8.3 El Niño and other large-scale climatic systems affect fisheries

During the 20th century, scientists uncovered the influence of the El Niño Southern Oscillation (ENSO) (see Chapter 6) on fisheries. During an El Niño, the waters off the west coast of South America are warmed; during a La Niña, they are cooled (see Figure 6.5, page 161). The warm waters brought by El Niño to the coast of South America have been long associated with crashes in commercially important fish populations, such as anchovies, that also trigger widespread mortality among fish-eating seabirds and sea mammals. Why should the raising of sea surface temperatures by only a few degrees decimate these marine populations? A critical clue is provided by the most common cause of death: starvation.

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In the absence of El Niño, there is strong upwelling along the coast of western South America (see Figure 8.4, page 233), which supports high levels of primary production (see Figure 8.7, page 235) and one of the world’s most productive fisheries. However, the coming of El Niño warms the waters off western South America, and essentially shuts off the upwelling of nutrient-rich waters to the euphotic zone (Figure 8.9). As a result, primary production declines drastically, with consequences throughout the entire food web. For example, there is little plankton to feed the small fish, which are essential as food for larger fish, seabirds, and sea mammals. As a result, these consumers, living higher in the marine food web, suffer massive starvation and reproductive failure, and the fishing economy of the region also suffers.

EL NIÑO AND OCEAN CONDITIONS OFF THE WEST COAST OF SOUTH AMERICA
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FIGURE 8.9 The effect of El Niño on the coastal ecosystems of western South America results from its creating a barrier to upwelling, which delivers nutrients to the euphotic zone.

Think About It

  1. How does El Niño suppress marine primary production along the west coast of South America?

  2. Do the El Niño/La Niña phenomena exert density-dependent or density-independent controls on sea mammal populations along the west coast of South America? Explain (see Chapter 3, page 71).

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Many of the feeds used in aquaculture incorporate fish meal made from forage fish caught along the west coast of South America. How should the pricing of these feeds vary with the El Niño/La Niña cycle?

8.1–8.3 Science: Summary

The goal of fisheries managers is to estimate how much we can harvest from a fish stock while still keeping it productive.

Prevailing winds blow across the oceans, driving oceanic currents, which influence patterns of primary production and fish stock productivity by transporting warm or cooling waters from one region to another. Upwelling further modifies marine environments by bringing nutrient-rich cool water to the surface. Limited penetration of water by light restricts photosynthesis to the euphotic zone.

Many commercially significant fish stocks vary as a consequence of oscillations in large-scale climate systems. For example, the El Niño Southern Oscillation influences fisheries productivity by altering physical and chemical conditions that can influence fish populations directly or indirectly through its effects on rates of primary production.