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The continental shelf makes up just 8 percent of the ocean's surface area—yet these shallow waters contain 90 percent of the global catch of fish. People once assumed that when they fished farther out to sea, the catch would become many times greater. But compared to the densely packed continental shelf, the rest of the ocean is a vast, barren desert.
What makes the continental shelf so productive?
Shallow And Bright
Out beyond the continental shelf, fish and other living things are relatively few and far between. The waters close to shore support more life because they contain more nutrients.
The shallow waters of the shelf are bathed in sunlight. This light fuels the growth of algae, the main source of food for ocean animals. The tiny animals that eat algae in turn become food for large schools of fish and other organisms, and a food chain develops.
Over the deep ocean, when food sinks from the upper, sunlit layer, it falls to the deep-sea ecosystem below. But over the shallow continental shelf, food that sinks generally stays within the continental shelf system. Nutrients that collect on the bottom provide food for many small animals, which are eaten by fish. When these fish die, the whole cycle repeats.
Flush With Food
Continental shelves are flooded with nutrients not available in other parts of the ocean.
Not only does the shallow bottom keep fallen food from leaving the continental shelf—but new nutrients are also constantly being added.
Runoff from the land brings nutrients pouring into coastal waters, as rivers and rainwater keep the continental shelf flush with food.
Even more important sources of nutrients in some areas are currents of deep water that sweep over the shelf. This upwelling carries a wealth of nutrients that sank farther out to sea.
In waters enriched by upwelling, much more algae can grow, providing food for huge numbers of marine animals. In the richest areas, such as Georges Bank, currents of cold water mix with warm water, creating ideal growing conditions for lobster, scallops, oysters and many species of fish.
BY THE NUMBERS: Continental Shelf Facts
Modern trawlers are equipped to haul in 100 tons of fish in an hour—as much as a 17th century fishing boat could catch in a year. Nearly 1/3 of the total world catch is discarded as unwanted "bycatch," which includes animals that are too low in price, too small or that exceed legal quotas. Throughout the world, 200 million people rely on fishing for their livelihood. Dragging trawling nets over the sea floor affects 150 times more land than is cleared annually by clearcut logging. |
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