Effects of pool formation and flash flooding on relative abundance of young-of-year flannelmouth suckers in the Paria River, Arizona

Flannelmouth sucker, Catostomus latipinnis, a fish endemic to the Colorado River basin in the western United States, appears to experience poor recrui tment to adult size in the Colorado River, downstream of Glen Canyon Dam. L ack or impermanence of rearing areas for young-of-year (YOY) fish is hypoth esized to be the problem. Knowing the importance of tributary mouths as rea ring 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 betw een 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 wi th CPUE of YOY flannelmouth suckers (Spearman Rho = -0.9856, p = 0.0003) du ring their critical rearing period (15 March-30 June). Thus, it appears tha t YOY flannelmouth suckers rear longer in the Paria River in years when fla sh flooding is minimal. Recruitment of YOY flannelmouth suckers at the Paria River may also be impr oved by enhancing pool formation during spring and summer rearing seasons. YOY flannelmouth sucker was captured in a pool created by high Colorado Riv er flows (greater than or equal to 336m(3)/s) that inundated the mouth of t he Paria River during spring and summer, 1996. In 1997, high flows (about 5 50-750 m(3)/s) in the Colorado River during winter and spring initially inu ndated 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 P aria River to deposit in the mouth. Thus, despite higher flows than 1996, t he slackwater area formed only occasionally in 1997. Differences in pool fo rmation between 1996 and 1997 demonstrate that pool formation cannot be inf erred solely from Colorado River flows. Copyright (C) 2001 John Wiley & Son s, Ltd.