Larval stage controls on Pacific sardine recruitment variability: high zooplankton abundance linked to poor reproductive success

Sardines inhabit some of the most highly productive areas of the ocean (upwelling areas). Despite this, those in the eastern Pacific appear to thrive during warm years characterized by low productivity. Among the explanations for the apparent sardine success during warm years are: (1) lowered predation pressure and (2) access to waters with higher food availability. We examine zooplankton abundance in the sardine Sardinops sagax larval habitat off the coast of southern California. According to our results, increased food availability (represented here by zooplankton abundance) cannot alone explain successful sardine year classes. It appears that zooplankton abundance may also be an index of predation pressure on larval stages, combining potential effects of (1) direct predation by zooplankton predators and (2) increased attraction of zooplanktivorus nekton. Accordingly, decreased predation pressure during warm years may be a major factor allowing bursts in sardine recruitment. These results suggest a need for modification of the traditional rather linear view of food web energy transfers. This view may neglect potential nonlinear feedbacks introduced in the food web by mortalities on the early life stages of higher trophic level organisms inflicted or mediated by lower trophic level organisms.