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19951 DIETS OF FISHES IN BACKWATERS 97
<br /><3% were empty, mostly from fish <- 13 mm
<br />TL. After subsets with <6 specimens contain-
<br />ing food were eliminated from the data set,
<br />2297 specimens representing nine species
<br />remained for diet analyses. Native fish includ-
<br />ed 972 Colorado squawfish (7.5-73.0 mm TL,
<br />mean = 19.1), 35 speckled dace (Rhinichthys
<br />osculus; 23.1-39.8 mm TL, mean = 28.1), 42
<br />bluehead sucker (Catostomus discobolus;
<br />23.0-58.9 mm TL, mean = 35.9), and 21 flan-
<br />nelmouth sucker (C. latipinnis; 32.0-64.3 mm
<br />TL, mean = 47.9). Nonnative fish included
<br />729 red shiner (11.3-74.5 mm TL, mean =
<br />29.1), 92 sand shiner (Notropis stramineus;
<br />22.2-53.2 mm TL, mean = 31.0), 330 fathead
<br />minnow (Pimephales promelas; 11.0-65.9 mm
<br />TL, mean = 32.5), 58 channel catfish
<br />(Ictalurus punctatus; 22.5-70.0 mm TL, mean
<br />= 42.9), and 18 green sunfish (Lepomis
<br />cyanellus; 20.7-56.8 mm TL, mean = 39.6).
<br />Characterization of Diets
<br />No major or consistent seasonal differences
<br />in diet measures were observed within species
<br />for fish of similar size. Accordingly, summer
<br />and autumn data were combined for species
<br />and lengths by river reach. Trends in values of
<br />proportional importance of each food category
<br />were similar between the two diet measures
<br />for all fishes; therefore, only means of volume
<br />percentages are reported.
<br />Diets consisted mostly of insects, zooplank-
<br />ton, algae, seeds, and organic and inorganic
<br />debris; but relative importance of these food
<br />categories varied among fishes or subsets
<br />within species (Table 1). Based on total num-
<br />ber of food categories included in the diet of
<br />each fish species, diets of Colorado squawfish
<br />and red shiner were the most varied (18 and
<br />17 food categories, respectively), followed by
<br />speckled dace (15), fathead minnow, channel
<br />catfish, and green sunfish (12 each), sand shin-
<br />er (11), flannelmouth sucker (9), and bluehead
<br />sucker (6). Variety of food consumed was
<br />greater in the lower than upper reach for red
<br />shiner, Colorado squawfish, flannelmouth
<br />sucker, channel catfish, and green sunfish,
<br />whereas diets of sand shiner, fathead minnow,
<br />and speckled dace were more varied in the
<br />upper reach (diet of bluehead sucker was ana-
<br />lyzed for fish from the upper reach only). Diet
<br />variety relative to fish length was greatest in
<br />red shiner, sand shiner, fathead minnow,
<br />Colorado squawfish, speckled dace, and blue-
<br />head sucker 21-30 or 31-40 mm TL and in
<br />flannelmouth sucker, channel catfish, and
<br />green sunfish >40 mm TL. Mean percent full-
<br />ness of digestive tracts was highest in fish
<br />21-30 or 31-40 mm TL for all species.
<br />Aquatic insects were a principal part of
<br />diets for all fishes except fathead minnow and
<br />bluehead sucker. Of identifiable insects,
<br />immature dipterans (especially larval chirono-
<br />mids) were predominant in digestive tracts.
<br />Larval chironomids were represented by at least
<br />21 genera, the most common being Chironomus
<br />followed by Rheotanytarsus, Eukiefferiella, Poly-
<br />pedilum, Tanytarsus, Cricotopus, and Microp-
<br />sectra. Representative families of other imma-
<br />ture dipterans were (in order of importance)
<br />Ceratopogonidae, Simuliidae, Dolichopodidae,
<br />Empididae, Muscidae, and Tipulidae. Propor-
<br />tional contribution of immature dipterans to
<br />diets of red shiner, sand shiner, speckled dace,
<br />and flannelmouth sucker was higher in the
<br />lower than upper reach. Relative importance
<br />of immature dipterans in diets of red shiner,
<br />sand shiner, and speckled dace decreased and
<br />utilization of other insects increased as fish
<br />length increased. Conversely, relative impor-
<br />tance of immature dipterans in diets of
<br />Colorado squawfish and channel catfish
<br />increased or remained high with increasing
<br />fish length. Corixids, larval and adult aquatic
<br />coleopterans (predominantly Dytiscidae, Elmi-
<br />dae, Haliplidae, and Hydrophilidae), trichopter-
<br />an larvae (mainly Hydropsychidae and Hydrop-
<br />tilidae), and ephemeropteran nymphs (pre-
<br />dominantly Baetidae and Heptageniidae) were
<br />minor components of diets for all fishes (<10%
<br />of food volume) except larger red shiner,
<br />speckled dace, and green sunfish.
<br />Red shiner and sand shiner ate more semi-
<br />aquatic or terrestrial insects than other fishes.
<br />Semiaquatic insects consumed were primarily
<br />larval and adult coleopterans (predominantly
<br />Heterocercidae and Staphylinidae) and adult
<br />hymenopterans (Scelionidae). Terrestrial
<br />insects consumed were primarily hemipterans
<br />and formicids.
<br />All fishes ate zooplankton, but it was partic-
<br />ularly important in diets of Colorado squaw-
<br />fish <31 mm TL (especially <21 mm TL),
<br />green sunfish <31 mm TL, and, to a lesser
<br />extent, red shiner and channel catfish <31 mm
<br />TL and flannelmouth sucker. Cladocerans (many
<br />identified as Daphnia, Eurycercus, and Macro-
<br />thrix) and especially cyclopoid copepods
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