Physical, Biotic, and Sampling Influences on Diel Habitat Use by

Abstract.—We used daytime and nighttime underwater observation to assess microhabitat use by bull trout Salvelinus confluentus (N 213) in streams of the intermountain western USA during the summers of 2001 and 2002. We recorded fish focal points and measured a set of habitat characteristics as well as habitat availability via line transects. Bull trout were benthic and solitary; most (88%) were observed at night. We developed a conditional logistic regression model to account for the effect of fish movement in response to snorkeling, and we fitted 18 candidate models to evaluate the relative influences of biotic and abiotic factors on habitat use. The candidate models were also fitted with a naı̈ve logistic regression (i.e., no movement) to evaluate the effects of movement on inferences of microhabitat use. The most plausible model describing bull trout habitat use was the same for the conditional and naı̈ve regressions and included depth, velocity, percent rubble substratum, and the day3 depth, body size3 depth, and body size3 day3 depth interactions. The presence of brook trout S. fontinalis and the abundance of conspecifics did not strongly influence microhabitat use by bull trout. The relative rankings of the remaining models differed substantially between the conditional and naı̈ve models. Relative to the conditional models, the naı̈ve models overestimated the importance of diurnal differences in habitat use and overestimated the use of deepwater habitats, particularly during the day. Both model types suggested that all sizes of bull trout were generally found in deeper, low-velocity habitat a