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~~-~. I r I t i ~~ ~~ a; i :~ 11 i~ t ~~ f~i is r ~; 1F.. ='~ = ~! -%J ;~ ~;~ ., ~~:,~: ~~ ' r ;~ g: of exotic species. The introduction of P. tnexicana resulted in a decrease in the population density of Moapa, ap- parently through an increase in para- sitism (35) and possibly through direct competitive interaction. A primary danger to Moapa is the possibility that additional introductions will cause an- other population decline from which it might not recover; such circumstances are not predictable. The White River springfish, Crenich- thys baileyi, presents still a different problem. This species occurs as a num- ber of disjunct populations along the course of the Pluvial White River, in Nevada. No careful taxonomic evalu- ation of all populations of the species has been made, yet marked physiolog- ical differences are known, and morpho- logical differentiation is apparent. Some populations exhibit the highest toler- ance to high temperatures and to low concentrations of dissolved oxygen known in fishes (37). Some stocks of C. baileyi, like stocks of Moapa, have become depleted following the intro- duction of aquarium fishes. In this case, too, a major factor in the decline seems to be the original species' greater sus- ceptibility to existing parasites in the presence of competition from exotic species, or the introduction of new parasites along with the introduced fish (35). Populations have been severely reduced-and in one instance the popu- lation became extinct-following the in- troduction of largemouth bass, Microp- terus sal~noides, and mollies, Poecilia n:exicana. During the past 3 years, three springs is Nevada have been se- lected for comparative ecological re- search as "controls" because they were uncontaminated by exotic fishes. In each instance, after no more than 2 months of work, exotic species suddenly appeared. This further illustrates the magnitude of the problem. It is difficult to decide which popu- lations of a fish like Crenichthys baileyi are to be preserved. In many respects this fish is intermediate between cate- gories 3 and 4. Yet, known physiolog- ical and morphological differences in- dicate that several populations provide an exceptionally high amount of infor- mation and that each may be scien- tifically important. Numerous other examples of fishes in category 4 could be cited. The pre- carious status of Gambusia ~ngei, which was almost destro)~ed by the introduc- tion of G. u)~nis into its warm-spring habitat in Big Bend National Park, ++as 1130 documented by Hubbs and Broderick (38). Cyprinodon diabolis, a unique species represented by no more than 700 individuals in Devil's Hole, Ne- vada, now is protected •in Death Valley National Monument. This species has been affected, but scarcely disturbed, by man. Some changes effected by man are not automatically detrimental to native fishes. Placement of the desert dace, Eremichthys acros, on the list of en- dangered species (2) resulted from a premature judgment concerning the im- pact of irrigation deve}opment (39) rather than from an objective evalua- tion. Subsequent work ~ in Soldier Meadows, Nevada, indicates expansion of populations of E. acros into the re- cently constructed irrigation ditches. This species belongs to category 4, but is not endangered, both because it suc- cessfully extends its populations into habitats built by man and because sev- eral populations exist. On the other side of the slate, a reminder that a number of fishes have become extinct in recent years seems appropriate. Some are Em- petrichthys merriami, Lepidomeda alti- velis, Lepidomeda mollispinis pratensis, the Crystal Spring population of Cy- prinodon nevadensis mionectes, the Hiko Spring population of Crenichthys baileyi, the Pahranagat Valley popula- tion of Pantosteus intermedius, Empe- trichthys latos pahrump, and E. 1. con- cavus. Exotic fishes obviously contrib- uted to the extinction of the first six, whereas habitat destruction is clearly re- sponsible for the extinction of the two last-named species. Conclusions Declines in the populations of native fishes in the American Southwest are largely due to habitat changes associ- ated with man's modification of various aquatic environments. Early decimation of the fauna was mainly a result of large-scale physical change, such as the diversion and impoundment of rivers and downcutting of streams in their formerly stable floodplains [that is, ar- royo cutting, a possible result of a combination of man's actions and cli- matic phenomena (4, 29)]. More sub- tle physical or chemical changes, the lowering of water tables through the use of subsurface water for irrigation, eutrophication and other pollutional effects, and biological phenomena as- sociated with the ever-increasing intro- duction of exotic species-all are a celerating the extirpation of remna populations. Present populations of most nati~ fishes are locally dense, especially isolated habitats occupied by. fish ~ category 4. Given a reasonable degrc of environmental stability, .these fi~ certainly are capable of maintainir themselves. However, some species b longing to category 3 present a diffe eat problem. They hav~~-t present r treated to the most inaccessible parts their ranges, where simple surveillam Hof their status is a major operatio Some of these fishes can spread rapid when water conditions improve. 'I'ht populations may be greatly depleted one year, or perhaps they withdra from a major part of their pos~;:~- range over a longer period, but a= a few years of high precipitation a~ stabilized stream flow they may spt , and repopulate almost all available l,~t itats. Such a population resurge.~ was recently documented by Minckl and Carufel (40) for the formerly d pleted Little Colorado spinedace, L pidomeda vittata, in the period 196: 66, and is known for other forms. It seems to us that many people al agencies currently involved in the slut and promotion of "endangered" speci are only partially realistic. This is e~ dented, for example, by their conce for "peripheral" species, those repr sented in a given state or country by isolated or remnant population perip eral to the main body of the gene pot In fishes, the inclusion of the Mexic: stoneroller (Campostoma ornatun the Atlantic salmon {Salmo salary, al a number of other forms in the U. list of endangered species (2), even "peripheral" species, seems unw~ ranted. One invariably meets oppositi~ on suggesting that each population a desert fish such as Crenichthys baffle should be preserved. But if each isolat• spring population of C. baileyi is n "worth saving," why then be concern with the different river populations S. salar? The distinction is apparer Salnto is well known to many peol and is of importance to sportsme Crenichthys is neither. We are simF dealing with an interaction of supF (meaning maintenance in nature, this context) and demand (meaning t interest of the people concerned). The t•alidity of a decision as whether or not a species is "endr ~ered" depends on many factors. F example, working style, prior inforrr SCIENCE, VOL.