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<br />numbers of larvae. Verification of this interaction is <br />difficult. <br />The greatest degree of overlap in habitat use was among <br />juveniles of native and nonnative fish species. If there is <br />competition for resources (e.g., food, cover, space), this native <br />age-class appears to be the most vulnerable. Several factors <br />appear to support this hypothesis. First, juvenile density is <br />typically highest in the late summer and autumn when most species <br />have completed spawning (Schlosser 1982; Yant et ale 1984; Bart <br />1989; this study). Therefore, resources may be limiting during <br />this time. Second, water temperature is usually high and <br />dissolved oxygen low while discharge is at a minimum (K. Gido and <br />D. Propst, unpublished data). Although productivity may be high, <br />fish require more resources to maintain basal metabolic rate and <br />sustain growth in warmer temperatures (Moyle and Cech 1988) . <br />This also coincides with reduced volume of habitat caused by low <br />flows. <br />Grossman et ale (1985) suggested that competition among fish <br />species in streams with variable flow regimes should be reduced <br />because of low population densities resulting from harsh <br />conditions. While this appears partially true for the San Juan <br />River (particularly as a consequence of spring runoff), <br />population densities are quite high during late summer and autumn' <br />and in certain habitats exceeded 50 individual m-2. These high <br />densities are primarily caused by the reproductive strategies of <br />~. lutrensis and E. promelas, that spawn for an extended period <br /> <br />18 <br />