During the 1997--98 El Ni¿o event, the temperature and oxygen contents of the coastal waters off northern Chile were noticeably higher than during non-El Ni¿o conditions, but phytoplankton and zooplankton biomasses were not dramatically altered. In contrast, the herbivorous copepod Calanus chilensis, which showed a limited tolerance to low-oxygen conditions, exhibited greater abundances, higher growth rates, and a significant reduction in adult body size. These results indicate a positive effect of the changes in oceanographic conditions due to El Ni¿o on zooplankton growth and production, and provide evidence for lack of food limitation on secondary production in the coastal region during this period. ¿ 2001 American Geophysical Union