Status of the Worlds Coral Reefs
Executive Summary

HOW CORAL REEFS RESPOND TO STRESS

Natural Resilience of Coral Reefs to Stress

Recent reports from the Global Coral Reef Monitoring Network (GCRMN), Reef Check and many other projects indicate that coral reefs are under considerable stress and are experiencing considerable damage. Coral reefs have been resilient ecosystems since the Mesozoic (about 200 million years ago), surviving major environmental events such as ice ages, meteor strikes, and large changes in solar activity. Notwithstanding these events, coral reefs have recovered to form the extensive reefs we see today, although recovery may have taken thousands to hundreds-of-thousands of years. For example, during the last ice age (end of the Pleistocene) sea levels fell by over 100 m, killing all existing coral reefs, but corals continued to grow on continental margins and seamounts. When sea levels started to rise 10,000 years ago, corals invaded continental shelves and island slopes and, over the last 6000 years of relatively stable sea level, have been forming new reefs. Coral reefs also have the capacity to regenerate rapidly after catastrophic tropical storms, plagues of the coral-eating crown-of-thorns starfish, and severe bleaching. Recovery often takes 15 to 20 years. However, over the past 50 years, there have been major increases in stresses to coral reefs from direct and indirect human activities. These stresses are threatening the existence of reefs in some areas, and will diminish the value of reefs in other areas. Fortunately the corals on vast areas of remote reefs are unlikely to be severely affected. The same cannot be said about valuable reef fisheries resources.

Natural Stresses to Coral Reefs

The major stresses to reefs are storms and waves, particularly tropical storms and cyclones (called hurricanes in the Atlantic; cyclones in the south Pacific and Indian Oceans; and typhoons in the north Pacific). These cause major intermittent damage to reefs, particularly to those reefs that rarely experience these storms. For example, Guam in the northwest Pacific is hit by one typhoon a year on average, such that the corals are stunted and robust; whereas reefs in the eastern Pacific, such as in French Polynesia and the southern Caribbean, rarely experience such storms, with the result that strong waves from the infrequent storms smash the fragile coral communities.

Freshwater runoff damages reefs in semi-enclosed bays and lagoons of the larger Pacific islands (e.g. Kaneohe Bay in Hawaii) by lowering salinity and depositing large amounts of sediments and nutrients. Reefs are also damaged by volcanic activity (earthquakes, volcanic lava flows, severe uplifting) in the Pacific, for example in Vanautu, the Solomons, Papua New Guinea, the Philippines and Indonesia. Coral bleaching has been particularly notable recently, and particularly damaging from 1997 to 1998. While bleaching is a response by corals to many stresses, the recent apparent increase in incidence and severity may be a foretaste of global climate change (see below).

The major biological stresses on reefs are predation by the crown-of-thorns starfish, and diseases. Starfish plagues can outbreak on reefs in the Pacific and Indian Oceans, and often reappear at 12 to 20 year intervals. In the Caribbean, coral diseases have been particularly devastating in some areas. There is now considerable speculation that the incidence of both these stresses has been exacerbated by human activities.

Human Stresses to Coral Reefs

Increases in human populations and economic activity in the tropics over the past 50 years have resulted in increasing pressures on adjacent reefs. The major damaging factors to reef corals are: pollution from excess sediment and nutrients because of poor land-use practices on high islands; agriculture; industries; urban sewage; and over-fishing. The major stress to remote reefs is from over-fishing, particularly the use of destructive methods in the Indo-Pacific over the past 10 to 20 years. In 1997, the first GCRMN/Reef Check global coral reef survey revealed that most reefs show clear evidence of local extinction of target species, and obvious damage from blast and poison fishing in the West Pacific. The value of, and demand for, reef fisheries products has increased rapidly, particularly for export to East Asia. The surveys showed that key indicator species, such as giant clams, lobsters, sea cucumbers, pearl shell and trochus, and reef sharks have been removed. Now high-priced fish such as grouper, humphead wrasse, snappers, and parrot fish are being removed from reefs throughout the Indo-Pacific through the use of fine-mesh gill nets and traps, dynamite (usually home-made), and poisons, such as cyanide and bleach. Subsistence fishing is depleting fish stocks in the Caribbean, particularly through the use of fine-mesh traps and nets.

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Last updated - 7 December 98

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