Effects of habitat area and complexity on Colorado River cutthroat trout density in Uinta Mountain streams

Habitat degradation has reduced the complexity and connectivity of streams on the north slope of the Uinta Mountains in northeastern Utah. These chang es have diminished the historical range of Colorado River cutthroat trout O ncorhynchus clarki pleuriticus, isolated the populations of this subspecies , and perhaps increased its risk of extinction. We assessed the effects of fragment area and habitat complexity on Colorado River cutthroat trout dens ity. We studied 88 reaches in 4 isolated stream fragments. At the fragment scale, both the density of adults and habitat complexity increased signific antly as fragment size increased. In the smaller fragments, the density of adults was lower while that of juveniles was higher. Habitat differed subst antially among fragments. At the reach scale, the density of adults was pos itively related to elevation, the percentage of undercut banks, and mean su bstrate particle size and negatively related to residual pool depth and the extent of large woody debris. The density of juveniles was positively rela ted to the extent of large woody debris and negatively related to residual pool depth and stream width. The habitat complexity index was weakly relate d to adult density at the reach scale. We were not able to distinguish the influence of habitat area or complexity on the density of adults, but a pop ulation living in an isolated stream fragment with low habitat complexity p robably requires more area to persist than a population of the same size li ving in a highly complex habitat.