Rainforests of the Sea

by Kieran Mulvaney

Originally published in BBC Wildlife, August 1997.

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Many of the world's coral reefs are in a critical condition - all are under threat. In the International Year of the Reef, Kieran Mulvaney assesses the problems facing the ocean's richest ecosystems.

To some, the open ocean exists as a vast wasteland, an aquatic desert largely devoid of life beyond the occasional playful dolphin or breaching whale. But this image, though common, is wrong: there is life in the ocean, teeming through the waters in a dazzling array of diversity. Much, however, is invisible: literally, as in the billions of microscopic organisms found throughout the water column, and figuratively, as in the many larger species that dwell in the ocean's darkest depths or the farthest reaches.

Coral reefs are a different matter. No need here for any expensive equipment to scan the tiniest water droplets or to plumb the ocean depths. As long as you're on the coast in the right parts of the world, you need nothing more than a mask and snorkel to enter the realm of the reef.

And a coral reef can be host to an amazing richness of life. Indeed, reefs are home to by far the greatest abundance of species in any marine ecosystem. A recent estimate suggests that there may be about 93,000 species already described from coral reefs. Many more remain to be identified, and the total number of species on coral reefs could be as high as a million. For example, about one third of the world's marine fishes are found on reefs, and, at One Tree Reef in Australia, nearly 150 species of fish have been recorded in an area of less than 50m2.

Reefs, both by themselves and in combination with ecosystems such as mangroves and seagrasses, provide important feeding, breeding and nursery areas for a wide range of marine life They create sheltered lagoons, protecting coastlines against wave erosion. Millions of people depend on reefs for their livelihood, and the combination of their spectacular array of life and their proximity to the coast has increasingly made them attractive tourist destinations

Unfortunately, this very same combination is also causing reef systems around the world to be degraded and destroyed at an alarming rate. According to the World Conservation Union (IUCN), as much as 10 per cent of the world's coral reefs has been degraded beyond recovery, and another 30 per cent is likely to decline within the next 20 years. Those at greatest risk are in South and South-east Asia, East Africa and the Caribbean, but the problem is widespread: out of 109 countries where reefs are known to occur, significant reef degradation has occurred in 93.

The abundant fish life on reefs has long been exploited for food. In earlier times, the scale and nature of the fishing meant that fish populations didn't suffer: but in more recent years, changes in fishing techniques and the commercialisation of fisheries have, among other factors, led to sharp declines in many species.

Dynamite fishing, for example, takes place in many areas. One blast can destroy an area of reef 10 metres in diameter. More recently, the use of sodium cyanide has become widespread as a means of stunning fish to supply the aquarium trade and also to satisfy increased demand for live fish in Asian restaurants

On reefs around the world, fisheries that once concentrated on valuable fish such as grouper and snapper now yield less desirable parrotfish, triggerfish and damselfish. Many of the spawning grounds of the Nassau grouper, where huge numbers aggregate for a few days each year, have been found by fishermen, who can remove the major part of an entire breeding population in a very short space of time. This species is now scarce throughout the Caribbean and. together with the Napoleon wrasse, was among the first species of coral reef fish to be listed as globally threatened on the 1996 IUCN Red List.

The queen conch, once an important part of the subsistence diet in the Caribbean, has also been virtually eradicated from many reefs in the region and is now an expensive, luxury food. Similarly, the giant clam, which filled an important dietary and ceremonial role in the Pacific, has been over-exploited across vast areas by an industry seeking to feed an Asian appetite for sushi and aphrodisiacs.

Perhaps even more serious than such direct exploitation, reef habitats are also under enormous threat from construction and development. More than half of the world's population lives on or near the coast, and both the proportion and absolute numbers are expected to continue growing. This increasing crush of people has had predictable consequences for reefs. In some places, new development has literally been built directly on top of reef systems; elsewhere, reefs have been blasted, bulldozed or mined for use as materials in, or to make way for, construction. More than 500,000 tonnes of coral sand are excavated annually from Mauritius alone.

There are other impacts. Pollution from small-scale, but repeated, oil spills, agrochemical run-off and heavy metals is a problem in many areas. Domestic and industrial pollution from Bangkok and Pattaya, for example, has almost totally destroyed the reefs in the northern Gulf of Thailand. And, when sewage, animal waste and fertiliser are carried downstream or simply discharged straight into coastal waters, seaweed and phytoplankton thrive, which reduces the sunlight reaching the corals.

Sedimentation, which arises from activities such as deforestation and dumping sand to make beaches, is another serious threat. The sediment smothers reefs and reduces the amount of sunlight penetrating the water, which is devastating for corals, whose cells contain photosynthesising algae. Death for corals can come in a matter of days or even, in extreme cases, hours.

Ironically, the growth of the tourist industry in reef regions, prompted by the corals' spectacular beauty, can add to the problems those reefs face. Damage by careless swimmers, divers and boaters can all destroy chunks of reef; of even greater concern are the additional sewage, wastes and beach erosion resulting from siting tourist facilities and resorts so close to reefs. Coral cover off the western coast of the Gulf of Thailand is reportedly declining by 20 per cent annually, due to tourist-related coastal clearing and sewage pollution.

A number of steps are being taken to try to limit, arrest and even reverse reef damage. 1997 has been declared the International Year of the Reef in an attempt to raise awareness and generate support for a global coral reef monitoring programme. Because reefs are found in so many different countries and are subject to a varying combination of pressures, each with their own specific social, environmental and economic causes, it is impossible to write a blanket prescription that will cure all corals' ills at a stroke.

Scientists, therefore, recommend an integrated approach which includes the establishment of marine protected areas, management of tourism and development activities, education, the generation (where appropriate) of sources of income other than those based on exploiting reefs, and strict regulation of the reef exploitation that does take place.

But, even as steps are being taken to address and implement such important measures, the difficulty of the task is highlighted by new problems, while our response is hampered by our poor knowledge of the coral reef world. Since the 1950s, plagues of the crown-of-thorns starfish have been reported from coral reefs across the Pacific. These huge starfish eat coral, and, when they occur in plague proportions, they can destroy an entire reef. Scientists are still unclear as to whether these plagues are entirely natural or have been exacerbated by human activities.

A rather different story was observed in the Caribbean in I983 when an unknown pathogen wiped out the long-spined sea urchin on some reefs, the effects were devastating - these urchins are important grazers, and their loss meant the prolific growth of algae, which killed vast areas of coral. The worst effects appeared to be in areas where overfishing had removed most of the grazing fishes, which might have been able to do the work of the urchins.

The corals themselves, too, are susceptible to poorly understood problems, such as black-band and white-band disease, which have destroyed vast areas of reef. Another issue of concern is coral bleaching. This is a decline of photosynthetic algae, or algal pigment, in the cells of reef corals, which often results in the coral's death. Though this can occur naturally, for example in coral damaged by storms, it is now happening much more frequently and, at times, over much larger areas. There are a number of possible causes, but the one attracting the most attention is increased sea temperatures as a result of global climate change.

Coral reefs have been called 'rainforests of the sea', a tribute to the abundance and diversity of life they contain. The comparison is, unfortunately, apt in another way. Unless action is taken soon, and applied widely, reefs, like rainforests, will continue to disappear before our very eyes.