Storms predators & climate change
Reefs are frequently disturbed by natural events. Hurricanes and tropical storms can reduce reefs to rubble. Corals are often affected by diseases, such as black-band disease, which spreads over colonies, progressively killing the polyps. The crown-of-thorns starfish feeds voraciously on corals and, if present in large numbers, can reduce a reef to a mass of dead coral skeletons in just a few weeks. Global climate change, or the enhanced greenhouse effect, may cause increases in sea temperature and sea level, as well as changes in ocean current patterns, that could damage coral reefs.

Shells on shelves
Corals and shells are collected all over the world to sell as souvenirs or to make into jewellery and other handicrafts. Several kinds of mollusc are now rare on reefs because of over-collecting. In many countries, coral is also mined from the reef and used to build houses and make roads, or is burnt to make lime. Where this happens, reefs are often stripped bare. Sustainable management of reefs will mean regulation of these industries and better enforcement of existing legislation to ensure that both the resources and the people who depend on them have a long-term future.

A paradise for tourists 
Millions of tourists visit coral reefs each year and, in many countries, reef-related tourism is now the main source of foreign exchange. But corals are easily broken by trampling when people walk out to the reef; snorkellers and divers may kill polyps simply by touching coral colonies; and anchors and ship groundings can destroy large areas of reef. Though tourism can damage reefs, carefully managed, it can play a major role in saving them. The tourism industry can promote and assist in the development of marine parks, mooring buoy programmes to prevent anchor damage, public awareness campaigns, and local initiatives to promote sustainable coastal management.

Coral Bleaching
When corals undergo certain kinds of stress, much of the zooxanthellae -- the symbiotic algae that provide coral polyps with nutrients -- are expelled from the coral tissue. Ultimately, weakened corals may die. Bleaching is a frequent symptom of pollution-induced stress, as well as a response to natural factors such as changes in water temperature, salinity levels, and possibly ultraviolet light.the reef; snorkellers and divers may kill polyps simply by touching coral colonies; and anchors and ship groundings can destroy large areas of reef. Though tourism can damage reefs, carefully managed, it can play a major role in saving them. The tourism industry can promote and assist in the development of marine parks, mooring buoy programmes to prevent anchor damage, public awareness campaigns, and local initiatives to promote sustainable coastal management.

During the El Niño of 1982-83, large areas of coral reef around the world were severely damaged by high water temperatures, which resulted in coral bleaching. Scientific studies have linked bleaching events to temporary "hot spots," local areas of unusually high temperatures caused by changes in atmospheric circulation during the El Niño oceanographic events.The effects of the severe El Niño that started in 1997 have yet to be documented.

Cyanide Fishing
The use of cyanide to stun and capture live reef fish began in 1960 in the Philippines to supply the market for aquarium fish in Europe and North America, a market worth more than $200 million a year. Since the late 1970s, the poison has also been used to capture larger live reef fish (primarily grouper species) for sale to specialty restaurants in Hong Kong and other Asian cities with large Chinese populations. Selected and plucked live from a restaurant tank, some species can fetch up to $300 per plate, and are an essential status symbol for major celebrations and business occasions. As the East Asian economy boomed over the past several decades, live reef food fish became a business worth some $1 billion annually.

Sediment Sewage & Spills
Changes in land use, such as forest clearance and intensification of agriculture, cause increased erosion. Soil is washed into rivers and out to sea, silting up reefs along the coast. The water becomes murky and stops sunlight, vital for coral growth, reaching the reef. If sediment settles on top of corals, it may smother them. This can also happen during the dredging of channels and harbours. Sewage and agricultural fertilisers, both of which often reach the sea, can damage reefs. An increase of nutrient levels in sea water, encourages seaweed growth at the expense of corals. Many other wastes from factories, towns and ships are also discharged at sea where they may poison corals and other reef life. Numerous activities, taking place even far inland or on the high seas, may have a deleterious impact on reefs. Managing reefs thus means improving forestry and agriculture methods and disposing of waste and effluent in better ways.

Coastal Development
The growth of coastal cities and towns generates a range of threats to nearby coral reefs. Where space is limited, airports and other construction projects are built upon reef communities. Dredging of harbors and shipping channels and the dumping of spoils result in the outright destruction of these habitats. In many areas, coral ecosystems are mined for construction materials -- sand and limestone, which is made into cement -- for new buildings.

Overfishing
Many reef species, including giant clams, sea cucumbers, sharks, lobsters, large groupers, snappers, and wrasses, are now fetching high prices both on domestic markets and internationally. In order to capture these "target species," commercial fisheries operations are moving further and further afield, and now regularly visit even the most remote reefs in the world. In many areas such harvesting is clearly unsustainable.

Fishers sweep reefs of their valuable species and then move on, eliminating entire populations within the areas they leave behind. Two examples from the Philippines illustrate this threat. During the 1960s and 1970s, several giant clam species became locally extinct due to overharvest for food and for their large shells, which are popular as decorations, sinks, and bird baths. As recently as 1987, the sea urchin Tripneustes gratilla was found in dense populations across a 24-square-kilometer sea grass bed on a reef flat in Bolinao. Exploitation rates increased suddenly with the appearance of a trader from China and by 1995 the sea urchin was believed to have become locally extinct.