Unit C8 - Coral Reefs


Unit C8


Many developing countries have coral reefs in some of their coastal areas, and many rural people depend on coral reefs for some or all of their livelihood and food.

What are coral reefs?

Coral reefs are built near the surface in tropical seas by the accumulated skeletons of tiny animals (mostly corals) and plants (coralline algae). Wherever sunlight reaches a suitable bottom along a coast, or on the side of a volcanic island, corals will start to grow. Over many years they build a reef that fringes the shoreline (a fringing reef).

If the reef grows out farther from land, or the land sinks and the reef keeps growing, then a barrier reef is formed with a lagoon behind it.

If a volcanic island that started with a fringing reef sinks completely beneath the surface, while the reef keeps growing, then only a ring of reef around a lagoon is left behind, making an atoll.

Sometimes an atoll or coral reef is lifted out of the water, making a flat raised coral island.

Why are coral reefs important?

Coral reefs are a very productive type of ecosystem with many kinds of plants and animals crowded together. Even though the clear waters of the tropical ocean are very poor in life, coral reefs are able to collect and recycle the materials they need to grow. They are thus to the tropical oceans as an oasis is to a desert. This makes them of great scientific interest.

The reefs are important to the life of many coastal and island people. The fish, shellfish and other reef animals are an important source of food, providing essential protein for island diets. Many different kinds of fishing and food collecting are possible in reef areas, producing very high catches, although these can be reduced by overfishing if care is not taken.

As coral reefs grow, they add to the size of islands, making sand, rubble and rock that build the island or shoreline. They also take the force of the waves, protecting the land behind. They thus help to protect the land from storms and keep it from washing away. Many tropical islands are made entirely of coral reefs, and most others have been added to or protected by them. The relatively calm and productive lagoons of many islands are also created by the surrounding reefs. Since reefs are built by living plants and animals, anything that damages the life on the reef also slows or stops its growth, threatening the long-term future of the land.

It is therefore important to know something about what makes up a coral reef and how it works.


Corals are made up of tiny animals living in colonies of many thousands that grow together to produce the coral skeleton. Each coral animal or polyp has a central mouth and stomach surrounded by a ring of arms or tentacles with which it catches its food (tiny plankton animals) floating in the water.

Corals also get food from single-celled plants called symbiotic algae that live inside them and make food from sunlight just like other plants.

Reef-building corals only live in shallow water where there is enough light, and they will die if the water gets too dirty for light to reach them. Corals grow very slowly, so if they are broken or killed by pollution or dynamiting for fish, the reef will take many years to recover.

Many corals have branches and look like plants. Indeed, they are to the reef what trees are to the forest. Just as the forest birds disappear when the trees are cut, so do most reef fish go away when the corals are broken or killed.


Algae are simple plants that make most of the food for the coral reef. Some of the larger seaweeds, often called limu in the islands, are also eaten by people.

Many algae have stony skeletons like the corals, and they may be as important as corals in building the reef and in producing the white sand of coral beaches. The pink coralline algae help to cement the coral skeletons together to make a strong reef, and often form an algal ridge just where the waves break, thus protecting the more fragile corals behind.


There are many kinds of animals and fish that live on the reef and are important to reef ecology. Some eat mostly algae and other plants. Others eat corals or other animals. Still others eat dead material (detritus) and thus help to keep the reef clean.

Reef animals such as octopus, shellfish and fish are often collected at low tide, or at night with torches or lamps, and provide protein for island diets. Reef shells are used for handicrafts and are popular with collectors, but if too many are taken, this can upset the balance of reef life.

Coral reef ecology

The coral reef ecosystem is both very old and highly efficient, with many hundreds of kinds of fish, plants and marine animals crowded together in complex communities rather like modern cities. Reef life is efficient at recycling food and nutrients within the system, and each species has a special role to play, just like the many occupations of people living in a city.

The reef ecosystem requires a stable environment without much change in water quality or temperature; if that environment is disturbed, the reef may die or may take many years to recover.

A coral reef is built up over thousands of years by the skeletons of generations of corals and algae, yet man can destroy the living system of the reef in a few moments. Evidence from around the world suggests that many coral reefs are declining steadily in quality and productivity. Damaged and degraded reefs are now the rule around centres of population. If this trend continues, an important resource for many coastal people will be lost.

What is destroying coral reefs?

Life on a coral reef is a delicate balance of many factors, and as with any living community, it may go through natural changes. These may be random changes, cycles of growth and destruction that repeat over time, or progressive evolutionary developments, say from one type of reef or structure to another. Human activities can interfere in many ways with this balance. Some problems come from the use or misuse of reef resources. Overfishing or heavy spear-fishing (which selectively removes the large predatory fish) may lower fish populations, particularly of the most useful species. Collecting of shells, corals or aquarium fish can also upset population balances. Destructive fishing techniques like breaking coral, or using dynamite and poisons destroy the structure of the reef and can kill everything including the baby fish and the food on which the fish depend. Too much use of such techniques can turn a rich productive reef into a desert.

Many kinds of pollution can also damage or destroy a coral reef. The most important on continents and high islands is sediment and muddy water from soil erosion on land. As land is cleared the soil washes away and generally ends up in the lagoon or on the reef. The muddy water keeps light from reaching the plants on the bottom, and the sediment smothers the corals and other animals. Many cities and villages also empty their waste water and sewage onto the reef, changing the balance of nutrients and making the seafood unsafe to eat. Runoff from agricultural land may include pesticides and fertilizers. Ports, industries and power stations may cause serious local pollution from oil, industrial wastes, heat or heavy metals.

The reef can also be disturbed or damaged directly by dredging or construction. Even recreational activities like diving and boating can cause broken coral; in areas of heavy use, much of the coral can be broken over time. The reef can also be disturbed biologically by the introduction of species not previously found there. Sometimes there can be population explosions such as the coral-eating "crown-of-thorns" starfish, Acanthaster, in the Pacific, or population collapses like the epidemic that killed most Diadema sea urchins in the Caribbean, which may have both natural and human causes. There are an increasing number of diseases killing off important corals, and unusually warm water, perhaps due to global warming, can cause coral bleaching.

Use and Management of Coral Reefs

Traditional coastal fisheries made good use of coral reef resources, based on detailed knowledge of many reef fish and other animals. In many cases the amount of fishing was controlled, often by taboos and other practices, and some of each kind of fish was taken at different times so that the balance of reef life was maintained. The limitations of traditional fishing technologies and sailing canoes also meant that many fish escaped and that more remote regions were rarely fished.

Modern technologies like nets, boats and diving equipment have made fishing easier today, but they have not made it possible for more fish to live on the reef, and after an initial period of success, catches have often declined with overfishing, sometimes even to less than traditional levels. Many subsistence fisheries seem to have approached the maximum sustainable catch on local reefs, and attempts to develop large-scale commercial or export fisheries in coral reef areas have generally failed.

The increasing damage to coral reefs by development, pollution and destructive fishing has also reduced the amount of fish a reef can produce, while the demand for fish has generally increased. In the Pacific, the risk of ciguatera fish poisoning has also made many fish resources unusable.

Coral reefs are difficult to manage because they cannot be fenced in or protected like land areas. They are influenced both by the open ocean and often by the adjacent land, as well as tides and currents that can bring damaging factors from far away.

Often traditional management rules can serve as guidelines for modern management practices in reef fisheries. When new techniques are introduced, their use must be kept within the limits of the resource. The prohibitions on destructive techniques like dynamiting must be strictly enforced, especially by the fishermen themselves.

Where reefs are subject to damage from the land, such as erosion, fresh-water run-off or pollution, controlling activities on the land is the only solution.

The creation of coral reef reserves or other protected areas can provide reservoirs where healthy populations can multiply and spread to replace fish caught in adjacent areas. The breeding sites and migration routes for important fish species may also need to be protected.

Many island coral reefs have already been damaged by human activities. Only careful management can prevent further damage to reef resources and maintain the productivity of coral reefs on into the future.

What can you do?

If you are a user of coral reef resources, you must realize that the reef is a living community and learn to respect its limits.

Fishermen should just catch what they need, being sure to leave some fish behind to replace what they have taken. Avoid catching animals while they are reproducing; wait at least until they have left a new generation behind. If the catch size gets smaller and smaller, fish for something else and let that area or species recover.

Villages or family groups with traditional fishing grounds can decide together on the rules needed to manage their reef fishery wisely. Such rules might include a ban or limit on spear guns, requiring nets with a mesh size large enough to let little fish escape, and the prohibition of damaging fishing techniques. Access to certain areas can also be limited, and reserves or closed seasons established to protect the breeding areas and times of vulnerable species.

Even if you do not live on the coast, your actions may be damaging coral reefs. Cutting too much forest, clearing the land without erosion controls so that it washes away in heavy rains, overuse or spillage of fertilizers or pesticides, pollution of water by village or city wastes, are ways in which everyone living near a coast or on an island may contribute to hurting coral reefs and other coastal resources. Only when everyone observes the principles of good environmental management can resources such as coral reefs be preserved.


What do coral reefs do for coast and islands?

How are coral reefs made?

What do the coral reefs look like in your area?

Are there many fish on your reefs?

If not, what has happened to them?

What was fishing like in your grandfather's time?

Are any traditional fishing practices or controls still used today?

Have reefs in your area been damaged?

If so, what has damaged them?

What can be done to manage your reefs more wisely?

Instructions for trainers in the use of this unit

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Last updated 21 July 2007