Fish Ecology

Delta Smelt life history

Delta Smelt on the brink of extinction

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.

Reconstructing life histories from otoliths

Otoliths (ear bones) provide a life-long archive of physiological (growth) and environmental conditions (chemistry) that a fish has experienced and thus can be used to reconstruct complex life history patterns. 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.