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.