Stable isotopes demonstrate the importance of freshwater fisheries in Late Holocene native Californian diets in the California Delta

•New historical ecological data for extirpated California Delta faunal species.•Faunal collagen stable isotope data used to model human paleodiets.•Human paleodiets 1000 to 400 years ago heavily dependent on local fisheries.•Sacramento perch are currently extirpated but were major source of dietary protein. Estimating paleodiet from macroscopic faunal and floral remains alone is difficult. Taphonomic biases make direct quantitative comparisons of different categories, such as fish bone, ungulate bone, and charred seeds challenging. Stable isotope data can help in this regard. To highlight this process, we present new bone collagen stable isotope data (δ13C, δ15N, and δ34S) from a series of animal and human remains from a Late Holocene (ca. 1000–300 cal BP) archaeological site in the California Delta. We contrast stable isotope data from humans with that from canid (Canis sp.), deer (Odocoileus sp.), elk (Cervus sp.), pronghorn (Antilocapra americana), and Sacramento perch (Archoplites interruptus) remains from the site. Most of these species are endangered or extirpated locally, making archaeological sites a primary source of paleoecological data. Of fourteen human individuals, twelve show isotope values consistent with heavy exploitation of freshwater wetland resources, which we interpret as a “local” dietary signature. By contrast, two individuals display a pattern more similar to precontact humans in other parts of interior California, which we interpret as recent immigrants to the site. Three subadults display elevated δ15N values relative to adults, consistent with these individuals still consuming significant amounts of breast milk near the time of death. A linear mixing model provides a quantitative estimate the role of different food groups in local diets. Together, we estimate that local adults gained over 60% of their protein from freshwater fishes. This heavy reliance on local fisheries is consistent with large numbers of fish bone from the site.