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<br />U's withdrawn from greater depths,
<br />contains higher concentrations of
<br />these nutrients.
<br />Releases of cold, clear water and
<br />reduced transport of organic mate-
<br />rial from the upper Colorado River
<br />basin have dramatically changed
<br />conditions for the aquatic macro-
<br />invertebrates downstream from the
<br />dam (Stevens et a!. 1997b). Thus, the
<br />food supply available for native fish
<br />in the Grand Canyon has changed
<br />greatly. Observations made in the
<br />less regulated upper Colorado River
<br />basin indicate that high densities of
<br />aquatic insects currently exist on
<br />gravel bars and that native fish feed
<br />on a variety of terrestrial and aquatic
<br />invertebrates (Dill 1944, Vanicek
<br />1967, Tyus and Minckley 1988). Pre-
<br />dam river runners in the Grand Can-
<br />yon described large accumulations
<br />of woody debris in eddies, and the
<br />decomposition of this wood prob-
<br />ably supported a suite of aquatic and
<br />terrestrial invertebrates.
<br />The Colorado River benthos in
<br />the Grand Canyon is productive but
<br />depauperare in species. Chironomids,
<br />simuliids, oligochaetes, and an in-
<br />troduced amphipod, Gammarus
<br />lacustris, are the most common
<br />[macroinvertebrates. More than 65%
<br />of the aquatic plant and invertebrate
<br />standing biomass of the entire Grand
<br />Canyon is produced upstream from
<br />Lees Ferry (Stevens et a!. 1997b).
<br />The occurrence of terrestrial macro-
<br />invertebrates in adult humpback
<br />chub stomachs increases downstream
<br />because of resupply from unregu-
<br />lated tributaries. Since construction
<br />of the dam, decomposition of woody
<br />debris has a minor effect on the in-
<br />vertebrate communities because rela-
<br />tively little of this matter is stored on
<br />the channel banks and none is resup-
<br />plied from upstream.
<br />
<br />Fish assemblage. The fish assemblage
<br />of the Colorado River through the
<br />Grand Canyon has altered dramati-
<br />cally during the past century, but
<br />this change is related only partly to
<br />dam-caused variations in discharge,
<br />sediment transport, temperature,
<br />nutrients, and food base. Before the
<br />late 1800s, 74% of the 35 fish spe-
<br />cies native to the entire Colorado
<br />River basin were endemic, the high-
<br />est percentage among North Ameri-
<br />can river basins (Miller 1959). Eight
<br />
<br />740
<br />
<br />of these species once lived in rhe
<br />Grand Canyon (Miller 1959). The
<br />eight native warm-water fishes dif-
<br />fered little in their temperature pref-
<br />erences but had different feeding
<br />strategies and spatial fidelities (Fig-
<br />ure 4).
<br />Three of the native fish species,
<br />Colorado squawfish (Ptychocheilus
<br />lucius), bony tail (Gila elegans), and
<br />roundtail chub (Gi/a robusta), were
<br />extirpated from the Grand Canyon
<br />by the 1970s (Minckley 1991). Ra-
<br />zorback suckers are currently rare in
<br />the Grand Canyon; only ten speci-
<br />mens were reported between 1944
<br />and 1990 (Valdez and Ryel 1997).
<br />By contrast, the humpback chub
<br />population in the Grand Canyon is
<br />the largest of six extant populations
<br />in the Colorado River basin.
<br />Cold water releases impede re-
<br />production of native fish. Native
<br />speckled dace (Rhinichthys osculus),
<br />blue head sucker (Catostomus disco-
<br />bolus), and flannelmouth sucker
<br />(Catostomus /atipinnis) continue to
<br />reproduce in several tributaries in
<br />the Grand Canyon, but there is very
<br />little reproduction by any of the na-
<br />tive species in the mainstem. For-
<br />successful spawning, these fish need
<br />a minimum temperature of about 16
<br />oC, and in the Colorado River these
<br />temperarures occur only immediately
<br />upstream from Lake Mead for a short
<br />time in the summer.
<br />At the same time that reproduc-
<br />tion of native fish has been reduced,
<br />competition and predarion by non-
<br />native fish have increased (Minckley
<br />1991, Douglas et a!. 1994). There
<br />had already been a marked decline in
<br />populations of many native fishes by
<br />the late 1950s (Miller 1959), pre-
<br />sumably because of pressure from
<br />non-narive fish and blockage of fish
<br />migration caused by the first
<br />mainstem dams. Non-native carp
<br />(Cyprinus carpio) and channel cat-
<br />fish (Ictalurus punctatus), which are
<br />warm-water species, were introduced
<br />to the basin in 1890 or so and were
<br />dominant in the lower Colorado
<br />River by 1911. Cold-water species,
<br />such as rainbow trout, brown trout
<br />(Salmo trutta), cutthroat trout (Oncor-
<br />hynchus clark ii) , and brook trout
<br />(Salvelinus rontina/is), were intro-
<br />duced after 1919. Warm-water
<br />centrarchid game fishes were intro-
<br />duced into Lake Mead in the 1930s,
<br />
<br />~
<br />
<br />and other non-native species gained
<br />access as incidentals or bait fish.
<br />Currently, there are 11 principal
<br />non-native species-three cold-wa-
<br />ter and eight warm-water-in the
<br />Grand Canyon (Valdez and Ryel
<br />1997). Each of these species was
<br />already in the Grand Canyon at the
<br />time the dam was completed. Non-
<br />native warm-water fishes fill eco-
<br />logical niches similar to rhose filled
<br />by the remaining narive species, and
<br />the non-native cold-water species oc-
<br />cupy new thermal niches (Figure 4).
<br />Niche overlap has increased selec-
<br />tion against native fishes, further
<br />threatening their existence.
<br />The cold temperatures of the regu-
<br />lated Colorado River in the Grand
<br />Canyon restrict the distribution of
<br />non-native warm-water species.
<br />Channel catfish and carp are less
<br />abundant than in the upper Colo-
<br />rado River basin, where summer river
<br />temperatures are warm. These spe-
<br />cies presently spawn only in the Linle
<br />Colorado River, because they require
<br />temperatures of over 20 oc. Popula-
<br />tions of fathead minnows (Pime-
<br />phales prome/as), black bullhead
<br />(Ictalurus melas), and green sunfish
<br />(Lepomis macrochirus) are also low
<br />in the mainstem and occur primarily
<br />downstream from the Little Colo-
<br />rado River. Few of the warm-water
<br />fishes that are common in Lake Mead,
<br />such as striped bass (Marone saxa-
<br />ti/is), ascend into the Grand canyon'i
<br />The trout fishery between the dam
<br />and Lees Ferry is maintained by pe-
<br />riodic releases of hatchery-reared
<br />fish. There is considerable natural
<br />reproduction in this fishery, although
<br />water temperature during spawning
<br />between December and February is
<br />usually 10 oC, below the optimum
<br />for trout. Trour upstream from Lees
<br />Ferry do not appear to mix with the
<br />self-sustaining rainbow trout popu-
<br />lations that occur downstream. The
<br />trout fishery is not only of recre-
<br />ational importance: These down-
<br />stream populations have been preyed
<br />on by wintering bald eagles since
<br />1982 (Brown and Stevens 1992).
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<br />Riparian vegetation. Glen Canyon
<br />Dam and its operations have altered
<br />the riparian ecosystem (Turner and
<br />Karpiscak 1980, Johnson 1991).
<br />Early photographs of the Grand Can-
<br />yon show that channel banks inun-
<br />
<br />BioScience Vol. 48 No.9
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