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180 The Southwestern A'a(ura(is( vol. 28, no. 2 There 6.-RrinNOduc4ons of comeback suckers i» Arizmra 1981-82 (/ok coon 1982, Johnson and Rinne 1982, orig. data provided by James Brooks, AGFD ). L.,relitl Darr r:o. I;RAHAM CO. Eagle Creek, S26, T45, R28E 6.80'81 1,000 Eagle Creek, 526, T45, R28E 7/19,'82 '3,000 Gila River, S3, T6S, R30E 6,'90.81 1,000 Gila River, S3, T65, R90E 9/9B1 1,899 Gila River, S8, T6S, R30E 7; 14'82 3,000 Gila River, S3, T6S, R30E 10'12/82 4,146 Gila River, 532, T6S, R91E 10!12;'82 8,297 Bonita Creek. Unsurv., T6S, R28E 9-'9%81 1,999 CILA CO. Chevy Creek, 53, 10, 15. T4 N, R15E 6/30,Bi 2.000 Chevy Creek, S3, 10, 15, T9N, RISE 4'6.'82 100,159 Coon Creek, SB-9, T9N, R15E 8%16-~82 2,500 Coon Creek, 58-9, T9N, R15E 4~6; 62 25,500 Sah River, S2, T3N, RISE 9/9,'81 1,399 Salt Riser, 59, T3N, R19E 9,'9!81 1,844 Sah River, S9, TSN, R14E ; 9/6B2 100,000 Salt River, 54-5, TSN, R19E 9'16'82 62,500 Sah River, S6-7, T3N, R14E 9.%16'82 277,500 YA\'APAI CO. Oak Creek, 523, T16N, R9E 7'1!81 1,000 Oak Creek, 529, T16N, R4E 7/14/82 6,000 Oak Creek, 512, T16N, R9E 7/1 %81 1,000 Nest Clear Creek, 519, T13N, R6E 7'1!81 1,000 West Clear Creek, S19, T19N, R6E 1/14'82 9,000 Verde River, 57, TI9N, R5E 9'15%82 6,763 Verde River, S9, I1, TI IN, R6E 9'10'81 2,688 L'rrde River, S3, I I, TI IN, R6E 9- IS%82 6,762 suggest a highly successful spawn just after impoundment of southwestern mainstream reservoirs, then long persistence of adults. A similar pattern exists in some other large cyprinoid fishes elsewhere in western United States (Moyle, 1976). Size variation in a single cohort of razorback suckers (Fig. 3) may well be adaptive to the predictabl}• variable Colorado River. Low adult mortality would lead to selection for iteroparity and large size, yet high mortality after a brie[ period o! growth would favor reproduction b}• young (smaller) size classes (G. R. Smith, 1981 a). In razorback suckers, fast-grooving (large) fish that could reproduce at a young age might be more fit in cycles of high dis- charge, a+•hile slow-growing (smaller) fish might best survive under intermit- tent conditions during long periods of drought, then reproduce a+'hen condi- tions of higher flow are again realized. G. R. Smith (1981a) demonstrated an intraspecific tendency for large-volume aquatic habitats in the intermoun- tain deserts to produce larger fishes than small volume ones, and proposed the evolution of alternative stategies as follows: If aduh mortality is variable bec'ausr of unpredictable variation in the severity of seasonal fluctuations, individuals that invest in early reprodunion sill Iravr more descendents after destructively dry years; those that groH' larger and produce mwe offspring later will leave more descendants in a series of weurr years. May' 1989 A4inckley-Status of Razorback Sucker 191 Postpluvial habitat instability in western North America may have allowed maintenance of alternative genotypes that result in growth and reproductive placticity among and within populations of western fishes. Perhaps the sta- bility of reservoirs has allowed such inherent variation to be expressed in }'ear classes of razorback suckers that have achieved adulthood. Destruction of the native fauna of the lower Colorado River has before been attributed mostly to physical modifications o[ the environment, such as de-watering, channelization, and impoundment (Beland, ]953; Miller, 1961; Minckley and Deacon, 1968; Vanicek et al., 1970; Holden and Stalnaker, 1975; Moyle, 1976; Behnke, 1980). Considering the great age of the Colorado River, and correspondingly great ages of at least some of the genera of Fishes inhabiting it (dating at least from Pliocene; Uyeno and Miller, 1963, 1965; G. R. Smith, 19$la-b; M. L. Smith, 1981), sufficient time has been available for them to have experienced as much, and likely far more physical change than has recently been effected by man. Desertification has de-watered much of western North America, undoubtedly in a cyclical fasion, for millenia, yet stream fishes persist in desert basins (Hubbs and Miller, 1948; Hubbs et a1., 1974; G. R. Smith, 1978, 1981a-b). Channel-straightening floods (Burkham, 1972) produce changes in habitat similar to man's channelization projects. Tectonic or volcanic events have repeatedly impounded desert rivers, even avithin the Grand Canyon (McKee et al., 1967), as evidenced by strandlines, terraces, and lacustrine deposits (Nations et al., 1982). De-watering kills fishes directly when complete, and thus requires no further discussion. Channelization or major floods decrease environmental heterogeneity and speed time-of-flow in more uniform channels. Impound- ment also obviously suppresses variability, and in doing so may, more than channelization, modify environmental cues required by an animal to feed, grow, and reproduce to fulfill its life cycle. Yet fishes of the genera Ptycho- cheilu.r, Gila, and Calostomus, plus other forms, lived under lacustrine con- ditions in Pliocene times (Uyeno and Miller, 1965; G. R. Smith, 1975, 1978, 1981a-b). High dams on canyon rivers of southwestern North America differ from natural impoundments in their release of water from cold hypolimnia of upstream reseraoirs. Physiological tolerances of native biotic elements ma}' therefore be exceeded by direct effects of temperature downstream; cold water precludes reproduction in many lowland southwestern fishes. Yet, long reaches o[ stream exist that warm sufficiently in summer to duplicate condi- tions in natural systems (Minckley, 1979). Blockage of spawning runs by dams and diversions also has been died as a factor in the decline of riverine species, a direct effect in the case of salmonids, but far more subtle avith most other groups. A "run" is obviously not necessar}• for successful spawn- ing in razorback suckers since they do so under reservoir conditions. At pres- ent, if native catostomids did move upstream from reservoirs to spawn, movement of juveniles downstream after hatching would end in a reservoir, where concentrations o[ predators would minimize or preclude swvival. Also, as pointed out by Moyle and Nichols (1973), dams may be more important in blocking re-dispersal of native, warmwater fishes back into areas where local extirpation has occurred. Re-establishment o[ stocks may therefore be physically impossible.