Coral reefs are under attack by enormous numbers of starfish of the variety called 'crown-of-thorns', technically Acanthaster planci. These fish eat the soft tissues of the reefs by inverting their stomachs, liquifying, and absorbing the coral. Another threat is mats of green bubble algae produced by nutrification from sewage and farm runoff. Black band disease and an unexplained plague has virtually wiped out reef-cleaning sea urchins throughout the Caribbean. Coral bleaching is the result of mass death of individual corals and the colourful algae that share their exoskeleton, usually from heat stress to which they are sensitive within one or two degrees. Other diverse causes of coral decline are general marine pollution and environmental disturbances, soil erosion from land, diseases, tourism, physical damage from ships and human activities, and weather extremes (hurricanes, cold snaps, high waves, tides).
Coral reefs are increasingly under threat from human activities, particularly from coastal development and overexploitation as well as blast fishing and land-based pollution.
More than half of the world's coral reefs are threatened by human activities, with up to 80 per cent at risk in the most populated areas. While some may yet be saved, it is too late for many others.
The Great Barrier Reef off Australia; the reefs of Hawaii in Kaneoke Bay and Waikiki Bay; reefs off Guam, the Virgin Islands, Jamaica, and Bermuda; and reefs in the Indian Ocean: all are in various stages of destruction. African states with endangered reefs in the western Indian Ocean include Kenya, Tanzania, Mozambique, Mauritius and the Seychelles. Florida's barrier reef was calculated in 1986 to be dying at the rate of 4% a year. In 1991, parts of the reef were dying at the rate of 10% a year.
Record high sea temperatures in the first half of 1998 (the hottest year in six centuries) triggered the die off of tropical corals, more than 70% in some areas and from the Carribean and Indian Ocean to the eastern Pacific. El NiÃ±o and global warming were blamed in a US State Department study, which adds that even under the best conditions these reefs will need decades to recover.
Scientists also believe global warming is responsible for the destruction of coral reefs around the Pacific island nation of Fiji.
In 2002, the Australian Institute of Marine Science completed an atlas of sea temperatures over the past decade and amalgamated it with historical data to show 2002 was the warmest year for water temperatures off north-east Australia since 1870. While sea temperatures around the reef have not risen greatly over the past century -- about half a degree Celsius -- the coral have a very low tolerance factor of only one or two degrees. Unless the corals can adapt and become acclimatised then they will bleach. The Indian Ocean contains about 15 per cent of the world's mapped coral reefs, of which more than one-half is estimated to be at risk from human activities. Coral reefs in the northern Red Sea (in the Gulf of Aqaba and near the Gulf of Suez) and along the coast of Djibouti are also considered to be under a high degree of threat.
The use of cyanide for fishing, which has been increasing since the mid-1980s, has become so widespread, that it is destroying reef ecosystems and wiping out broad expanses of what ecologists say is the global epicenter of oceanic biological diversity.
As the largest archipelagic state in the world, Indonesia has now only seven percent of its coral reefs in good condition. According to a 1997 survey, estimated 58 percent of Indonesia's coral reefs had been heavily damaged and 35 percent partly damaged, largely because of human activity. Because the reefs provide vital shelter and breeding grounds for fish, and many of the poorest communities among Indonesia's population of 200 million depend heavily on fish for protein, the economic and social consequences of wholesale reef destruction could be devastating. For example, the Mentawai Islands along the southern coast of Sumatra in Indonesia are a picture of tropical paradise, but below the surface of the crystal-clear azure waters, there is an underwater wasteland. According to a 1996 survey, most of the reefs along the 1,280 kilometres long chain of islands have been completely destroyed and marine life was nonexistent. Possible causes of the destruction included dynamite and cyanide fishing, infestation by the coral-eating crown of thorns starfish and sediment runoff due to logging on some islands.
Greenpeace Australia claims that if current rates of climate change continue then the world's coral reefs might be wiped out in 100 years.