Archaeological evidence, including riverine and lake settlements, as well as fishing and netting artifacts, suggests that there was an increased reliance on inland fisheries during the mid-Holocene (ca. 4500–1800 cal BP) in northeastern North America. Unfortunately, more direct lines of evidence investigating this idea have not been thoroughly examined due to several factors, including inconsistent excavation techniques, and limitations in destructive analysis of human material remains. Here, we measured stable carbon and nitrogen isotope values from one human female and fifteen terrestrial and aquatic faunal taxa from deposits within the mid-Holocene site, Dutchess Quarry Cave 1 (Orange County, NY), to assess dietary source proportions and determine whether anadromous fish were a prominent dietary component for this individual. Using cluster analysis, potential prey species were grouped into three sources consistent with anadromous fish (“prey 1”), carnivores and omnivores (“prey 2”), and terrestrial herbivores and game birds (“prey 3”). We evaluated the relative contributions of the stable isotope values of the three prey groups using Bayesian analysis with MixSIAR. Our results indicate that animals within prey 3 made up the largest component of this individual’s diet, implying that terrestrial herbivores and game birds likely dominated her diet. Fauna from isotope groups prey 1 (anadromous fish) and 2 (carnivores and omnivores) supplemented the diet. Based on these data, it appears that anadromous fish were a seasonal component of human diet and that the incorporation of these resources did not involve the dramatic reduction of other year-round protein sources.