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<br />Plate j, Fish of the Colorado system: (a) Gila cypha, (b) Gila elegans, (c) Gila cypha, showing
<br />nuchal hump, (d) Xyrauchen texanus, (e) Ptychocheilus lucius, (I) Salmo gilae and (g) Salmo gairdneri
<br />from the Lee Ferry fishery. (Photos a, b: Behnke & Benson 1980; c, d, e: R, Valdez; f: U,S, Forest
<br />Service; g: Arizona Game & Fish Dept),
<br />
<br />The reproductive habits of the rare big-river fish are gradually being docu-
<br />mented (Miller et al. 1982), as surviving populations are discovered (see
<br />Stanford & Ward ] 986a: Fig. ]), It appears that all are spring spawners.
<br />responding to the warmer spring runoff, Both minnows and suckers move into
<br />small streams to spawn, The squawfish migration was spectacular before regu-
<br />lation, with many large, light-coloured spawners (hence "white salmon"),
<br />Stanford (unpublished) has observed pre-regulation runs of squawfish and
<br />round tail chub in tributaries of the lower Gunnison River. The longheld view
<br />that these fish moved great distances from the mainstream to spawn is challenged
<br />by thc limited movement of radio-tagged spawners in the Yampa River (Tyus
<br />
<br />392
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<br />
<br />et al. ]982), Some populations may have been restricted to specific locations
<br />because habitats combining suitable food, backwater nurseries and spawning
<br />sites were limited. This could have been an additional isolating mechanism that,
<br />with the great age of the Colorado River, may help explain the diverse mor-
<br />phologies of the big-river fish, particularly Gila, Rhinichthys and Catostomus.
<br />The ecology of the headwater fauna is typical of rhithron streams (cf. Hynes
<br />1970). Ward et al. (]986) described the productive zoo benthos of the cold,
<br />Upper Basin streams, the major prey of Cottus, Salmo and Rhinichthys, and,
<br />Rinne (1980, 1982) described the ecology of native salmonids in tributaries of
<br />the Gila, The spinedace (Lepidomeda vittata), an endangered species, occurs in
<br />most north-flowing tributaries of the upper Little Colorado Basin. These
<br />streams often are reduced to pools, yet this remarkable species persists, though
<br />not in association with exotic fish (Minckley & Carufel 1967).
<br />Fish in the Virgin and Pluvial White rivers and desert segments of the Gila
<br />system are remarkably well-adapted to life in extreme temperatures (35-450C),
<br />high salinities (to c, 90g1-1) and drought (Deacon & Minckley 1974), During
<br />dry periods, the longfin dace (Agosia chrysogaster) is able to persist beneath
<br />algal mats and debris that are only slightly moist. This fish has a high repro-
<br />ductive capacity, and a few adults may repopulate the stream within a few weeks
<br />(Minckley & Barber ]97]). The desert pupfish seemingly appears from the sand
<br />following wet weather (Minckley ]973), and prefers highly saline habitats like
<br />the Salton Sea. However, extreme flooding or prolonged drought may eliminate
<br />desert fish from stream segments. Collins et al. (198]) reported that an intro-
<br />duced population of the endangered Gila top minnow (Poeciliopsis occidentalis
<br />occidentalis) was eliminated from one desert stream by flooding, Ephemeral
<br />streams, when re-watered, rapidly produce plant material (Busch & Fisher] 98];
<br />Fisher et al. ] 982), providing a food base for omnivorous fish which in turn can
<br />reproduce rapidly to ensure survival through the succeeding dry period, On the
<br />other hand, fish in the desert springs of the Pluvial White River occupy hom-
<br />ogeneous, perennial environments. These omnivorous fish use a limited, but
<br />reliable food supply, and have developed related adaptations of anatomy and
<br />behaviour (Cross 1976; Constantz 1981; Deacon & Minckley 1974).
<br />
<br />The fish fauna of the regulated system
<br />
<br />The present fauna of the Colorado system is dominated by exotic species
<br />(Table 2), except in a few isolated locations (sse Stanford & Ward 1986a:
<br />Fig. I). Of the 32 native freshwater fish listed in Table I, one is extinct and 15
<br />occur in sparse populations. In the headwaters of the Upper Basin, the range of
<br />the Colorado River cut-throat trout (Salmo clarki pleuriticus) is limited to two
<br />or three isolated populations (Behnke & Benson 1980). Less than 8000 speci-
<br />mens of S. gila (Plate If) remain in its native habitat in the upper Gila Basin,
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<br />393
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