Archaeological evidence across four millennia indicates recent erosion of Chinook salmon age structure in California

Chinook salmon Oncorhynchus tshawytscha provide crucial ecosystem services, but their populations are in steep decline throughout their native range. Recent decades have seen widespread erosion of age structures, but the lack of long-term baselines makes it difficult to assess this change. We collaborated with the Estom Yumeka Maidu Tribe of Enterprise Rancheria and used otoliths to reconstruct changes in age structure for California Central Valley Chinook salmon over the last 4 millennia. Specifically, we compared the returner age structure of present-day hatchery and natural populations in the Feather and Yuba rivers (2002-2020) with archaeological data from the same watershed, spanning the Middle-Holocene (1800-1000 BCE), Late-Holocene (500-1770 CE), and post-European-contact (1770-1870 CE) periods. We observed a shift to younger ages, from dominantly age-4 returners in the archaeological samples to age-3 fish in both hatchery and natural populations today. The recent time period also shows reduced variance and diversity in return ages compared to the post-European-contact time period, which has the most robust sample size of the archaeological collection. The shift toward younger ages in returning fish may have caused losses in productivity, while the decrease in variance and diversity may have reduced their resilience to environmental stochasticity. The erosion of age structure since European contact suggests anthropogenic factors—such as loss of freshwater and estuarine habitats, and industrialized ocean fishing and hatcheries—as potential contributors. Incorporating archaeological data into ecological assessments can help guard against hidden shifting baselines and inform restoration targets for more resilient populations.