El Niño-Southern Oscillation (ENSO) is a major climate phenomenon centered over the equatorial Pacific Ocean that significantly alters global weather and ocean conditions. El Niño, the warm phase of this cycle, dramatically affects the normally cool waters off the western coast of South America, particularly near Peru and Ecuador. This cyclical warming causes the massive marine productivity of the region to crash, forcing species like the Peruvian anchoveta to abandon their traditional feeding grounds. The resulting physical changes create conditions of thermal stress and food scarcity, which are the fundamental reasons for this large-scale ecological displacement.
The Anomalous Productivity of the Humboldt Current
The cold, low-salinity Humboldt Current (or Peru Current) flows northward along the South American coast. This current creates one of the world’s most productive marine ecosystems, supporting a fishery that accounts for an estimated 18 to 20 percent of the total worldwide marine fish catch. This biological engine is powered by coastal upwelling, the constant mechanism of renewal for the surface waters.
Prevailing coastal winds push surface water westward, away from the continent. The displaced surface water is then replaced by deep, cold water rising from depths, often between 100 to 300 meters. This deep water is rich in dissolved inorganic nutrients, specifically nitrates and phosphates, accumulated from the decomposition of organic matter.
When these nutrient-dense waters reach the sunlit surface layer, they fuel explosive growth in phytoplankton, the microscopic plants that form the foundation of the entire food web. These massive blooms are consumed by zooplankton, which in turn support vast populations of small pelagic fish, such as the anchoveta and sardines. This continuous supply of cold, nutrient-rich water maintains the region’s high productivity year-round.
The Mechanism of El Niño Warming
El Niño events begin with a shift in atmospheric circulation across the equatorial Pacific, known as the Southern Oscillation. The trade winds, which normally blow strongly from east to west, weaken significantly or reverse direction. This atmospheric disruption allows a massive pool of warm surface water, typically banked near Indonesia, to slosh eastward along the equator toward the South American coast.
This influx of low-density, warm water dramatically alters the vertical structure of the ocean near Peru and Ecuador. Normally, the boundary between the warm surface layer and the cold, deep water—known as the thermocline—is shallow. The arrival of the warm water mass deepens the thermocline considerably, sometimes exceeding 600 meters in strong events, creating a thick, stable layer of warm water at the surface.
This deep thermocline acts as an impenetrable cap, effectively suppressing the upwelling process. Coastal winds can no longer draw the cold, nutrient-dense water from below the surface layer to the euphotic zone where sunlight penetrates. The surface water quickly becomes abnormally warm, sometimes by 5 to 8 degrees Celsius above average, and is severely depleted of the nitrates and phosphates that sustain primary production.
The Ecological Collapse and Fish Migration
The immediate biological consequence of the suppressed upwelling is the widespread starvation of the marine food web. With the nutrient supply cut off by the deepened thermocline, the vast phytoplankton blooms that once characterized the coast cease to flourish. The primary food source for small pelagic fish, particularly the anchoveta (Engraulis ringens), vanishes, leading to a collapse of primary production.
Anchoveta and sardines are cold-water species adapted to the consistently cool temperatures of the Humboldt Current. The sudden and prolonged increase in sea surface temperature, sometimes reaching 25°C or higher, places immense thermal stress on their physiology, impacting their metabolism, growth, and reproductive cycles. The combination of thermal intolerance and the disappearance of their food supply leaves the fish with only two survival options: move or perish.
The massive-scale migration away from the coast is a survival mechanism, as the fish actively seek out more suitable habitat. Many schools move southward, following the retreating boundary of the cold water, or they disperse further offshore into deeper waters to find cooler temperatures. This movement drastically reduces the availability of fish biomass in the traditional fishing grounds, a phenomenon that has historically caused landings to drop to less than a third of normal levels during strong El Niño events.