El Niño is a recurring climate pattern representing the warm phase of the El Niño-Southern Oscillation (ENSO) cycle. This phenomenon is defined by the warming of sea surface temperatures in the central and eastern equatorial Pacific Ocean. The resulting shift in the ocean-atmosphere interaction dramatically alters global weather systems and the marine environments that support fisheries around the world.
How El Niño Alters Ocean Conditions
The fundamental shift caused by El Niño involves the redistribution of heat and the suppression of upwelling. Normally, strong trade winds blow surface water westward away from the coasts of the Americas, allowing cold, nutrient-rich water from the deep ocean to rise to the surface. During an El Niño event, these trade winds weaken considerably, preventing this cold, deep water from reaching the surface layer.
The weakening of the trade winds allows warmer surface water to accumulate and spread eastward across the equatorial Pacific. This warmer layer deepens the thermocline, the boundary separating the warm surface water from the cold, deep water below. With the thermocline now deeper, the limited upwelling that still occurs cannot draw up the necessary nutrients from the deep ocean.
The lack of nutrient-rich water at the surface creates nutrient poverty, specifically reducing nitrates and phosphates. These nutrients are foundational for the marine food web, as they support the growth of phytoplankton. Consequently, the eastern Pacific coastal ecosystem transitions from a highly productive, nutrient-rich system to a warm, nutrient-poor environment.
Effects on Marine Life and Species Migration
The lack of foundational nutrients triggers a cascade of effects throughout the marine food web. The collapse of primary production, meaning the failure of phytoplankton blooms, removes the food source for zooplankton and small forage fish. This decline immediately impacts small, schooling pelagic species, such as anchovies and sardines, which depend directly on the productivity of the surface waters.
In addition to starvation risk, elevated sea surface temperatures place physiological stress on fish adapted to cooler waters. This thermal stress can reduce growth rates, impair reproduction, and increase the susceptibility of many species to disease. Marine organisms respond to these deteriorating conditions by seeking refuge or migrating away from the affected area.
Many cold-water species seek cooler environments by moving deeper in the water column, below the deepened thermocline. A more dramatic response is geographical migration, where entire populations shift their distribution to avoid the warm waters. For instance, the Peruvian anchovy, central to one of the world’s largest single-species fisheries, often experiences severe population decline or moves south toward cooler waters off Chile during El Niño years.
Warm-water species, conversely, expand their ranges into previously cooler, higher-latitude areas along the coasts of North and South America. Tropical fish like yellowtail, albacore tuna, and mahi-mahi are frequently observed much farther north than their typical distribution, such as off the coast of California. This shift in distribution entirely alters the species composition available in traditional fishing grounds.
Impacts on Commercial Fishing Operations
The biological shifts caused by El Niño translate directly into operational and economic consequences for commercial fishing fleets. The disappearance of traditional target species from their usual locations leads to reduced catches, creating immediate financial hardship for fishers. For fleets that target forage fish, like the anchovy fishery off Peru, the population collapse can lead to the complete closure of fishing seasons.
Fleets must adapt by traveling farther offshore or poleward to find the migrated fish populations, dramatically increasing fuel consumption and operational costs. Some operators must also invest in specialized gear to target different, warmer-water species that have moved into the area. This need to change target species and fishing locations introduces unpredictability that can destabilize local fishing economies.
The global seafood market also experiences volatility due to these large-scale environmental changes. The reduction in anchovy catches, a primary source for fishmeal and fish oil, causes prices for these commodities to rise significantly. Since fishmeal is a major component in feed for aquaculture and livestock globally, this price increase affects sectors far removed from the Pacific Ocean fishing grounds.
Governments and regulatory bodies often respond to the stress on fish stocks by implementing temporary fishing bans or reducing catch quotas to protect the remaining populations. These regulatory actions, while necessary for conservation, compound the economic pressure on fishing communities. The result is a period of market disruption, increased operational expenses, and socioeconomic strain across coastal regions dependent on the affected fisheries.