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examBoard: Cambridge
examType: IGCSE
lessonTitle: Major Ocean Currents Distribution
Ocean currents are like massive rivers flowing through our seas, moving enormous volumes of water around the planet. These currents play a crucial role in distributing heat, nutrients and marine life across the globe. For world fisheries, ocean currents are absolutely vital - they create the conditions that allow fish populations to thrive in specific regions.
Key Definitions:
Surface currents are driven primarily by global wind patterns and affect the upper 400 metres of the ocean. The Coriolis effect (Earth's rotation) causes these currents to move clockwise in the Northern Hemisphere and anticlockwise in the Southern Hemisphere, forming large circular patterns called gyres.
Deep ocean currents (also called thermohaline circulation) are driven by differences in water density, which is affected by temperature and salinity. These currents move much slower than surface currents but transport massive amounts of water. They form a global "conveyor belt" that plays a crucial role in climate regulation.
The world's oceans contain several major current systems that are critical for fisheries. Understanding their patterns helps explain why certain regions are so productive for fishing.
Five major gyres dominate ocean circulation patterns:
Includes the Gulf Stream, which carries warm water from the Gulf of Mexico to Western Europe. The North Atlantic is home to historically important cod, herring and mackerel fisheries.
Contains the Benguela Current along Africa's southwest coast, creating one of the world's most productive fishing areas due to strong upwelling.
Features the Kuroshio Current (Japan's equivalent of the Gulf Stream) and the California Current. These support massive sardine, anchovy and tuna fisheries.
Includes the Humboldt (Peru) Current, which supports one of the world's largest fisheries, particularly for anchovies. The cold, nutrient-rich water creates exceptional fishing conditions.
Contains seasonal monsoon-driven currents that affect fishing patterns throughout the year. Supports significant tuna and small pelagic fish populations.
Upwelling zones are areas where deep, cold, nutrient-rich water rises to the surface. These zones represent just 1% of the ocean surface but support about 50% of the world's fishing harvest!
When deep water rises to the surface, it brings nutrients like nitrates and phosphates that have accumulated at the ocean bottom. These nutrients fuel phytoplankton growth, which forms the base of the marine food web. More phytoplankton means more zooplankton, which attracts small fish, which then attract larger predatory fish. This creates incredibly productive fishing grounds.
The four major coastal upwelling systems are found along the eastern boundaries of the world's oceans: the Benguela Current (southwest Africa), the Canary Current (northwest Africa), the California Current (North America) and the Humboldt Current (South America). These regions support massive fisheries for sardines, anchovies and other species.
The Humboldt Current flows northward along the west coast of South America and creates one of the world's most productive marine ecosystems. Cold water from the Antarctic rises along the coast of Chile and Peru, bringing nutrients to the surface. This upwelling supports an enormous anchovy fishery that, at its peak, accounted for about 20% of the world's total fish catch! The fishery has experienced dramatic collapses during El Niño events, which temporarily disrupt the upwelling pattern by bringing warm water to the region. This case demonstrates how closely fisheries depend on consistent ocean current patterns.
Ocean currents influence fish distribution in several key ways:
Many commercially important fish species migrate along current systems. Tuna, for example, follow warm currents across entire ocean basins. Atlantic bluefin tuna spawn in the Gulf of Mexico and their young use the Gulf Stream to reach feeding grounds in the North Atlantic. Salmon navigate ocean currents during their years at sea before returning to their natal rivers to spawn.
Where different currents meet, they create convergence zones that concentrate food sources and attract predatory fish. The meeting of the warm Kuroshio Current and the cold Oyashio Current off Japan creates one of the richest fishing grounds in the world. Similarly, the mixing of the Brazil Current and the Falkland Current creates productive fishing grounds off Argentina.
Climate change is altering ocean currents in ways that will significantly impact world fisheries:
As oceans warm, some currents are slowing down, particularly parts of the Atlantic Meridional Overturning Circulation (which includes the Gulf Stream). This could reduce nutrient mixing and affect fisheries in the North Atlantic. Meanwhile, warming is causing many fish species to shift their ranges poleward, following their preferred temperature zones.
Melting ice sheets are adding freshwater to the North Atlantic, potentially disrupting the density-driven currents that bring nutrients to important fishing grounds. Changes in rainfall patterns are also affecting coastal currents and estuaries where many fish species spawn.
The North Sea cod fishery has been heavily affected by changing ocean conditions. Warming waters have pushed cod populations northward, with many moving out of traditional UK fishing grounds. The warming is linked to changes in the North Atlantic Current (an extension of the Gulf Stream). Scientists have found that cod recruitment (the number of young fish entering the population) is strongly linked to water temperature and plankton availability, both of which are affected by current patterns. This shift has forced fishing fleets to travel further north or target different species, showing how current changes can have real economic impacts on fishing communities.
Understanding ocean currents is becoming increasingly important for sustainable fisheries management:
The distribution of major ocean currents creates the foundation for the world's most productive fisheries. By understanding these patterns and how they're changing, we can better manage our marine resources for future generations.
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