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<br />VIi, DESERT FISHES
<br />
<br />433
<br />
<br />of intense sunlight coupled with high primary productivity. At night, how-
<br />ever, conditions may be otherwise.
<br />Bottom muds of many desert habitats have relatively high concentrations
<br />of hydrogen sulfide, as based on olfaction after they are disturbed; Pup-
<br />fishes, and apparently some other cyprinodontids, spend relatively long
<br />periods of time, especially in winter, within such sediments. Toxicity some-
<br />times must be near-lethal. Martin (1968) suggested possible damage to
<br />sheepshead pup fish exposed to hydrogen sulfide from a bottom agitated
<br />by collecting activities. Minckley (in Hubbs and Miller, 1965) reported
<br />a total kill of fish (Cyprinodon atrorus) resulting from agitation of a deep
<br />silt bottom in a hot -( 41.5 OC), drying pool. However, factors other than
<br />hydrogen sulfide certainly could have been involved under such severe
<br />conditions. It seems entirely probable, however, that Cyprinodon and its
<br />relatives may have evolved a physiological means of coping with hydrogen'
<br />sulfide in their severe habitats.
<br />
<br />IV. Biological Responses to Desert Water Conditions
<br />
<br />A, DISPERSION RELATIVE TO ENVIRONMENT
<br />
<br />Fishes native to deserts, almost without exception, have their distribu-
<br />tions limited either by geographic barriers, or by a need (or tolerance)
<br />for some special environmental feature. Only one of the 200 or so native
<br />fishes in western North America, the speckled dace, may be considered
<br />widespread (Miller, 1959), and it consists of a myriad of local, vari-
<br />ously differentiated forms that may well represent a species-flock rather
<br />than a polymorphic, single entity. The relatively depauperate nature of this
<br />western fauna, resulting from long years of vast geologic and climatic
<br />change, makes it particularly instructive. The relationships of morphology
<br />and environment are perhaps more clearly seen here than in places where
<br />speciose faunas saturate the habitat, and biotic factors obscure biologic-
<br />environmental interrelations.
<br />
<br />}, Stream-Dwelling Fishes
<br />
<br />Under relatively stable conditions of discharge, or as an average over
<br />a period of time, fishes and other organisms in the linear system provided
<br />by a stream tend to distribute themselves into different sections according
<br />to their requirements or preferences. Many factors other than current,
<br />temperature, substrate. competition from other organisms, and so on, may
<br />be involved, especially in situations such as streams fed by springs. But
<br />the physical gradient, to which current is intimately related, is the factor
<br />against which distributions of fishes are usually compared .,~Minckle~,
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