El Niño, a climate phenomenon known for causing unusually warm ocean waters off the western coast of South America, continues to have significant impacts on marine ecosystems, particularly in the South Pacific. This recurring event, named by fishermen centuries ago for its typical arrival near Christmas, alters oceanic and atmospheric circulation patterns that influence marine life and fisheries.
The Earth’s rotation plays a central role in shaping the dynamics of ocean currents and winds. The planet’s spherical shape and axial spin generate prevailing easterly trade winds in the tropics and large circulating systems called oceanic gyres. These gyres, rotating clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere, drive cold, nutrient-rich waters along eastern coastlines. Off South America, the Humboldt Current transports cold water northward along the Pacific coast, supporting productive fisheries such as the anchoveta, a small pelagic fish vital to regional ecosystems and economies.
During normal, or “neutral,” years, the Humboldt Current’s upwelling brings abundant nutrients to surface waters, sustaining large populations of anchoveta, which in turn support seabirds, predatory fish, and human fisheries. However, El Niño events disrupt this balance. Weakened trade winds and increased atmospheric pressure over Asian coasts cause warm water to spread eastward across the Pacific. In strong El Niño phases, warm surface water extends along the equatorial coast of South America, suppressing the upwelling of cold, nutrient-rich waters by pushing them into deeper layers.
This warming reduces the habitat and food availability for anchoveta, leading to sharp declines in their populations. The consequences extend through the marine food web, causing seabird starvation, predatory fish displacement, and reduced yields for commercial fisheries. Historically, severe El Niño episodes have precipitated dramatic fishery crashes, particularly when combined with intensive harvesting.
The ongoing climate crisis further exacerbates these pressures by increasing ocean temperatures and acidification, which intensify El Niño’s effects and threaten marine biodiversity. The United Nations Food and Agriculture Organization’s 2025 report identified over one-third of the world’s fisheries as overfished, including the anchoveta stocks in Peru, which, though currently classified as "maximally sustainably fished," remain vulnerable due to historically low population levels and increased exploitation.
Modern fishing technology capitalizes on anchoveta behavior, which involves daytime schooling to avoid predators. While this natural defense reduces predation, it also makes anchoveta highly susceptible to large-scale capture by fishing vessels using extensive nets to encircle and harvest concentrated fish shoals. This method can rapidly remove significant portions of the population, complicating accurate assessments of stock health, as the fish are unevenly distributed in dense clusters.
Anchoveta serve as a key ecological link between plankton and higher predators, including commercially important species and seabirds. Their declines can destabilize marine ecosystems, as seen in other regions where the collapse of small pelagic fish has led to overgrowth of phytoplankton, oxygen depletion, and subsequent harm to marine life.
Research projects a decrease in anchoveta abundance by 8 to 14 percent per decade throughout the 21st century, driven by rising sea temperatures, changing salinity and oxygen levels, and shifts in distribution and reproduction patterns. These changes complicate management efforts aimed at sustainability, particularly as early life stages become more vulnerable to fishing mortality.
The anchoveta’s outlook underscores the complex challenges facing marine ecosystems amid climate change and human exploitation. The interplay of natural climate variability such as El Niño with anthropogenic impacts threatens the resilience of vital fish stocks, raising concerns for biodiversity, food security, and economic stability in affected regions.