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. I B. GILA RIVER I I A. GUNNISON RIVER I !,.I!!!!- - __ . ODO:.h:ob"uiti" (....'01 forll'l') _ .R D.cul~' e. clark. 1. cobhtl -~ i.l:u'~~': I 4000 ~ ~ ~ 3000 .g ~ G. rDbvlto G. "'lIOnl X.IUG"'" ---P."iVclul --- --=:.---!~ ! 2000 0-24' 0-26' o 225 300 0 200 Disfance From Headwaters (km) 800 75 .50 400 Figure 1. Altitudinal distribution of native fish in (A) the Gunnison River and (B) the Gila River before flow regulation, Approximate annual ranges in water temperatures also are shown (after Deacon & Minckley 1974; Dr R. Behnke, Colorado State Univ" pers, commun.; Stanford & Ward unpublished) The distributions of S. gilae and S, apache in the Upper Gila River are like those of two morphologically distinct, more stenothermic forms of the minnow ~ila robusta, perhaps indicating similar isolation mechanisms (Minckley 1 973; RIn~e 1976), Prior to regulation, the historical distribution of Gila in the Upper Basin barely overlapped the predominant range of Salmo (cf. F,ig, la), Thus, t,he N-S latitudinal gradient interacts with altitudinal gradients In each sub-basin. In the south, desert conditions have advanced high into the tributaries, where isolated water bodies harbour a diverse endemic fauna adapted to a broad range of temperatures. In the north, wetter, less extreme environments favour species commonly found in adjacent drainages. Prior to regulation, the native fauna of the mainstream river from the delta into Colorado and the Green River into it'S middle segment was limited primarily to six endemic minnows and suckers (Table 1, Plate 1): bony tail chub (Gila elegans), round tail chub (G. robus/a), humpback chub (G. cypha), squawfish (Ptychocheilus lllcius), flannelmouth sucker (Catostomus latipinnis) and razor- back sucker (Xyrauchen /exanus). The non-endemic speckled dace (Rhinichthys osculus) also occurred throughout the mainstream. The woundfin (Plagopterus argentissimus) was limited to the Lower Gila and Virgin rivers. Desert pupfish (Cyprinodon macularis) were found primarily in backwaters and marshes. Roundtail chub and flannelmouth sucker were more common in areas above the confluence with the Virgin River. Several euryhaline marine species, including tenpounder (Elops affinis) and spotted sleeper (Elot~is picta), apparently, fre- quented the estuary and delta areas (Gilbert & Scofield 1898), but only stnpcd mullet (Mugil cephalus) were abundant in the main channel (M~nc~ley. 1973. 1979). Thus, the predominant fauna of the lower main segment (Vlrgm Rtver to below the Gila) was three species (squawfish, bony tail chub and razorback sucker; Mincklcy 1979). This reflects the severity of the environment prior to regulation. All big-river species except bony tail chub (Tyus et al. 1982) were 390 abundant in the lower reaches of major Upper Basin tributaries (Stanford & Ward 1986a; Fig. 1). . The trophic structure of the big-river fish communities probably was simple, especially in the lower mainstream (Minckley 1979). The sparsity of zoobenthos in the sandy, unstable river bottom (Ward et al. 1986) may have forced the fish to forage in specific locations. Attached algae, insects, snails and other foods would have been prevalent in areas of stable substrata, such as debris dams or rock-rubble riffles and rapids in canyon segments. ~. The humpback chub (Gila cypha) now is commonly found in the most turbulent canyon streams (Valdez & Clemmer 1982); its bizarre nuchal hump (Plate la, c) may assist in swift waters by hydraulically maintaining the head near the substratum. Bony tail and humpback chub feed on zoo benthos and terrestrial insects, with algae and detritus (Vanicek & Kramer 1969), although bony tails are inclined to feed on drifting seston and may even subsist on plankton, as in Lake Mohave. Dense vegetation along the pristine river, with incessant desert winds, may have linked the ecology of bony tails with terrestrial production of insects, especially in the lower river where bony tails were once more common than humpbacks (Minckley 1979). Speckled dace, round tail chub and flannelmouth suckers are omnivorous (Minckley 1973), but are more abundant in rubble areas harbouring algae and zoobenthos (Stanford unpub- lished). Razorback suckers (Plate Id) have numerous gill rakers as a fine bran- chial sieve to trap seston and fine Aufwuchs. The keel-like antero-dorsal hump : > acts as a lateral stabiIiser as the fish moves through the water with its mouth open, filtering fine detritus and plankton (Minckley 1973). Squawfish (Ptycho- cheilus lucius) were the top carnivores of this simple, uniquely specialised big-river fauna. Squawfish (Plate Ie) feed voraciously on other fish (Vanicek & Kramer 1969), and may attain 1.5-2.0m and 35-45 kg (Miller 1961). Called "white salmon" in the Colorado Basin, the squawfish is one of the world's .~ largest minnows (Minckley 1973). Despite the simple food chain, the river alone could not provide a food base to support large populations of big fish such as squawfish and razorback suckers. (Minckley 1979), Prior to regulation, numerous backwaters occurred through- out the range of the big-river fish. In aggraded segments of the Lower Basin extensive marshes were reworked and refilled during river spates (Sykes 1937; Dill 1944). These habitats probably subsidised the river trophic base by accumulating detritus and producing plankton and zoo benthos, All the big- river species use backwaters as refugia, nurseries and feeding areas (Minckley 1973; Valdez & Wick 1983), Valdez & Wick (1983) showed that radio-tagged squawfish and razorback suckers frequently move between mainstream and backwater, with a remarkable ability to sense the depth and location of the connecting channel (which often fills with sediment between floods, isolating the backwater). 'Backwaters and marshes in the Lower Basin also harboured large populations of the desert pupfish. Seasonal migrations of small mullet into the lower river probably provided additional prey for squawfish (Minckley 1979). 391 .ot&. . ~J