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Changing Fish Fauna of the Southwest 389 Clara to its upper part and has, at times, been very difficult to find there. On the other hand it was "abundant everywhere" in 1912-13 in the then flowing streams of the Los Angeles Plain, just to the south (Culver and Hubbs, 1917). It still occurred in the Los Angeles River as late as 1929 (USNM 94343), but disappeared there within the next decade; it was probably during this same period that it vanished from the Santa Ana River, for the last record there of which I am aware is for 1923 (CAS 18558), and none was found during careful investiga- tion along the river in the late 1930'x. By 1939, this stickleback oc- curred on the Los Angeles Plain -apparently only in the San Gabriel River system near Whittier, where it was last taken in April 1942 (UMMZ 139009). Changing water conditions, especially the disappearance of former perennial flows (see p. 371), and competition with introduced fishes have evidently brought about the extinction or near extinction of this fish over that part of its range where it once was most abundant. Gambusia affinis affanis has very likely been the principal exotic re- sponsible for the disappearance of the stickleback; the mosquitofish was introduced and widely established in California in 1922 (Dill, 1944, p. 162), and was common in the Los Angeles area by 1930. Whether G. a. williamsoni still survives on the Los Angeles Plain is questionable; it was still present in the upper part of Santa Clara River in 1951 and probably persists also in the Mohave River at Victorville (where it has been introduced, and was last collected in 1955). DEPLETION AND SPEC= REPLACEMENT The Arizona native trout, which I have tentatively referred to Salmo gilae (Miller, 1950, p. 34), once inhabited suitable tributaries of Salt River, Gila River (in New Mexico as well as Arizona), and the headwaters of the Little Colorado River (Fig. 1). It is the only trout indigenous to Arizona.s Testimony from old-timers and records I have examined specimens of the trout recorded by Jordan and Evermann (1896, p. 496) as Salmo clarki pleuriticus fmin the Little Colorado River and identify them as the Arizona native trout. The occurrence of the latter in the Little Colorado drainage is the result of introduction by way of an 1897 canal diversion (from a tributary of White River), or by transplanting activities of early stockmen, or both (letter from Jack E. Hemphill, November 10, 1960). This fish is well portrayed in color on the cover of the pamphlet by Mulch and • Gamble (1954). 0