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Stable isotope analysis (SIA) is a powerful tool for assessing food webs and diet. However, the underlying assumptions that stable isotope ratios in an organism's tissues reflect that of its present diet may be complicated by isotopic turnover and retention time. Quantification ofin situisotopic turnover of study organisms and different tissues via field studies is essential to inform scientifically sound food web and diet studies utilizing SIA. Rainbow Trout white muscle and liver tissue were collected monthly from North Twin Lake, Washington during the 2013 growing season. Using ANCOVAs, we estimated stable carbon retention time of 154 (95% CI: 106–224) and 292 (95% CI: 257–763) days for Rainbow Trout liver and white muscle, respectively. For nitrogen, we estimated a retention time of 186 (95% CI: 114–329) days for both white muscle and liver. Our analyses suggest longer isotopic retention times (and therefore slower isotope turnover rates) for adult fish in a natural system relative to those commonly reported in the literature. These findings can improve interpretation of SIA in future aquatic food web and diet studies involving adult coldwater fish in freshwater systems. [ABSTRACT FROM PUBLISHER]