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Brown Trout were introduced to the Trinity River in Northern California in the 1890’s. Since 1932, Brown Trout have sustained their population without additional stocking. Over the last 15 years, fisheries managers have been concerned that predation by piscivorous Brown Trout may impede efforts to restore native salmonids, in particular endangered Coho Salmon. I investigated predation by Brown Trout on native fish in the 64 km of the main stem Trinity River below Lewiston Dam. Using a bioenergetics approach parameterized with field measurements of Brown Trout abundance and growth, I estimated the amount of energy needed to sustain the 2015 Brown Trout population and used stable isotope analysis and gastric lavage to quantify the biomass of prey consumed over the course of a single year. I found that Brown Trout, particularly large individuals, primarily ate hatchery fish. Invertebrates were the next most popular prey followed by wild salmonids and ammocoetes. I estimated that in 2015, Brown Trout ate 6.5% of the biomass released from Trinity Hatchery (95% CI 4.1 to 9.6 %) and the wild consumption was equivalent to 23% (95% CI 1.4 to 88%) of the biomass of wild salmonids which survived to emigrate out of the study reach.