El Niño is a complex, naturally occurring climate pattern defined by the sustained warming of the surface waters in the central and eastern tropical Pacific Ocean. This phenomenon is one phase of the larger El Niño-Southern Oscillation cycle, which significantly shifts atmospheric and oceanic conditions across the globe. While El Niño’s influence is felt worldwide, its most powerful negative effect is on the delicate ecological balance of marine environments. The resulting disruption to ocean productivity directly undermines established fishing patterns and the stability of commercial fisheries.
The Critical Role of Upwelling Suppression
The foundation of a highly productive marine ecosystem relies on a process called upwelling, which is critically disrupted by El Niño. Under normal conditions, prevailing winds push warmer surface water away from the coasts of continents like South America. This action allows cold, deep water from the ocean floor to rise to the surface to replace the displaced water.
This deep water is rich in dissolved inorganic nutrients, such as nitrates and phosphates. When this nutrient-dense water reaches the sunlit surface layer, or photic zone, it fuels massive blooms of microscopic marine plants.
During an El Niño event, however, the thick layer of unusually warm surface water acts as a thermal barrier, effectively capping the ocean. This warm layer is less dense than the cooler water below, making it difficult for the deeper, nutrient-laden water to ascend. The strong upwelling currents are thus suppressed or completely halted by the warm water mass.
This physical change, often referred to as the deepening of the thermocline, prevents the essential supply of nutrients from reaching the organisms that rely on sunlight for photosynthesis. The resulting nutrient deprivation in the surface waters immediately triggers a decline in primary productivity, the initial energy source for the entire marine food web.
Collapse of the Marine Food Web Base
The abrupt loss of nutrient supply following upwelling suppression has immediate and cascading biological consequences, beginning at the lowest trophic level. The primary producers of the ocean, phytoplankton, cannot flourish without the influx of nitrates and phosphates from below the surface. Satellite data often shows a marked drop in chlorophyll concentration—a proxy for phytoplankton biomass—in affected regions during El Niño events.
This significant reduction in phytoplankton means a corresponding scarcity of zooplankton, the tiny animals that graze on the plant life. Since plankton organisms form the base of the entire marine food web, their collapse leads to widespread starvation throughout the ecosystem.
Small, schooling forage fish, such as anchovies and sardines, are directly dependent on plankton as their primary food source. When the food supply vanishes, these populations experience high mortality, reduced growth, and a sharp decline in reproductive success. For example, the Peruvian anchoveta fishery has historically seen its productivity severely compromised by strong El Niño events.
Higher-level consumers, like larger fish, marine mammals, and seabirds, soon find their food sources depleted. This ecological disruption chokes off the energy flow to top predators, leading to widespread population stress and declines.
Migration and Commercial Fishery Relocation
The altered ocean conditions force commercially valuable fish species to make dramatic shifts in their distribution, creating significant logistical and economic problems for the fishing industry. The warm surface waters of El Niño cause cold-water species to seek out more suitable habitats.
Many species migrate poleward, away from their traditional equatorial or coastal fishing grounds, in search of cooler water masses. For example, species like Pacific whiting and market squid have been observed to shift their distribution northward along the North American West Coast during El Niño years. Other species often move to deeper offshore waters, where the thermal barrier has not yet fully penetrated, in an attempt to find a more temperate environment.
This relocation directly harms commercial fishers by drastically reducing catch rates in established areas. For instance, the 2023 El Niño event led to a roughly 50% reduction in industrial landings for the Peruvian anchoveta fishery. Fishing vessels must travel much farther, often increasing fuel consumption and operational costs significantly, to locate the shifted fish populations.
Coastal Habitat Damage and Aquaculture Impacts
Beyond the open ocean, El Niño causes extensive damage to nearshore environments and the aquaculture sector through a combination of warming and altered weather patterns. The prolonged presence of abnormally warm coastal waters significantly increases the probability of coral bleaching, where corals expel symbiotic algae. Since coral reefs act as critical nursery habitats and shelters for countless commercial fish and shellfish species, their destruction reduces the overall productivity of coastal fisheries for years to come.
Changes in weather, particularly intensified rainfall in some regions, lead to increased freshwater runoff from land into coastal estuaries and bays. This influx of polluted runoff and altered salinity gradients can stress or directly kill sensitive organisms like oysters and clams, damaging important shellfish beds. Conversely, El Niño-induced drought in other areas can lead to hypersalinity, also harming estuarine life.
The aquaculture industry, which relies on controlled conditions, is also vulnerable to these environmental stressors. Warmer water temperatures can stress cultured species like shrimp and finfish, leading to reduced growth rates and making them more susceptible to disease outbreaks. Additionally, the farming of marine plants, such as sea moss, can be negatively affected by the temperature and salinity changes.