Ancient DNA and Stable Isotope Analysis Provide Insight into 19th Century Green Sea Turtle Dietary Ecology

This post is by Cyler Conrad and co-authors of the recently published paper “Ancient DNA Analysis and Stable Isotope Ecology of Sea Turtles (Cheloniidae) from the Gold Rush-era (1850s) Eastern Pacific Ocean.” Cyler holds a PhD in anthropology from the University of New Mexico, Albuquerque, New Mexico.

The full paper can be read in Open Quaternary:

During the 20th century, sea turtle populations in the Eastern Pacific, especially green sea turtles, plummeted. Today, green sea turtles are recovering after the implementation of preservation and conservation plans. Of the primary causes for this historic sea turtle crash, anthropogenic over-exploitation and by-catch are significant factors. In this paper, however, my colleagues and I approached this record differently and examined whether habitat and dietary changes may have influenced declines in green sea turtle populations in the eastern Pacific.

From the onset, this project was multi-faceted and collaborative. In 2012-2013 Kale Bruner (U. of Kansas), Allen Pastron (Archeo-Tec) and I analyzed a Gold Rush-era archaeological faunal assemblage from a site in downtown San Francisco. A sea turtle bone appeared in the assemblage. Only a few years later, Michael Stoyka (Sonoma State U.) analyzed another Gold Rush-era archaeological faunal assemblage in San Francisco. The story was the same; at this second site additional sea turtle bones were recovered. Unfortunately, our combined morphological zooarchaeological analysis failed to accurately identify these specimens to taxon. We were left with a major question – to what species do these 1850s sea turtle specimens belong?

Fig. 1 Turtles

Fig 1. Sea turtle carapace specimens recovered at CA-SFR-195H in San Francisco, California. Photograph courtesy of Michael Stoyka.

At this point, our colleagues Marie Labonte and Brian Kemp from the University of Oklahoma Laboratories of Molecular Anthropology and Microbiome Research, provided a key piece of analysis which helped shape this research. Through their ancient DNA investigation, they identified that both Gold Rush-era sea turtles are green sea turtles (Chelonia mydas). This proved invaluable. We now had well-dated green sea turtle specimens from two archaeological sites in San Francisco. Naturally, our next question followed: do these 1850s green sea turtles share dietary and habitat characteristics with their modern counterparts?

A review of historic documents, analysis of stable isotopes in the green sea turtle bone collagen and apatite tissues, and collaboration with Laura Pagès Barceló (Biology) and Emily Jones (Anthropology) at the U. of New Mexico and Jeffrey Seminoff and Calandra Turner Tomaszewicz at the National Oceanic and Atmospheric Administration, helped provide an answer. Investigation of a range of stable isotope systems, including carbon, nitrogen, hydrogen and oxygen, indicated that we have very little evidence for any dietary or habitat differences in our 1850s sea turtle samples and modern counterparts. Based on historic documentation we identified that these Gold Rush green sea turtles were captured and imported from the Baja California region of the Eastern Pacific Ocean, and yet, when compared with a large modern dataset of green sea turtles from this same region, we failed to find any statistically significant differences through time.

Fig. 2 Turtles

Fig 2. A green sea turtle from the Eastern Pacific. Photograph courtesy of Boyd Lyon.

While this research may suggest that green sea turtle diets and habitats have not changed since the 1850s in the Eastern Pacific, we suspect this story is much more complicated. Our historic 1850s sample only includes two individual sea turtles, we lack specific information on the exact time of year, age of turtle, and location of capture – all of which influence stable isotope values – and our hydrogen and oxygen stable isotope values do not match known, modern sea turtle values. Our hope is that by someday establishing a larger historic green sea turtle dataset, and one that spans the historic (and prehistoric) period, we can clarify the patterns present within this study and understand if 1850s green sea turtles truly foraged in similar ways to their modern relatives, or if our Gold Rush-era samples simply match modern green sea turtle stable isotope signatures for some other, unknown, reason.

We have only scratched the surface on an intriguing and exciting long-term ecological record for an important, and endangered, species.

The full paper can be read in Open Quaternary:

Conrad, C., Pagès Barceló, L., Seminoff, JA., Turner Tomaszewicz, CN., Labonte, MJ., Kemp, BM., Jones, EL., Stoyka, M., Bruner, K., & Pastron AG., (2018). Stable Isotope Ecology and Ancient DNA Analysis of Sea Turtles (Cheloniidae) from the Gold Rush-era (1850s) Eastern Pacific Ocean. Open Quaternary. 4(1), 3. DOI:

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