During an El Nino, warm water gathers along the West coast of South America, so the nutrient rich water is no longer being upwelled. keeping in mind that cold water sinks and warm water rises, usually the warm water near coastal regions would not be full of nutrients compared to the cold water where the nutrients fall. therefore, more fish will be located or migrate to areas of this nutrient rich cold water instead. So, many fishermen would not have high yields of fish ultimately affecting their way of life and money. also a lot of these fish that remain at the surface would not be as healthy or nutrient rich.
hope that helps! :))