Application of a data‐assimilation model to variability of Pacific sardine spawning and survivor habitats with ENSO in the California Current System

The Pacific sardine ( Sardinops sagax ) showed significant differences in spawning habitat area, spawning habitat quality and availability of survivor habitat as the Pacific Ocean went through the La Niña state in April 2002 to a weak El Niño in April 2003. During another El Niño/Southern Oscillation transition period in 2006–2007 when the El Niño state retreated and the La Niña returned, a similar pattern in spawning habitat quality was seen. The coupling between the atmospheric forcing, the physical ocean states and the properties of the sardine egg spawning are investigated using dynamically consistent data‐assimilation fits of the available physical oceanographic observations during these months. Fits were executed using the Regional Ocean Modeling System four‐dimensional variational assimilation platform along with adjoint model runs using a passive tracer to deduce source waters for the areas of interest. Analysis using the data‐assimilation model runs reveals that unusually strong equatorward wind‐forcing drives offshore transport during the La Niña conditions, which extends the spawning habitat for sardine further offshore. A statistical model of sardine spawning habitat shows better habitat quality during the El Niño conditions, which is associated with higher egg densities and corresponded to higher daily egg production. Concentration of eggs is also increased by convergence of water. The results of the source waters analysis using the adjoint data assimilation model support the idea that offshore transport extends the spawning habitat, and show that higher levels of nutrient are brought into the spawning habitat with high concentration of sardine eggs.