Biotic Resistance by Native Bonneville Cutthroat Trout to the Establishment of Exotic Brown Trout Is Influenced by Population Density

Invasive species are a leading cause for native species declines worldwide, 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.  However, despite being competitively dominant and capable of rearing juveniles throughout the watershed, they have not expanded into headwater sections occupied by Bonneville cutthroat trout (Oncorhynchus clarki Utah).  We designed multi-scale laboratory and field experiments to test whether cutthroat trout may limit brown trout establishment through biotic resistance.  Our results demonstrated that competition between cutthroat and brown trout is strongly influenced by the density of native trout, but somewhat surprisingly, the effect was manifested on cutthroat trout.  Cutthroat trout lost less weight and had higher survival in laboratory and field experiments at high cutthroat trout densities.  Furthermore, laboratory experiments suggest that cutthroat trout, sympatric with brown trout at low densities, are at greater risk of disease than at high densities. These results suggest that efforts to conserve high density populations or restore impaired populations are likely to increase resistance by cutthroat trout to the establishment of brown trout.