this is not a comprehensive review and we have not considered open ocean ecosystems but for a fuller treatment see stenseth et al catches of small pelagic fishes have been as high as million tonnes per year representing more than of world capture fish production fao however these show pronounced fluctuations often associated with enso effects on upwelling dynamics and productivity lehodey et al the strength of upwelling off the south american coastline is strongly determined by the enso phase with warm phases associated with a lower thermocline which reduces phytoplankton production through inhibited upwelling of nutrients