Influence of El Nino and La Nina on the population dynamics of Calanus chilensis in the Humboldt Current ecosystem of northern Chile

From a time-series of sea surface temperature (SST) and zooplankton data fr om December 1991 through January 1998, and using a temperature-dependent mo del. ar studied the growth rate (g), female size. generation time (GT), and number of generations per year of Calanus chilensis from the Mejillones Pe ninsula, northern Chile. Female size uas negatively related to SST on both seasonal and interannual scales, and abundance was positively correlated to SST through seasons, though nut associated with warm (El Nino) or cold (Lt r Nina) years. The model predicted that cold and warm years may have a subs tantial effect on annual abundance of this species. Mean annual abundances of C, chilensis were not consistent with those predicted by the model, howe ver, suggesting that factors other than temperature may regulate interannua l differences in population size. For instance, changes in mortality rates between warm and cold years and advective losses under strong coastal upwel ling may be implicated in the variation. Despite smaller population size in years subject to El Nino conditions, predicted annual means of g and GT sh owed little variability. as expected for fairly stable/constant annual mean SST. The overall g was 0.33 d(-1), while GT was 18.1 d under an overall me an SST of 17.7 degreesC. The lack of evidence for food-shortage effects dur ing El Nino conditions and the influence of temperature on a seasonal scale suggest that temperature-dependent growth may be a mechanism regulating po pulation size by adjusting growth and development to stable temperature reg imes on an annual basis. (C) 2000 International Council for the Exploration of the Sea.