8173

• UCREFRP Catalog Number: 8173
• Author: Gido, K. B. and D. L. Propst.
• Title: Habitat Use and Association of Native and Nonnative Fish in the San Juan River, New Mexico and Utah.
• USFW Year: n.d.
• USFW - Doc Type: \
• Copyright Material: NO

late summer-autumn when flows are low and species abundances are

high. At this time, native and nonnative juveniles,

particularly, may compete for resources.

The majority of the species and age-classes of San Juan

River fish that we examined occupied similar low velocity,

shallow to moderate depth, silt-bottomed habitats. Coevolved

stream fish assemblages have typically been reported to have

relatively high levels of resource partitioning (Gorman and Karr

1978; Ross 1986) indicating structured assemblages. The fish

assemblage on the San Juan River, however, is a combination of

native and nonnative species with a brief history of sympatry in

a modified environment. Thus, the stability of these

associations is equivocal because of the short evolutionary time

'this assemblage has been together and overlap in resource use may

reduce the fitness of interacting species.

Our ability to detect habitat overlap may have been effected

by the spatial scale of sampling. The sampling scale we used

(<300m study sites) was too small to detect large scale patterns

of habitat partitioning caused by environmental factors that vary

on the level of stream reach (e.g., conductivity, temperature,

river gradient). Additionally, differences among sites in

available habitat (Table 3) may result in reach level variation

in the fish assemblage. Thus, because we pooled data from all

sites, a negative association in habitat use may be attributed to

reach level habitat characteristics rather than differences on

the scale of individual habitats. Our intent, however, was to

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