WRECKING THE REEFS

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The good news is that the Philippine government has started to crack down on dynamite and cyanide fishing. The bad news is that those destructive practices are just the latest in a string of insults to reefs, and not necessarily the most serious. Far more troubling to biologists is the fact that groupers and other valuable reef fish are being harvested at a critical point in their reproductive cycle. With satellite navigation systems to guide them, fishing boats are homing in on areas where large numbers of the fish have gathered to spawn. Already, says University of Hong Kong fish biologist Yvonne Sadovy, fishermen have eradicated entire groups of sexually mature adults, which jeopardizes the future of the fish and also the future of the reef.

Large fish, it turns out, are among the most critical of all reef inhabitants, especially herbivores like parrotfish that eat seaweed. Fast-growing seaweed is always threatening to engulf the reefs, but it is usually kept in check by grazing fish. About a decade ago, a team of marine scientists conducted an experiment off the coast of Belize that proves this point. To keep fish out, the researchers surrounded a section of coral the size of a small pasture with a chicken-wire fence. Within 10 weeks, they found, the area inside the fence had been completely overgrown by seaweed and many of the corals had died.

A reef is like a complex machine with many redundant components; when one malfunctions, another usually takes over. By the middle of the past century, for example, Jamaica's expanding human population had devastated the stock of seaweed-eating fish. Still, the reefs that surrounded the island looked healthy. Why? The answer, says the Smithsonian's Jackson, is sea urchins, which are also herbivores and which temporarily filled in for the missing fish. In 1983, however, the urchins succumbed to a mysterious disease. All of a sudden Jamaica's reefs crashed. With no urchins to crop back the seaweed, Jamaica's corals, once considered the jewels of the Caribbean, were soon smothered by a vegetative thicket.

Unhappily for reefs, humans upset the balance between corals and their competitors in many ways. Consider the erosion that accompanies deforestation and agriculture. No longer restrained by tree roots, tons of soil laden with nitrogen and phosphate washes into rivers and then sweeps into the sea, forming a muddy plume that may be hundreds of miles long. As this nutrient-rich water flows over a reef, it stimulates the growth of all kinds of algae--including the microscopic diatoms and dinoflagellates that nourish such reef animals as the crown-of-thorns starfish. In recent years hordes of these coral-devouring starfish have infested Australia's 1,200-mile-long Great Barrier Reef, and soil-borne nutrients are at least partly to blame.