Scripps Institution of Oceanography

Populations of Pacific sardine (Sardinops sagax) and northern anchovy (Engraulis mordax) have responded differently to oceanographic changes in the California Current Ecosystem (CCE) over the past century. Similar multi-decadal scale variability has been observed in sardine and anchovy populations around the world. Although correlations between ocean temperatures and fish biomasses are evident, the underlying processes relating ocean conditions to fish production remain unknown. Here, I examine the ecological differences between sardine and anchovy in the CCE and consider the oceanographic conditions that affect the planktonic prey utilized by the two species.

Sardine and anchovy are planktivorous fish that consume a wide range of prey items. However, direct comparison of the gill rakers of the two species indicate that sardine are better adapted to retain smaller plankters than anchovy. Oceanographic conditions influence the size spectrum of zooplankton communities in the CCE, with larger individual plankters relatively more abundant in the eutrophic region nearshore. I hypothesize that this relationship results from size-dependent trophic interactions between zooplankton and their phytoplanktonic prey. Model results indicate that changes observed in the biomasses and size structure of zooplankton communities in offshore, oligotrophic waters influence the potential growth rate of sardine, while anchovy growth is uniformly negative in the offshore region. Positive growth by anchovy is possible only in the nearshore, eutrophic zone. Different atmospheric conditions control the nutrient supply to these regions of the CCE. The nearshore area is enriched by rapid upwelling of nutrient-rich waters resulting from coastal upwelling, while slow upwelling resulting from positive wind-stress curl supplies nutrients to the offshore area. Estimates of curl-driven upwelling are associated with pycnocline and nutricline shoaling in the southern CCE over the passed decades, and sardine production is more strongly related to changes in curl-driven upwelling than with coastal upwelling or sea-surface temperature. Upwelling rate is a fundamental determinant of the biological structure and production in coastal pelagic ecosystems, and future changes in the magnitude and spatial gradient of wind stress may have important and differing effects on these ecosystems.