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However, successful recruitment every five to ten years may be sufficient for a long-lived species such as the razorback sucker that lives to 44 years (McCarthy and Minckley 1987) or possibly longer, once the species is recovered. Sparks (1995) stated that creating an ideal water regime annually may be unnecessary for certain species because the reproductive potential of most warmwater fishes enables them to produce strong year classes. The four endangered Colorado River fishes are known to be long- lived (Henrickson and Brothers 1993; Kaeding and Zimmerman 1983; Miller et al. 1982; McCarthy and Minckley 1987; Vanicek and Kramer 1969). The Colorado squawfish and bonytail were reported to produce strong year classes that were correlated with streamflows (Vanicek and Kramer 1969) . Vanicek and Kramer also reported strong year classes in the roundtail chub (Gila robusta) that is endemic to the Colorado River system but is not federally listed as threatened or endangered. McCarthy and Minckley (1987) concluded that 24 to 44 year-old razorback suckers in Lake Mohave were hatched prior to or coincidental with the construction and filling of the reservoir and that some year-classes were stronger than others. Finally, Kaeding and Zimmerman (1983) provided evidence of differences in numbers of humpback chub juveniles between years. Therefore, recruitment of the endangered Colorado River fishes was not an annual event but occurred when environmental conditions favored survival of larval and juvenile fishes. Endemic fish species may have evolved a life history strategy where strong year-classes sustained stocks in the dynamic, natural streamflow of the upper Colorado River and its tributaries. I I I I I , I I Duration of Floodinq. Flooding depression ponds along Upper Basin rivers should provide an adequate duration of time for razorback sucker to survive to the juvenile or subadult life stage when they would be large enough to escape predation and competition by most nonnative fishes. Razorback suckers should grow to 100 and 200 mm TL (- 4 to 8 in) in one growing season and 300 mm TL (- 12 in) in two growing seasons, based on present information (Figure 3). I I If the duration of inundation in floodplain terrace habitats is short, razorback suckers will not grow to a size where predation and competition by nonnative fishes is no longer a principal factor in their survival. If the duration of flooding these habitats is longer, nonnative fishes may flourish and continue to adversely affect larval and juvenile razorback suckers through predation and competition (Nelson et al. 1995). I I I Contaminants may cause high mortality of larval and Age-O razorback suckers in some depression floodplain ponds. Bioaccumulation through the food chain in floodplain habitats may increase mortality of Age-O razorback suckers (Hamilton 1995; Hamilton et al. 1996) and elevated levels in milt and eggs may inhibit successful reproduction in adult razorback suckers (Hamilton and Waddell 1994). C. Control of Nonnative Fishes. In Section IV, predation on and competition with endangered fishes by nonnative fishes were identified as important problems (Tyus and Saunders 1996) that must be addressed concurrently with other Recovery Program actions. Nonnative fishes in floodplain habitats and backwaters along the main channel will also benefit from increased production from enhanced/restored floodplain habitats (Nelson et al. 1995). Recovery of the endangered fishes is dependent on sufficient survival of larvae and juveniles that will provide adequate recruitment to develop and maintain self-sustaining populations. Therefore, adverse nonnative fish interactions may limit the recovery of the razorback sucker unless adequate control measures can be implemented in immediate upstream reaches, and particularly in downstream reaches of the main channels, where advanced larvae or early juvenile razorbacks occupy backwater habitats upon leaving floodplain habitats. I 34 I J I I I