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m; 'I - Case Review 257 et al., 1986; Li and Moyle, 1993). Competition is often difficult to docu- ment because freshwater fish lack specialization in food habits and there is generally much overlap between species (Larkin, 1956). Low numbers of zooplankton occur in the main channel and backwaters of rivers (Le., food is limited for early life stages of fish) in the Upper Basin (Table 9.7). This suggests that food for larval razorback suckers is limited in riverine habitats. Razorback sucker larvae that reach a total length of about 25 mm are too large for minnows such as the red shiner to prey on because the mouths of adult red shiners are gape limited (T. Crowl, Utah State University, 1995, personal communication). Two months or more are required by razorback suckers to become large enough to escape predation by the numerous non- native minnows in backwaters of the rivers (Figure 9.3). Because of this, it may be necessary to enhance floodplain depressions in the Upper Basin. Periodic inundation (< 2 weeks) under present regulated streamflows may not be enough for sufficient growth by razorback sucker larvae to escape predation by abundant nonnative fishes. A phenomenon of riverine fishes is the passive downstream drift of larvae after swimup because larvae lack the ability to swim against strong currents (Muth et al., 1998). Drifting larvae would be expected to occupy habitats with low water velocity such as backwaters along the main river channel and off-channel habitats such as floodplain habitats. Predation on razorback sucker larvae by nonnative fishes in backwaters apparently results in extremely high mortality because razorback larvae constitute the largest portion of drifting organisms during the spring. Physical factors (Le., habitat) regulate the carrying capacity of a partic- ular aquatic envuonment (Le., the ability to support aquatic life). Because the amount of physical habitat available is finite, increases in the number of species present in a particular habitat usually result in smaller popu- lations of most species (Allee et aI., 1949). However, native fish species' that are adapted to the physical features of natural aquatic habitats are likely to increase if their habitats are enhanced. Many nonnative fishes are extremely adaptable to altered habitats (Li and Moyle, 1993) and have become prevalent in the Upper Basin to the detriment of several native fishes. However, habitat enhancement may provide some recruit- ment of native fishes even in the presence of well-established nonnative fishes. Year-Class Strength of Endangered Colorado River Fishes 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). Recruitment of all endangered Colorado River fishes was not annual but OCcurred when environmental conditions favored survival of larval and juvenile fishes. Periodic high survival of fish larvae occurred when the quantity (Le., density) and quality (Le., right size) of food organisms were adequate in floodplains during their critical period. This adaptation may have been a life history strategy that evolved in the dynamic natural stream- flow regime in rivers of the Upper Basin. Because the endangered Colorado River fishes are long-lived, adequate survival in one of 5 to 10 years should