Summary

The hypothesized mechanisms reviewed in this chapter have been selected as having potential for generating low frequency variability in sardine (Sardinops spp.) and anchovy (Engraulis spp.) populations such as those experienced in the California Current, Humboldt Current, and the Benguela and Kuroshio Current systems. No generally accepted theory yet exists. An initial framework for such a theory is proposed, in which sardine productivity is linked to low frequency variability in boundary current flows, which is also related to the characteristic sea surface temperature anomalies associated with sardine productivity in these systems. During periods of weaker flow, planktonic sardine larvae are able to gain swimming capacity before being flushed from the system, allowing sardines to inhabit the main body of the boundary current. During periods of stronger flows, successful sardine reproduction is restricted to coastal waters, and productivity is relatively low. Anchovies are always restricted to coastal waters, and are more influenced by upwelling and coastal productivity; these characteristics tend to be correlated with boundary current fluctuations, giving rise to a tendency (but not requirement) of sardine and anchovy alternations. The Japanese system lacks coastal upwelling, but the cold, nutrient-rich Oyashio Current provides an analogous function.

A wide variety of mechanisms can be added to this framework as appropriate to individual systems. Physical processes include patterns of boundary current flow, including current meandering and formation of cyclonic eddies. A latitudinal shift in the source water coming from the North Pacific has been identified in the California Current, and contributes to the characteristic temperature and nutrient anomalies.