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INTRODUCED FISHES OF THE LOWER COLORADO 87
<br />;had, but black crappie ate a1.1981), with fewer animals and biomass where currents were present (Minck-
<br />ite threadfin shad, rainbow ley 1979, Cowell and Hudson 1967), presumably in response to changes in
<br />Ambystoma 49 inum, and sediments (Schulback and Sandholm 1962). Shoreline populations of benthic
<br />'amander is a common bait invertebrates were locally dominated by Asiatic clams, reflecting high plankton
<br />'Ind a single specimen also, populations in the lake. These were eaten by carp, channel catfish, and redear
<br /> sunfish, at frequencies comparable to those in the river channel (based on
<br />)wer Colorado River were qualitative examination of stomachs).
<br />')s were found among de- Epilimnetic penstock intakes in Parker Dam allow warm water to flow down-
<br />depended upon a broader stream, thus enhancing habitat for warmwater fishes in the river below Lake
<br />'?ic invertebrates, also had Havasu. Particulate materials, including plankton, in turn enhance filter-feeding
<br />piscivores tended to have benthic animals, as do hard bottoms and an abundant micro- and macroflora.
<br />catfish, 39.10/6). Edwards Macrophytes, benthic algae, and phytoplankton made up a significant part of the
<br />!ied bass as empty. Small- diet of all but centrarachids near Parker, Arizona, with phytoplankton being
<br />.1 an incidence of empty derived in part from the pseudofeces of Asiatic clams. Detritus, both from auto-
<br />r .played a wider food base and allochthonous sources, also was present in stomachs of many species at high
<br /> frequencies, especially threadfin shad, carp, and channel catfish. Benthic insects,
<br /> consisting mostly of chironomid dipteran larvae, comprised major parts of the
<br /> diet of all species present. Other invertebrates, excepting clams and crayfish,
<br />the lower Colorado River were broadly represented, but far less significant than chironomids. Clams were
<br />,ear Davis Dam, waters of eaten by specialists (carp, channel catfish, and redear sunfish), forming major
<br />major proportion of basic parts of their diets. Crayfish were generally taken by piscivores, with the excep-
<br />itus pass. through the dam tion of smallmouth bass, who appeared to feed selectively upon them. Other
<br />eding invertebrates (e.g. fishes were important in the diets of six species, and especially so for channel
<br />hreadfin shad also drawn and flathead catfishes, largemouth bass, and black crappie. Fishes lowest in the
<br />d supply for striped bass food web of the system, threadfin shad and red shiner, were eaten by other
<br />Cold water resulting from fishes most frequently, and were the most abundant species in the river (Minck-
<br />temperate (and obviously, ley 1979). Other prey species were.mostly juvenile centrarchids, for the most
<br />eduction is relatively high, part secondary consumers in the system.
<br />data available), but essen- In the lowermost reaches, detritivory became a major mode of life for sailfin
<br />food web. molly, striped mullet, and mouthbrooder. Accumulation of organic materials
<br />,retinue to be influenced by from upstream, resulting from high rates of production, lack of flooding, and in
<br />ickwaters provide habitat part from diminution in discharge as a function of progressive water use, allows
<br />relatively devoid of foods these fishes to maintain and expand their populations. However, constraints of
<br />n shad almost non-existent temperature upstream (too low in winter or near Davis Dam) for the molly and
<br /> mouthbrooders, and distance from the sea plus intervening barriers for the
<br />-rvoir that differs greatly mullet, undoubtedly limit their over-all distribution more than food.
<br />Few stomachs fishes
<br />
<br />?'ed
<br />in Table 1), but food DISCUSSION AND CONCLUSIONS
<br />.e, Minckley, and Bersell Although the Colorado River provides a relatively low food diversity, food
<br />re distributed relative to habits of introduced fishes of the Colorado River do not differ substantially from
<br />post abundant near nutri- those of the same species within their native ranges (see reviews in Calhoun
<br />-ations, and piscivorous 1966). Many abundant forage species are introduced (probably not so for most
<br />'iic predators were more insects and oligochaetes) and are characterized by high reproductive rates and
<br />ith a more general distri- high-density, monospecific populations. These features are also evident in the
<br />t studied in Lake Havasu, new fish fauna, with many species now populating transitory habitats where
<br />:trient inputs at the upper- populations may explode, stunt, then eventually stabilize at low levels or disap-
<br />m (Portz 1973, Minckley . + pear.
<br />vo similar in diversity and The food web of the Colorado River is based upon autochthonous materials.
<br />impoundments (Rinne et Fishes in other large rivers often depend upon allochthonous inputs. In the
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