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Changing Fish Fauna of the Southwest 397 tory. Even when exceptionally heavy runoff occurs, the expected ad- ditional recharge of the water table by downward percolation now fails to be effective-probably because of shallow penetration and subsequent evaporation (Smith, 1940, p. 41). Introduction of exotics.-With the establishment of the U. S. Fish Commission in 1872 the march of exotics began with the introductior. of the carp. Today the number of native and introduced species in- habiting certain southwestern states shows that the indigenous fauna of any state does not outnumber the planted kinds by more than 3 to 1, generally only about 2 to 1; in Arizona, however, there are more exotics than there are natives. The following figures are derived from the references cited plus the author's manuscript checklists: Colo- rado, 54 native and 33 exotic species (Beckman, 1952) ; New Mexico, 61 native and 22 introduced kinds (Koster, 1957) ; Arizona, 28 native, 37 stocked fishes; Utah, 26 native, 24 planted species; Nevada, 41 native and 21 exotic kinds (La Rivers and Trelease, 1952) and California, 67 native fresh-water and anadromous species and 32 in- troduced species (Shapovalov, Dill, and Cordone, 1959). The period of intensive introductions occurred chiefly between 1930 and 1950, along with the construction of major dams and diver- sions and the marked increase in human population. Most of these alien species-numbering approximately 36-carne from eastern United States, in large part as intentional plants of game fishes. Only 8 exotic species are known to have become established in the West prior to 1930. Many of the established aliens have affected the native species either directly-through predation, competition, and hybridization (Hubbs and Miller, 1953, and earlier references cited) -or indirectly, by altering the habitat (as carp have done). In the Mohave River of eastern California the one native fish has been almost wiped out through competition and hybridization with a better-adapted exotic species (Hubbs and Miller, 1943, 1948, and subsequent unpublished studies). The western mosquitofish, Gambusia a. affinis, has ousted native species and subspecies through competition and predation, so that some indigenous forms are now threatened with extermination. The complete replacement of Poeciliopsis by Gambusia at Arivaca Creek, Arizona, possibly in less than 2 years, has been described above. Ellis (1914, p. 129) attributed the scarcity of the yellowfin V%