The largest population of endangered humpback chub Gila cypha inhabits the lower Little Colorado River (LCR) and the main-stem Colorado River near its confluence with the LCR in Grand Canyon, Arizona. At present, fish in both rivers spawn almost exclusively in the LCR. Flows in the main-stem Colorado River are regulated by Glen Canyon Dam, and water temperature approximates predam winter temperatures year-round. The LCR continues to provide a relatively natural hydrograph and seasonal warming patterns. Length-weight relationships among adult humpback chub from the lower Colorado River basin showed a seasonal pattern of declining condition during spring spawning season followed by recovery of condition during summer through early winter. Fish from the main stem recovered condition more rapidly after reproduction than did fish from the LCR and may have benefited from dam-mediated environmental changes. Grand Canyon Colorado River fish had the greatest weight at length of eight locations sampled in the upper and lower basins. Records since 1978 indicate a decline in condition of lower basin humpback chub coincident with a reported decline in population size in Grand Canyon. We recommend increased monitoring of all populations, including mark-recapture studies to provide population estimates during the spawning season and condition monitoring during October-November to determine more subtle changes in fish health.The humpback chub Gila cypha is one of four endangered big-river fish species endemic to the Colorado River Basin; the other species are the Colorado pikeminnow Ptychocheilus lucius, razorback sucker Xyrauchen texanus, and bonytail

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Flannelmouth sucker, Catostomus latipinnis, a fish endemic to the Colorado River basin in the western United States, appears to experience poor recruitment to adult size in the Colorado River, downstream of Glen Canyon Dam. Lack or impermanence of rearing areas for young-of-year (YOY) fish is hypothesized to be the problem. Knowing the importance of tributary mouths as rearing areas in other river systems, we studied use of the mouth of the Paria River, a tributary of the Colorado River, by YOY flannelmouth suckers, and the availability of rearing area in the mouth at different flow levels in the Colorado River in 1996 and 1997. We also examined the relationship between flash floods in the Paria River and catch-per-unit-effort (CPUE) of YOY in the Paria River between 1991 and 1996.Maximum mean daily discharge in the Paria River was inversely correlated with CPUE of YOY flannelmouth suckers (Spearman Rho = −0.9856, p=0.0003) during their critical rearing period (15 March-30 June). Thus, it appears that YOY flannelmouth suckers rear longer in the Paria River in years when flash flooding is minimal.Recruitment of YOY flannelmouth suckers at the Paria River may also be improved by enhancing pool formation during spring and summer rearing seasons. YOY flannelmouth sucker was captured in a pool created by high Colorado River flows ( ]336 m 3 /s) that inundated the mouth of the Paria River during spring and summer, 1996. In 1997, high flows (about 550-750 m 3 /s) in the Colorado River during winter and spring initially inundated the Paria River and formed a pool in the mouth. However, these high flows eventually caused 0.5-1.0 m of suspended sediment from the incoming Paria River to deposit in the mouth. Thus, despite higher flows than 1996, the slackwater area formed only occasionally in 1997. Differences in pool formation between 1996 and 1997 demonstrate that pool formation cannot be inferred solely from Colorado River flows.

Estimates of the age and growth of roundtail chub Gila robusta were made by analyzing thin cross sections of sagittal otoliths from 280 specimens collected in the upper Verde River, Arizona, from January 1997 to December 1999. Marginal increment analysis indicated that a single annulus completed formation between January and May. The use of otoliths to age roundtail chub was further validated using otoliths of knownage, hatchery-reared fish for which (upon examination) the number of annuli agreed 100% of the time with the true age of the fish. In addition, daily increments were observed on sagittal otoliths of age-0 roundtail chub and, when counted, agreed with the true age of fish 70% of the time; daily ring counts were within 1 and 2 d of the true age 80% and 90% of the time, respectively. Ages assigned to wild-caught fish by three independent readers resulted in a coefficient of variation (100 ϫ SD/ mean) of 8.2. Ages of roundtail chub varied from 1 to 7, the largest fish being a 427-mm total length (TL), 836-g total weight (TW), age-7 female. The largest male was 413 mm TL, 629 g TW, and age 6. The growth of age-1-7 roundtail chub (both sexes combined) was greatest during the first year and was best described by the equation L t ϭ 499.30 [1 Ϫ e Ϫ0.23(tϪ0.147) ], where L t is length at time t. *