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examBoard: Cambridge
examType: IGCSE
lessonTitle: Cold and Warm Currents Effect on Fisheries
Ocean currents are like massive rivers flowing through our seas, moving enormous amounts of water around the planet. These currents have a huge impact on marine life and are particularly important for the world's fisheries. They determine where fish live, feed and breed, making some areas of the ocean incredibly rich in fish while others remain relatively empty.
Key Definitions:
Cold currents flow from polar regions toward the equator, bringing cooler water to warmer regions. They typically flow along the western edges of continents. Examples include the Humboldt (Peru) Current off South America and the Benguela Current off southwest Africa.
Warm currents flow from the equator toward the poles, carrying warmer water to cooler regions. They typically flow along the eastern edges of continents. Examples include the Gulf Stream in the North Atlantic and the Kuroshio Current off Japan.
Ocean currents don't just move water around - they create the perfect conditions for thriving marine ecosystems. Understanding how they work helps explain why certain areas are so good for fishing.
One of the most important ways currents affect fisheries is through upwelling. When winds push surface water away from coastlines, deep, cold water rises to replace it. This upwelled water is packed with nutrients that have accumulated at the ocean bottom from sinking organic matter.
Cold currents bring nutrient-rich water to the surface through upwelling, providing essential nutrients like nitrogen and phosphorus.
These nutrients fuel explosive growth of phytoplankton (tiny marine plants), which multiply rapidly in the sunlit surface waters.
Small fish feed on the plankton, larger fish eat the smaller fish, creating rich fishing grounds that support commercial fisheries.
Cold currents create some of the world's most productive fishing grounds. The combination of nutrient-rich water and optimal temperatures creates perfect conditions for massive fish populations.
Running along the west coast of South America, this cold current creates one of the world's most productive marine ecosystems. It supports massive anchovy fisheries off Peru and Chile, which at their peak accounted for about 20% of global fish catches. The nutrient-rich upwelling here creates an explosion of plankton that feeds huge schools of anchovies and sardines.
Flowing northward along the southwest coast of Africa, this cold current creates another major upwelling system. It supports valuable sardine, anchovy and hake fisheries. The Benguela system is known for its high primary productivity and oxygen-depleted zones that create unique ecological conditions.
Warm currents support different types of fisheries, often characterized by greater biodiversity but sometimes lower overall productivity than cold current regions.
This powerful warm current in the North Atlantic influences fisheries from Florida to Norway. It carries tropical species northward and creates temperature boundaries that concentrate fish. The meeting of the Gulf Stream with the cold Labrador Current creates the famous Grand Banks fishing grounds off Newfoundland, historically one of the richest cod fisheries in the world.
Known as the "Black Current," this warm flow runs northeastward past Japan and merges with the cold Oyashio Current. Where these currents meet, they create highly productive fishing grounds for Pacific saury, squid, sardines and tuna. This convergence zone is one of Japan's most important fishing regions.
The Peruvian anchovy fishery, supported by the Humboldt Current, demonstrates both the potential and vulnerability of current-dependent fisheries:
Some of the most productive fishing grounds occur where different currents meet. These convergence zones concentrate nutrients and create thermal boundaries that many fish species prefer.
Where cold polar water meets warmer subtropical water, these boundary zones create highly productive fishing grounds. The Antarctic Convergence supports huge krill populations that feed whales, seals and commercially harvested fish like Patagonian toothfish (sold as Chilean sea bass).
Where strong currents like the Gulf Stream and Kuroshio run along continental edges, they create productive fishing zones. The eddies and meanders in these currents concentrate food and create habitat for pelagic fish like tuna and billfish.
Current-dependent fisheries face several challenges that threaten their sustainability:
Rising ocean temperatures are altering current patterns and causing fish populations to shift. Some traditional fishing grounds are seeing declining catches as fish move toward cooler waters.
Many current-dependent fisheries have been overharvested, reducing their resilience to natural fluctuations in current strength and nutrient upwelling.
Coastal pollution can disrupt the delicate balance of nutrients in upwelling zones, potentially reducing productivity even when currents are functioning normally.
The North Sea cod fishery illustrates how changing current patterns affect fisheries:
Managing fisheries that depend on ocean currents requires understanding both the natural variability of these systems and human impacts. Effective approaches include:
Understanding the relationship between ocean currents and fisheries is becoming increasingly important as climate change alters these systems. By recognizing how currents create productive fishing grounds and how these systems are changing, we can work toward more sustainable management of these vital marine resources.
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