Exclusion of Cutthroat Trout From Pool Habitats By Exotic Brown Trout Is Limited By High Densities of Native Cutthroat Trout in a Field Experiment

Invasive species are a leading cause for native species declines, but factors limiting invasions remain poorly understood.  Brown trout (Salmo trutta) are among the worst invaders worldwide, but little is known about brown trout invasions relative to other invasive salmonids.  Brown trout occur at low elevations in the Logan River, Utah, but they have not expanded into headwater sections occupied by Bonneville cutthroat trout (Oncorhynchus clarki Utah).  Results of laboratory and field experiments demonstrated that competition between cutthroat and brown trout is strongly influenced by native trout density.  In laboratory experiments, brown trout instigated aggressive behavior six times more frequently when cutthroat trout were held at low densities.  In field experiments, growth and survival of cutthroat trout was higher when held at high densities.  In the field, we used PIT tag arrays to determine whether high rates of aggression by brown trout led to the exclusion of cutthroat trout from favorable habitats at low cutthroat trout densities.  Our results suggest high densities of native cutthroat trout reduce the extent to which brown trout exclude cutthroat trout from pool habitats, and ultimately may provide biotic resistance to the establishment of brown trout in areas with high densities of native fish.