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result of impoundment, the implication being that the presence of a dam is directly responsible. Yet these fishes are becoming extirpated in areas, like the Salt River Can}•on, where such modifi- cations are yet to be made. Our lack of information on species requirements for reproductive success and on such mat- ters as the effects of introduced fishes on native species is discouraging. Another kind of big-river fish-the small, streamlined woundfin, Plagop- terus argentissimus, adapted to life in sandy, swift, turbid, downstream parts of the lower Colorado basin-occurred in the Salt River at Tempe (Table 1) and in the Gila River at Yuma, Dome, and Gila City, in the period 1890-95 (4, 23). Elsewhere in the system this species was not recorded by early (or later) collectors. The last reproducing popula- tion of the monotypic genus Plagopterus now lives in the lower Virgin River of southwest Utah, northwest Arizona, and southeast Nevada. A few stragglers have been caught in the lower Moapa River (Nevada) in'recent years (24). Plans by the U.S. Bureau of Reclama- lion to construct a tlam on the lower ~~irin River S miles above SL Georsc. litah, would affect about SQ of the ap- proximately 90 miles of river habitat suitable for this species. Planning -calls for flow in the Virgin River tiownstrcam to be maintained only b}' return irriga- tion flow and springs in and below "the narrows," about 13 miles below St. George (?5). The Bureau estimates that, downstream from the proposed dam, turbidity will decrease, salinity will in- crease, and flow in the river will be equalized. Equalization of flow means that, on the average, flow will be de- creased in every month, but the de- crease will be less in the summer than in the winter. Assuming that the Virgin River Dam is funded and constructed and that predictions of the downstream effects are borne out, eve are still unable to confidently predict what will happen to Plagopterus. We do know, however, that Plagopterus disappeared from the Gila River early in this century, presum- ably because of the first man-induced changes; that despite its ability to invade the somewhat smaller Moapa River it has not become established there; and that it is fulfilling its life cycle only in the lower Virgin River. These facts sug- gest that any change in river condition is likely to be detrimental. Such change should he avoided until some attempt has been made to define habitat require- ments for the species. The Gila spinedace, Meda ftrlgida, is endemic in the Gila River basin, re- quires another kind of habitat, and demonstrates yet another type of sensi- tivity to man's activities. The spinedace frequents moderately swift currents flowing over gravel bottoms at or near the lower ends of riffles, and is mid- water in habit (23, 26). In this respect it resembles any number of small cypri- nids of more eastern drainages. At one time it occurred throughout the upper Gila River basin (Fig. 2B). Many streams in which it formerly lived still flow strongly, and the habitats seem totally suitable for its continued life, yet in recent years it has not been taken anywhere in the Verde River drainage, where it was abundant in the past. The aggressive, introduced red shiner, 0 Table ]. Fishes recorded from the Salt River, Maricopa County, Arizona, in the city of Tempe, in the period 1890-1967. Dashed lines span the period during which a species probably inhabited this segment of the stream; (O) occurrences documented by specimens in museums or recorded in the literature; (X) probable occurrence of a species at a given time, on the basis of collections made before that time or in other parts of the drainage, both upstream and downstream from Tempe. ~, Species 1900 Year of collection or probable occurrence 1920 1940 1960 . Native species Gila elegans O____ Meda fulgida O____ Plagopterus argentissimus O____ Prychocheilus Lucius X____ R/rinic/tihys osculus O____ Catostomus lalipinnis O____ X~~rauchen texamts O____ A.Snsia chrysogaster X--------------------- -------------------0---- Gila internredia X--------------------- -------------------0---- Cila robrrsta X--°----------------- -------------------0---- Pueciliopsisoccidentalis O_____________________ ___________________O____ C"rprinodo» ,nacularius 0--------------------- -------------------0------------------ ----X------------------0---- Cutnstornusinsigrtis 0--------------------- -------------------X----------------------X------------------0--------0-- Pantosteus clarki 0---- ----------------- -------------------X------------------ ----X------------------0--------0-- lrtlroduced species Gmnbusia affinis ----0------------------ ----0----------- -------0--------0-- Lepomis cyanellus ----0---------°------- =---X------------------0--------0-- Cyprinus carpio ----0------------------X--------0-- Ictalwus melas ----0------------------0___----_O-- Lepomis macrochlrus ____O__________________X________O__ ,- Pomoxisn' aculotus ----0------------------X_-----_-7C-- Poecilia latip ntta ~ ____p________O__ Mkropterus salmoldes ____ ____ O Doroson:a petenensis ____O__ Carassiusauratus ____ __ O Norernigontus crysoleucus • O Notro is lutrensis p ----0-- Pimephales promelas ~ ___ O Ictalurus natalis ____O~ Tctaltrrus punctatts ____ __ O Lebistes reticulatus * ----0-- Poerilia n:exicana • Xiphophorus variatus • ____~__ Lepomis microlophus ____O__ Tilapia,rtossambica * ____O__ ' These species were. taken prior to severe flooding in the Salt River channel at Tempe in the winter of 1965-66, but not subsequently. 29 MARCH 1968 t427 ~~ 'r i' ~~. 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