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110Q) bo) I r 11 r t c_I?\l 11-Y 7(0- 13 I
<br /> 78 CALIFORNIA FISH AND GAME
<br /> Calif fish and Came 68(2): 78 89 1982 of unusually high rr i s
<br />
<br />TROPHIC INTERRELATIONS AMONG INTRODUCED 1 impounded and sor,
<br />seldom exceeds 1
<br /> FISHES IN THE LOWER COLORADO RIVER than 40°C, and wing
<br /> ,
<br />SOUTHWESTERN UNITED STATES' few hours.
<br /> W. L. MINCKLEY j I
<br />
<br />Department of Zoology
<br />A
<br />i I
<br />i N v I -
<br /> r
<br />zona Slate University , j UT
<br /> Tempe, AZ 85281 t
<br /> Analysis of 1,050 stomachs of 18 species of fishes from the lower Colorado River ?- - -
<br /> mainstream indicates a relatively simplified food web based on autochthonous detri- f C A \
<br /> tal materials, algae, and macrophytes. Aquatic insects, clams, and crayfish eaten by
<br /> larger fishes fed directly on detritus or particles filtered from the water. Threadfin
<br />shad and red shiner, both depending principally on detritus as food, were major AZ
<br /> forage for piscivores.
<br /> 1
<br /> INTRODUCTION
<br /> The lower Colorado River has one of the most highly modified channels in
<br /> western North America. The most char teristic feature of aquatic habitats in
<br /> arid zones, extreme variability in time arrclrgpace, has been suppressed, and this f
<br /> notoriously fluctuating, formerly turbid stream now flows under essentially com-
<br /> plete control by mainstream and tributary impoundments
<br />Its native fishes
<br />hi
<br />hl
<br /> .
<br />,
<br />g
<br />y
<br />endemic but comprising only about nine species, are extirpated from the main-
<br />CAL.. I F0
<br /> stream, or are rare. A new fauna, consisting of exotic species, is becoming
<br /> established. So far, 44 non-native taxa have been recorded from the reach
<br />
<br />between Davis Dam and the U. S. and Mexican Boundary, 20 of which are I
<br /> locally or regionally abundant (Minckley 1979,"Nicola 1979). Most published
<br /> data on fishes of the river consist of faunal listings (Evermann 1931, Miller 1961
<br /> ,
<br />Miller and Lowe 1967, Bradley and Deacon 1967), general discussions of the
<br />+
<br /> fisheries (Moffett 1942, Dill 1944, Jonez et al. 1951, Wallis 1951, Kimsey 1958)
<br /> ,
<br />keys for identification (Miller 1952, Winn and Miller 1954, Minckley 1971 a), and
<br /> numerous shorter works dealing with species introductions
<br />and observations on
<br />
<br />t ,
<br />distribution and ecology (reviewed by Minckley 1973 and Moyle 1976)
<br />Part of
<br /> .
<br />an ecological survey of the mainstream lower Colorado River, conducted from ' La l
<br /> 1974 through 1976, was to identify general food relations within the fish fauna -- - - --- - - -
<br /> .
<br />Food habits of 18 species of fishes were studied to obtain an outline of trophic
<br />'. structure for the system. Edwards (1974) reported on foods of striped bass Mora I
<br /> ,
<br />Moron saxati/is and his data are also summarized herein.
<br /> DESCRIPTION OF THE AREA r`
<br /> The study area was delimited upstream by Davis Dam and below by the -
<br /> International Boundary. The Colorado River forms the border between Nevada FIGURE 1. Map o' the
<br /> and Arizona to the north, California and Arizona through much of the study name, mer
<br />j reach, and Baja California Norte, Mexico, and Arizona, at the southern extreme f Terrain along the
<br /> (Figure 1). The International Boundary lies about 120 km upstream from the Gulf "roded through mo,w
<br /> of California. Historically, the only perennial tributaries entering this 453-km are stony along moor,,
<br /> reach are the Gila and Bill Williams rivers. The former is now maintained below 1 of fine sands and sil,
<br /> dams by return flow from irrigation and domestic wastewaters except in periods i The sparse natural ,
<br /> 'Accepted for publication January 1981. (lower Colorado Ri,,
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