evidence of some genetic isolation between Lake Mojave stocks 15