Brown trout early life history in the Logan River, and potential effects on native Bonneville cutthroat trout

The introduction of exotic fish has proven detrimental to the persistence of native trout populations throughout the intermountain West. In the Logan River, Utah, brown trout Salmo trutta outcompete native Bonneville cutthroat trout Oncorhynchus clarkii utah when the two species exist in sympatry, but the factors that determine brown trout distribution, and thus limit these competitive interactions, remain unknown. Our goal was to better understand the role of brown trout spawning ecology and early life-stage survival in determining the spatial extent and structure of the population. We conducted spawning surveys throughout the fall and winter. We also planted brown trout eggs in incubation boxes at eight sites to test the hypothesis that high elevation conditions limit egg-to-fry survival and therefore determine, in part, the upper extent of brown trout distribution. We counted \u3e1,600 redds over a two and a half month period within a 50 km stretch of river and three tributaries. We had a high degree of success predicting potential spawning areas based on the USFWS habitat suitability index, with redds being at their highest densities in areas with adequate spawning substrate and moderate flows. We also observed a strong positive relationship between redd densities and adult brown trout population abundance indicated by long term monitoring; however, we did observe brown trout redds up to an elevation of 1,980 m, 2.3 km upstream from any observed accounts of adult brown trout during summer surveys. Egg-to fry survival varied across the longitudinal gradient of elevation, temperature, and discharge. Understanding the mechanisms that limit brown trout distribution within the Logan River will allow us to predict the potential for these fish to expand their range and further reduce cutthroat trout abundance in the Logan and similar river systems throughout the West