Factors Controlling Calanoid Copepod Biomass and Distribution in the Upper San Francisco Estuary and Implications for Managing the Imperiled Delta Smelt (Hypomesus transpacificus)

Delta smelt struggle to persist in a dramatically altered estuarine environment. Complex and incompletely understood relationships between food availability, environmental stressors, other components of the species' habitat, and the abundance of delta smelt impede the effective management and recovery of the species. The empirical modeling presented in this study quantitatively describes spatial-temporal biomass values of calanoid copepods, a key prey item for delta smelt, in relation to multiple potential controlling factors. The results underscore the role that river flows through the estuary have in determining prey availability, and demonstrate contributions of water temperature, salinity, and macronutrients in determining copepod biomass. The analysis also shows the importance of non-native, invasive bivalves in determining copepod biomass. Importantly, the analysis describes spatial-temporal shifts in the relative importance of modeled covariates across sampling locations in the Delta. Model results indicate that increasing flows in the fall of wetter years adversely affected copepod biomass, while increases in flows in the spring of drier years provided regional increases in biomass. The results of this analysis can inform resource management decisions and contribute to a comprehensive model that can meaningfully guide efforts to recover the imperiled delta smelt.