How El Niño Disrupts Marine Ecosystems

Published: May 26, 2026 · 8 min read

The Thermocline Collapse: Where It All Starts

Under normal conditions, the eastern tropical Pacific has a shallow thermocline — the boundary between warm surface water and cold deep water — that allows nutrient-rich water to well up along the coasts of Peru and Ecuador. These nutrients fuel phytoplankton blooms, which form the base of one of the most productive marine food webs on the planet, supporting the Peruvian anchoveta fishery, the largest single-species fishery in the world.

During El Niño, the thermocline deepens dramatically in the eastern Pacific. The upwelling still occurs mechanically — winds continue to push surface water offshore — but the water that rises to replace it is now warm, nutrient-depleted water from above the deepened thermocline rather than the cold, nutrient-rich water from below it. Primary productivity can drop by 80-90% in the most affected areas.

Fisheries: The Anchoveta Collapse and Its Ripple Effects

The Peruvian anchoveta (Engraulis ringens) fishery is the canary in the coal mine for El Niño impacts. Catches routinely fall from 6-10 million metric tons in normal years to below 1 million tons during strong El Niño events. The 1972-73 El Niño, which coincided with a peak in anchoveta fishing pressure, triggered a fishery collapse that took over a decade to recover from. The 1997-98 event reduced anchoveta biomass by an estimated 80%.

This collapse cascades through both the marine ecosystem and the global economy. Anchoveta are processed into fishmeal and fish oil, which feed the world's aquaculture and livestock industries. When the anchoveta fishery fails, global fishmeal prices spike, raising costs for salmon farms, shrimp operations, and poultry producers worldwide.

Beyond anchoveta, El Niño reshuffles entire pelagic ecosystems. Tuna, billfish, and squid shift their ranges by hundreds or thousands of kilometers following the warm water, often crossing into different nations' exclusive economic zones and triggering international disputes over catch allocations.

Coral Bleaching: When the Reefs Turn White

El Niño is the primary driver of mass coral bleaching events at the global scale. Corals live in a narrow temperature envelope — typically 23-29 °C depending on location. When sea surface temperatures exceed the local bleaching threshold by just 1-2 °C for several weeks, corals expel their symbiotic zooxanthellae algae, turning white and losing their primary food source.

The 2015-16 El Niño produced the third global bleaching event on record, affecting over 70% of the world's coral reefs. The Great Barrier Reef lost an estimated 29% of its shallow-water coral cover in 2016 alone, with the northern third of the reef experiencing mortality rates above 50%. The 2023-24 El Niño, superimposed on climate change-driven ocean warming, triggered the fourth global bleaching event.

Reef recovery takes 10-15 years under ideal conditions. With El Niño events now occurring every 2-7 years and climate change raising baseline ocean temperatures, many reefs are entering a state of chronic disturbance where full recovery between bleaching events is no longer possible.

Seabirds, Marine Mammals, and Top Predators

When the anchoveta disappear, the animals that depend on them follow. The 1982-83 El Niño caused massive die-offs of marine birds along the Peruvian coast — an estimated 17 million Guanay cormorants, Peruvian boobies, and Peruvian pelicans starved or abandoned breeding colonies. The Galápagos penguin population crashed by 77%, and the Galápagos fur seal lost nearly all of its pups that season.

California sea lion strandings spike during strong El Niño years because lactating females must travel farther and dive deeper to find prey. During the 2015-16 El Niño, NOAA recorded over 3,000 stranded sea lion pups on California beaches, primarily emaciated and dehydrated animals that could not find sufficient food after weaning.

The Indian Ocean and Beyond: Long-Distance Teleconnections

El Niño's marine fingerprint extends well beyond the Pacific. During El Niño, the Indian Ocean typically warms in the months following the event's peak — the "Indian Ocean capacitor" effect. This warming contributed to back-to-back bleaching events on Indian Ocean reefs in 1998 and 2016, affecting the Maldives, Seychelles, and Chagos Archipelago.

Explore more at the El Niño Guide — comprehensive climate science explained.