Research Projects

Salmon in Putah Creek

Putah Creek is a small river running from Lake Berryessa through the town of Winters, along the UC Davis campus into the Yolo Bypass. Salmon were once abundant here but became extirpated due to habitat alteration and water diversions. Following on-going restoration efforts Chinook salmon have returned to Putah Creek for the third year in a row. Chinook salmon are well established in many rivers in the Central Valley and are supplemented with several large hatcheries. We conducted in-river surveys and collected otolith (fish ear stones) from carcasses to determine the origin of these pioneering salmon. Otoliths have daily growth increments and can provide a life-long archive of age, growth and environmental signatures. Strontium isotopes measured in otoliths enable us to reconstruct how fish move through habitats and to determine their origin. This project is being carried out in the OGFishlab at UC Davis in collaboration with Nann Fangue, Andrew Rypel, Eric Chapman and Emily Jacinto.

Further reading

Geochemical Tools Identify the Origins of Chinook Salmon Returning to a Restored Creek. Willmes et al., 2021 Fisheries,fsh.10516.

Delta Smelt Life History

California’s endemic Delta Smelt (Hypomesus transpacificus) is rapidly approaching extinction in the San Francisco Estuary, placing it in the crossfire between human and environmental uses of limited freshwater resources. Though managed as an anadromous species, recent studies have challenged this simple life-cycle model for Delta Smelt, suggesting the species is estuarine resident with several localized “hot-spots” of abundance. Using laser-ablation otolith strontium isotope microchemistry, we discovered three distinct life history phenotypes: freshwater resident (FWR), brackish water resident (BWR), and semi-anadromous (SA) fish. We further refined life history phenotypes using a novel unsupervised algorithm and hierarchical clustering and found that in the last resilient year-class, the FWR (12%) and BWR (7%) comprised a small portion of the population, while the majority of fish were SA (81%). Furthermore, the SA fish could be clustered into four additional life history phenotypes that varied by natal origin, dispersal age and adult salinity history. Future conservation efforts should explicitly account for these diverse life history strategies in management actions to avoid Delta Smelt extinction in the wild.

Further reading

Hobbs, J.A., Lewis, L.S., Willmes, M., Denney, C., and Bush, E. 2019. Complex life histories discovered in a critically endangered fish. Scientific Reports 9(1): 16772.

Reconstructing Palaeo-mobility

Human mobility in recent history is well documented and often related to drastic external changes, including war, famine, and the discovery and exploration of new geographic regions and resources. Reconstruction of mobility patterns in prehistory is thus a crucial part of understanding the forces that drove our ancestors, but it is complicated by the fact that the archaeological evidence becomes scarce as we go back in time. The application of stable isotopes in archaeological research has revolutionised palaeomobility studies by providing independent data, which can be used to evaluate models of migration, trade, and cultural change.

This research project explores the use of strontium isotope ratios (87Sr/86Sr) to trace prehistoric human mobility patterns. Strontium isotope ratios vary across the landscape based on the age and composition of the underlying geology. Through diet humans incorporate strontium into their skeletal tissues such as bones and teeth. Teeth form during childhood and are resistant to weathering and geochemical alteration, often preserving the original isotope values. By comparing the strontium isotope ratios in teeth to the variations of strontium isotopes in the landscape it becomes possible to investigate mobility across geologically different areas between childhood and death.

Funding was provided by ARC DP110101415 (Grün, Spriggs, Armstrong, Maureille and Falguères) Understanding the migrations of prehistoric populations through direct dating and isotopic tracking of their mobility patterns. Part of this research was supported by the Australian French Association for Science & Technology through the ACT Science Fellowship program (2013).