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276 FISH CULTURE IN FISHERIES MANAGEMENT station. The former was brought back in 1984 because of its limited natural occurrence. The roundtail chub was brought back in 1985 to enable further hybridization studies with other members of this complex genus including Sonora, Gila, Rio Grande, Yaqui, and (:hihuahua chubs. Culturing Most efforts in developing culture techniques have been concentrated on four large Colorado River fishes: razorback sucker, Colorado squaw- fish, bony tail, and humpback chub. Marsh (1985) conducted studies at Dexter NFH to determine the influence of temperature on egg development, hatching success, and fry development of these big Colorado River fishes. He found the optimum temperature for egg incubation and fry develop- ment to be near 70 F and well water is heated for this purpose. Razorback suckers continue to decrease in the Colorado River system. There has been no doc- umented recruitment to the aging Lake Mohave razorback sucker populations since the mid-1950s (Minckley 1983). Based on otoliths, Michael McCarthy (pers. comm.), Arizona State Univer- sity, found that razorback suckers collected from Lake Mohave, Arizona-Nevada, in 1980-8], ranged from 24 to 43 years of age (1937 to 1957 year- classes). Only recently has sexual maturation of razor- back suckers been reported in the literature. Minckley (1983) reported that razorback suckers reared at Willow Beach NFH attained sexual maturity in six years, when the fish were 14 to 16 inches total length. Two-year-old male razorbacks (15.6 inches, 1.6 pounds) reared at Dexter were sexually mature in the spring of 1983; microscopic examination. of the milt revealed motile, thus presumably viable, spermatozoa. Ovaries from two-year-old female razorbacks (16.2 inches, 1.9 pounds) contained developing eggs. Three-year- old females spawned successfully in 1984 (Ham- man 1985). Sexual maturation ofrazorback sucker is more closely related to size than age. Female razorback suckers held at Dexter NFH ' became gravid and spawned when the water tem- perature reached 55 F. Females were injected with hormone to stimulate final maturation and ovu- lation of eggs. Inslee (1982) determined that 100 International Units (IU) of chorionic gonadotro- pin (CG) per pound of body weight injected intramuscularly was the preferred hormone, dos- age, and site of injection for female razorback suckers. Male razorback suckers received intra- muscular CG injections at 300 IV/pound to stim. ulate and/or maintain gamete production. ' Utilizing the wet method, eggs are hand-stripped into pans, milt added from two or more males, and the eggs stirred gently with a feather to ensure adequate mixing of sexual products. Razorback sucker eggs are very adhesive so they are "clayed'! with slurried bentonite following fertilization; The c1ayed eggs are poured into floating egg baskets, gently washed to remove the bentonite, allowed to water-harden, enumerated gravimetrically, and placed in Heath trays and in jars. Water-flow through the incubators is regulated at three gpm. At 70 F, razorback sucker eggs begin hatching at about 96 hours 'and continue through 144 hours; peak hatching occurs at about 120 hours. Fry are held in tanks until swim-up, then stocked in rearing ponds at rates ranging from 100,000 to 150,000/ surface acre. Artificial spawning of female razorback suckers in 1984 included 70 three-year-old domestic brood fish (Hamman 1985) and 25 wild-caught brood fish. The 70 three-year-old domestic brood av- eraged 2.8 pounds and produced 22,660 eggs per pound of body weight. Mean fecundity was 63,674 and egg viability 61.1 %. The 25 wild fish averaged 7.76 pounds and produced 15,787 eggs per pound of body weight. Mean fecundity was 123,110 and egg viability 78.8%. Commercial trout feed is fed to all production species; feed conversion .f9[ fingerling razorback suckers was 2.30 in 1984.; As with the razorback sucker, the numbers of Colorado squawfish have been reduced drastically throughout their historic range in the Colorado River system. They are extin~t ip th~ lower basin, and only remnant populations survive in the upper basin (Tyus et aI1982). North Ameri~'s largest cyprinid, Colorado squawfish had lor~erly at- tained lengths up to six feet and weights of nearly 100 pounds (Miller 1961). All available data sug- gest that squawfish are slow growing and long lived. Nine-year-old hatchery-reared squawfish at Dexter averaged only 20 inches and three pounds' in 1983. Considering that razorback suckers live over 40 years, it is easy to speculate that a six- foot, 100-pound Colorado squawfish could be more than a half-century old. Hatchery-reared Colorado. squawfish became sexually mature .in their sixth year (Hamman 1981b). " .., /: ;' 1 Colorado squawfish housed at Dexter, NFH spawn in late Mayor early June as the water temperature nears 70 F. Although some females ripen slightly ahead of others, all must be spawned within a one- to two-week period