8123 - Laserfiche WebLink

Home Browse Search

t high production of humpback chub juveniles with a reduction in nonnative fishes (R. Valdez, 1994, Personal Communication). Therefore, recruitment of all endangered Colorado River fishes was not an annual event but occurred when environmental conditions favored survival of larval and juvenile fishes. This adaptation may have been a life history strategy that evolved because of the natural streamflow regime that produced a hydrograph with high peaks during runoff with wide annual fluctuations. A phenomenon of riverine fishes is the passive downstream drift of larvae after swimup. This phenomenon has been documented for Colorado squawfish (Haynes et al. 1984; Nesler et al. 1988; Tyus and Haines 1991). Also, razorback suckers were reported to drift passively from spawning sites (Maddux et al. 1993; Marsh and Minckley 1989; Minckley et al. 1991). In addition, young humpback chubs in the Little Colorado River occupied habitats with slow current (Valdez 1989). It is logical to assume that the larvae of all four endangered Colorado River fishes in riverine environments drift downstream after swimup. Drifting larvae would be expected to occupy areas of low water velocity such as eddies behind rock protuberances in canyons, backwaters along the river, and in off-channel habitats such as flooded bottomlands. During years of high flow, backwaters were formed and bottomlands along the river system were flooded. It is possible that the flooded bottomlands may have been the primary nursery areas for some, if not all, of the endangered fishes. The strength of year classes in riverine fishes is known to be strongly correlated with the area flooded during wet years (Welcomme 1989). In the Lower Colorado River Basin, studies have demonstrated that predation by non-native fishes and the lack of food may be limiting the survival of larval and juvenile endangered fishes (Marsh and Langhorst 1988; Papoulias and Minckley 1990, Papoulias and Minckley 1992). The riverine environment of the Upper Colorado River Basin is not productive for zooplankton(Cooper and Severn 1994 a, b, c, and d; Grabowski and Hiebert 1989). Mabey (1993) demonstrated major differences in food organisms in the upper basin waters: the main channel of the Green River produced few zooplankton and benthic organisms, backwaters were more productive, and flooded bottomlands were most productive. The reason for high productivity in flooded bottomlands is that sediments settle out as the hydraulic gradient declines. This phenomenon adds nutrients to the bottomlands. In addition, sunlight can penetrate the clearer water to allow phytoplankton to flourish as primary producers and to stimulate production of the food chain. Secondary producers, such as zooplankton and benthic invertebrates, become abundant in flooded bottomlands and provide the necessary food resources for good survival and growth of larval and juvenile fishes. The shallow, quiet waters in flooded bottomlands also become much warmer than backwaters or the main channel. Warm water temperatures are needed for good growth of the warmwater endangered fishes (Kaeding and Osmundson 1988; Osmundson and Kaeding 1989). Conceptual management plans to enhance habitats in former flooded bottomlands in the Upper Colorado River Basin are scheduled to be completed by May, 1994. Proposed habitat improvement projects, contained in these plans, may result in sufficient recruitment to produce self-sustaining populations of some of the endangered fishes. 1 11