Are trout populations affected by reach-scale stream slope?

Reach-scale stream slope and the structure of associated physical habitats are thought to affect trout populations, yet previous studies confound the effect of stream slope with other factors that influence trout populations. We isolated the effect of stream slope on trout populations by sampling re aches immediately upstream and downstream of 23 marked changes in stream sl ope on 18 streams across Wyoming and Idaho. No effect of stream slope on ar eal trout density was observed, but when trout density was expressed volume trically to control for differences in channel cross sections among reaches in different slope classes, the highest densities of trout occurred in med ium-slope reaches, intermediate densities occurred in high-slope reaches, a nd the lowest densities occurred in low-slope reaches. The relative abundan ce of large trout was reciprocal to the pattern in volumetric trout density . Trout biomass and species composition were not affected by stream slope. Our results suggest that an assumption made by many fish-habitat models, th at populations are affected by the structure of physical habitats, is at ti mes untenable for trout populations in Rocky Mountain streams and is contin gent upon the spatial scale of investigation and the population metric(s) u sed to describe populations.