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<br />investigate these associations. <br />Reproduction by many native species 1n southwestern streams <br /> <br /> <br />1S closely tied to spring runoff (Baltz and Moyle 1993). Spring <br /> <br /> <br />snowmelt reduces water temperatures and thereby delays <br /> <br /> <br />reproduction of nonnative species that require warmer <br /> <br /> <br />temperatures to spawn than do native fishes of the San Juan <br /> <br /> <br />River. In this study, larval ~. latipinnis and ~. discobolus <br /> <br /> <br />were present when water temperature was <180C, but larval '~. <br /> <br /> <br />lutrensis and E. promelas did not appear until water temperature <br /> <br />exceeded 210C. <br /> <br /> <br />Although our analytical methods (based on presence/absence <br /> <br /> <br />data, with no consideration of sample date) indicate considerable <br /> <br /> <br />habitat-use overlap of larval natives and nonnatives, the actual <br /> <br /> <br />overlap is less because of temporal differences in spawning. The <br /> <br /> <br />negative correlation in abundance between larval ~. latipinnis <br /> <br /> <br />and k. lutrensis (Table 4) reflects this temporal segregation in <br /> <br /> <br />habitat use. One exception among nonnative San Juan River fishes <br /> <br /> <br />is ~. carpio that spawns during spring runoff on inundated flood <br /> <br />plains (K. Gido, personal observation). This accounts for high <br /> <br /> <br />overlap between ~. carpio and native fish larvae. <br /> <br /> <br />The occurrence of adult piscivorous nonnatives in habitats <br /> <br /> <br />used by larval native fish may negatively affect native <br /> <br /> <br />populations (Marsh and Langhorst 1988). In our sampling, adult <br /> <br /> <br />nonnative fish occurred simultaneously in habitats with native <br /> <br /> <br />larvae. Although we found no significant positive association <br /> <br /> <br />between these fish, a small number of predators may consume large <br /> <br />17 <br />