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86 CALIFORNIA FISH AND GAME lifted fishes. Only one redear sunfish took a juvenile shad, but black crappie ale them frequently, along with red shiner. Striped bass ate threadfin shad, rainbo~~~ trout, crayfish, centrarchids, carp, tiger salamander, Ambystoma tgrinum, and unidentified animal material (Edwards 1974). The salamander is a common bait animal along the Colorado River (Minckley 19716); and a single specimen alsn was eaten by a channel catfish (Table 1 ). Percentages of empty stomachs in fishes of the lower Colorado River were related to principal food habits. No empty stomachs were found among de- tritivores and facultative planktivores. Species that depended upon a broader food base, including significant frequencies of benthic invertebrates, also had relatively low percentages of empty stomachs. Large piscivores tended to have a high incidence of empty stomachs (e.g. flathead catfish, 39.1%). Edwards (i 974) reported 45% of 100 stomachs of adult striped bass as empty. Small- mouth bass, ~varmouth, and green sunfish also had an incidence of empty stomachs that exceeded 20%, but largemouth bass displayed a wider food base and all but 11.6°% contained foods. SPATIAL VARIATION Trophic structure within the fish community of the lower Colorado River differs substantially in different reaches of stream. Near Davis Dam, waters of the upstream reservoir (Lake Mohave) provide a major proportion of basic foodstuffs. Substantial amounts of plankton and detritus pass through the dam and this is reflected in a large proportion of filter-feeding invertebrates (e.g. simuliids) in stomachs of fishes from that reach. Threadfin shad also drawn through the dam form a major portion of the food supply for striped bass (Edwards 1974), and likely for larger rainbow trout. Cold water resulting from ~. hypolimnic releases from Davis Dam exclude many temperate (and obviously tropical) species of fishes from that area. Primary production is relatively high, since aquatic vegetation is abundant (no quantitative data available), but essen- '' tially no fishes are present to utilize that level in the food web. Downstream in Topock Gorge, deep, swift areas continue to be influenced by hypolimnic water from Lake Mohave. A fe~v major backwaters provide habitat for temperate fishes. It seems likely that this reach is relatively devoid of foods h for piscivores, since red shiners were rare and threadfin shad almost non-exisrem (Minckley 1979). - Lake Havasu is a relatively stable, mainstream reservoir that differs grearl~ ~~". from the remainder of the reach under consideration. Few stomachs of fiches ~~ were examined from the lake (none of those presented in Table 1), but focd ~~°~ relations seemed similar to those described by Rinne, Minckley, and Berseil ~• (1981) from reservoirs in central Arizona. Fishes were distributed relative to ~~~t their food supplies: planktivorous threadfin shad were most abundant near nutri- i,~. ent inputs, thus near phyto- and zooplankton concentrations, and piscivorous .T largemouth bass tended to be near threadfin shad. Benthic predators were more generally distributed within the reservoirs, in keeping with a more general distri- bution of benthic invertebrates. Zooplankton was not studied in Lake Havasu, but Chlorophyll a concentrations were highest near nutrient inputs at the upper- most end of the lake and in the Bill Williams River arm (Portz 1973, Minckley 1979). Benthic invertebrates in Lake Havasu were also similar in diversity and general abundance to those elsev<~liere in low desert impoundments (Rinne e1 al. 1.981), with fev~ ley 1979, Cowell sediments (Schulf; invel-tebrates werF populations in the sunfish, at freque qualitative examir Epilimnetic pen stream, thus enha Havasu. Particuia'. benthic animals, Macrophytes, ber diet of all but ce derived in part frc and allochthonou frequencies, espe consisting mostly diet of all specie were broadly rep eaten by speciali parts of their diet lion of smallmor: fishes were impc and flathead calf food web of the fishes most frequ ley 1979). Othe part secondary + In the lowerrr molly, striped from upstream, part from dirrini these fishes to n temperature ups mouthbrooders, mullet, undoubl Although the habits of introd~ those of tie sa 1966). Many al insects and olig high-density, it new fish fauna populations m< pear. The food wE fishes in othe