Healy, B.D., R. C. Schelly, C. B. Yackulic, E. C. Omana Smith, P. Budy. 2020. Remarkable response of native fishes to invasive trout suppression varies with trout density, temperature, and annual hydrology. Canadian Journal of Fisheries and Aquatic Sciences 77(9):1446-1462
Invasive species and habitat loss are among the greatest threats to riverine biodiversity. Novel habitats created through extensive river regulation, characterized by altered flow, sediment, and thermal regimes often favor invasive fishes over native species. Consequently, invasive fish suppression is a key component of endangered fish recovery programs in the Colorado River, but outcomes are often confounded by environmental variation. We studied the response of desert fishes to a brown trout (Salmo trutta) and rainbow trout (Oncorhychus mykiss) suppression program in a Colorado River tributary with natural flow patterns, and longitudinal variation in thermal characteristics similar to a tailwater. We investigated trends in fishes related to salmonid suppression, and drivers of native fish distribution and abundance across thermal and hydrological gradients using generalized-linear mixed-effects models. We observed declines in salmonid abundance of 89.3%, and concurrent increases in native fishes of ~480%; greatest increases occurred in warmer reaches and in years with spring flooding. Our results suggest recovery of imperiled native fishes can be achieved through suppression of salmonids, but outcomes may vary with temperature and flow characteristics. Thus, invasive species suppression, and efforts to mimic natural flow and thermal regimes may allow for the recovery of native fishes.