2.4.11 (HL)—ENSO mechanisms

El Niño weakens eastern-Pacific upwelling and can reduce fisheries; La Niña often strengthens upwelling and marine productivity.

Follow the chain: trade winds alter warm-water position, stratification changes nutrient delivery, phytoplankton changes, then fish and people respond. Remote rainfall effects are probabilistic, not guaranteed.

During El Niño, weaker upwelling can reduce nutrients near Peru, lowering phytoplankton and the fish that depend on them.

Warm, stratified surface water plus reduced nutrient-rich deep-water supply—not fish decline alone.

An ENSO association is not a deterministic forecast for every region; state the pathway and uncertainty.