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Table VIII. Percentage composition of native and non-native fishes in Walter Walker Wildlife Area Gravel Pit when isolated (I, n=734) from and open (0, n=607) to [he Upper Colorado River. Native Species Percentage Non-Native Percentage Composition Species Composition I 0 I 0 silt-laden river dissipates in the pond. A layer of silt 0.3- 0.6 m deep covers most of the gravel substrate of the pond, except for a narrow band of gravel and cobble along the wind- swept shore. Colorado squawfish and razorback sucker have been caught in relatively large numbers in this pit [5,6], but these numbers have steadily declined [3,7]. Fish composition, particularly the relative abundance of native and non-native species, changed dramatically with river access in 1980 (Table VIII). Over 96% of the fishes in WWWA during isola- tion were non-native. That composition changed dramatically to 58.2% non-native and 41.8% native when river flows in- undated the pit during runoff. Semi-open gravel pits such as WWWA are inh abited by large numbers of non-native fishes that flourish in the pit year around. Many native spe-cies occupy the pit and similar quiet areas during spring runoff, apparently as refuges from the physical rigors of the river or as staging areas for spawning, or both. Kennedy [4] caught gravid female razor- backs in large backwaters of the Lower Colorado River and suggests that spawning may occur in these habitats. Gravel flannelmouth black bullhead 32.3 22.1 sucker 0.5 29.3 common carp 28.5 16.8 roundtail chub - 10.5 green sunfish 25.1 6.4 Colorado channel catfish 6.8 5.6 squawfish 0.8 0.8 white sucker 2.0 1.5 razorback sucker 2.6 0.8 largemouth bass 0.7 0.5 bluehead sucker - 0.2 bluegill 0.4 - flannelmouth x black crappie 0.3 0.3 razorback - 0.2 northern pike - 0.2 Totals: 3.9 41.8 red shiner - 4.5 ,_ sand shiner - 0.3 Total: 96.1 58.2 pits may also serve as staging and possibly spawning areas for Colorado squawfish and razorback sucker; tuber culate squawfish and razorbacks releasing eggs and milt were caught in WWWA and Clifton Ponds in May and June 1980. The large complement of predaceous fishes in these gravel pits pr obably precludes any spawning s uccess by these rare species. Native fishes appear extremely sensitive to water levels in these pits and will leave t hem with descending flow to avoid being stranded in the i solated pits. Razorback sucke r were observed moving between Clifton Ponds and the Colorado River on a nearly daily basis with fluctuating water levels. HABITAT ENHANCEMENT Habitat requirements of the protected native fishes of the Upper Colorado River System are not completely understood. Those that are known differ by species and life stages; habitat features used by one species or life stage are not likely to satisfy the needs of another. For example, small, low-water embayments and backwaters are used by yoy and some small juvenile Colorado squawfish as well as some yoy and juvenile humpback chub. Medium-sized features are used by juveniles of both species at all flow stages. Large 532 5 '~'~ Figure 6. Aerial view of a large abandoned gravel pit at Walter Walker Wildlife Area on the Upper Colorado River, river mile 163.6. Note the narrow gravel berm separating the pit from the river and the single high-water opening.