Summary

Decadal-scale dynamics of small pelagic fish populations from five large marine boundary currents (Kuroshio, California, Humboldt, Benguela, and Canary Currents) and their possible links to climate variability are described and compared. Small pelagic clupeiform fish species such as anchovies, sardines, sardinellas, herring and sprat are characterized by decadal drastic fluctuations of biomass, which are often associated with regime shifts in large marine ecosystems and their dynamics are governed by long-term, decadal-scale physical processes. Consequently, small pelagics are excellent indicators of regime shifts. When occurring in the same system, anchovies and sardines usually fluctuate out of phase. These shifts between sardine-dominated and anchovy-dominated states seem to restructure the entire ecosystem, as concomitant qualitative and quantitative changes in ecosystem components other than sardines and anchovy populations have been observed. The small pelagic fish seem thereby to respond to a bottom-up forcing of the ecosystem which itself is driven by changing ocean conditions. Evidence is emerging that these ecosystem shifts are associated with large-scale changes in subsurface processes and basin-scale circulation. In the Humboldt Current ecosystem, the shifts seem to be linked to lasting periods of warm or cold water anomalies related to the approach or retreat of warm oceanic subtropical surface water (SSW) of high salinity to the coast of Peru and Chile. The famous collapse of the Peruvian anchovy around 1970 was the result of such a regime shift, not the consequence of the El Niño 1972/73, which happened after the anchovy decline was already initiated. Dynamics of Japanese sardines and anchovies and their Kuroshio Current ecosystem exhibit a surprising synchrony with processes in the Humboldt system.