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We used strong inference with Akaike's Information Criterion (AIC) to assess the processes capable of explaining long-term (1984-1995) variation in the per capita rate of change of mottled sculpin (Cottus bairdi) populations in the Coweeta Creek drainage (USA). We sampled two fourth- and one fifth-order sites (BCA [uppermost], BCB, and CC [lowermost]) along a downstream gradient, and the study encompassed extensive flow variation. Physical habitat availability varied significantly both within and among the sites. Sculpin densities in all sites were highly stable (coefficients of variation = 0.23-0.41) and sampling variability was low (coefficients of variation = 0.11-0.15). Population stability was positively associated with habitat stability, and the only significant correlations of population parameters among sites involved juveniles. Sculpin densities were significantly higher in BCB than in CC. The data suggest that, despite their proximity, the dynamics of populations within the sites are being determined by small-scale (i.e., 30-50 m) rather than broad-scale spatial processes. Both AIC and Dennis and Taper analyses indicated that simple density dependence had the greatest ability to explain variation in r for all life-history classes in all sites (AIC, seven of nine cases; Dennis and Taper, nine of nine cases). Multiprocess models had little explanatory power. When adults were removed from two sites, juvenile sculpin shifted into microhabitats formerly occupied by adults. No shifts occurred in control sites. Consequently, it is likely that the patterns of density dependence observed in all three sites were a consequence of intraspecific competition for space. Our findings argue for a multitiered approach to the study of population variation, one that encompasses long-term monitoring, spatial variation, and experimental testing of potential mechanisms. [ABSTRACT FROM AUTHOR]