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habit. Some springfishes in the White River live at temperatures exceeding 36.70 C. and at extremely low oxygen concentrations. Most populations are threatened by introduced fishes, but those living in hot, low-oxygen water have less of a problem since the exotic species cannot invade their special habitat. The Monkey Spring pup fish (Fig. 77) of southern Arizona was not so lucky. Largemouth bass were introduced into their habitat, and within a few months, the native species was gone. Figure 77. Extinct fishes from the Monkey Spring system, Arizona. Above: Monkey Spring pupfish, 55 mm; Below: Gila chub, 15 cm. DISCUSSION AND CONCLUSIONS Of 41 strictly freshwater fishes discussed here, 10 occur only in Mexico, 9 are only in the United States, and 22 species are shared by the two countries, the last serving to illustrate the regional nature of the fauna. Those found only in one or the other country represent in large part local endemics, which demonstrate in their isolation some of the uniqueness of fishes in arid lands. In 1989, 28 (68 %) of the 41 taxa were officially recognized (hereafter listed) as threatened, endangered, or of special concern by the American Fisheries Society. Three species from the region (Las Vegas speckled dace, Pahranagat spinedace, and Monkey Spring pupfish, not included in the above compilation) were reported to have suffered extinction. Definition of just how much trouble these fishes are actually encountering is a major problem. Fishes are difficult to assess from the standpoint of population status. In fact, they are even often difficult to discuss since they live in water, a medium inconvenient for humans to enter. Fishes are furthermore difficult to observe or capture, and even to identify after they are in hand. Lastly, for some reason they are often considered "second-class citizens" when compared with four-legged, furry or feathered beasts; humans identify more with warm- blooded animals that resemble themselves. Despite this, if one determines that a fish exists only as a hundred or so individuals in a single tiny spring, it can readily be defined as endangered since a single event could destroy it. Widespread species are another matter, and most fishes in this region were, in fact, relatively wide ranging. A fish may become greatly reduced in range, such as the Sonoran topminnow that lived from western New Mexico to the Colorado River then south through Arizona and Sonora. It now occupies fewer than 10 natural localities in the United States. One might also believe this sufficient to define endangerment. However, each existing population consists of hundreds or thousands of individuals, and overall population size may actually number in the millions. ' This is a substantial number when compared with some endangered terrestrial species such as California Condor or Mexican Wolf, which are reduced to only a few individuals. However, a short- lived species such as topminnow may disappear in a year or less when subjected to an efficient predator such as the introduced mosquitofish, so size of local populations may have little to do with its actual status. Others, such as bony tail of the Colorado River mainstream, may be equally as rare as California Condor. Fewer than 50 individuals have been caught despite intensive efforts since 1975. This fish is so rare that it cannot be studied in nature. Further, it appears to be hybridizing with other chubs because the few remaining wild individuals cannot find suitable mates. Biologists have no firm ideas why this species has disappeared. As with both the condor and wolf, artificial propagation has been initiated to perpetuate the bonytail, although it has been difficult to capture enough individuals to establish a viable broodstock. An intermediate state between the topminnow example and that of bony tail involves the razorback sucker, which enjoys one population that exceeds 50,000 individuals in Lake Mohave, Arizona- Nevada, yet is almost gone from the remainder of its 39