A Slickness Unto Death

By Sarah Richardson|Thursday, June 02, 1994
RELATED TAGS: ECOSYSTEMS
In which what should have been a ritual of renewal--the annual spawning of corals on an Australian reef--ends instead in mass destruction.

A little after 10:30 P.M. on March 27, 1989, biologist Chris Simpson was returning to shore after having watched the corals spawn on Ningaloo Reef. He and his colleagues from the Environmental Protection Authority in Perth had not been alone on the reef that night; there were also tourists in glass-bottomed boats equipped with underwater torches. All had come to this remote marine preserve, near Coral Bay on Australia's western shore, to see one of nature's most spectacular mating rituals. For a couple of evenings in early fall, between 8 and 10 P.M., the corals on Ningaloo Reef spawn en masse, spewing millions of sperm and eggs into the calm waters of the lagoon. The buoyant bundles rise like an upside-down pastel snowstorm toward the surface. There they break apart and form a coral slick: a cloud of jostling gametes, all looking for a mate.

The lagoon was very calm that night, Simpson recalls. The coral slicks were all over. As we got closer to shore, we noticed all the moray eels at the surface with their mouths out, gasping for air. That's very unusual. And when we got to the shore we noticed all sorts of fish flapping around in the shallows, obviously in major distress.

Over the next couple of days, as the corals continued to spawn, the slicks spread across the lagoon. And the death toll mounted. We noticed fish everywhere--fish were floating in the lagoon, and millions of fish were washed up on the beach, says Simpson. We couldn't dive, because the water was absolutely putrid with all this decaying matter. Eventually the water cleared up, allowing Simpson and his colleagues to survey the damage on the bottom. Most of the corals, most of the clams, most of the crabs and worms--everything was dead, he says. It was just complete devastation.

Corals are famously sensitive to environmental insult. Sewage outfalls kill them; clumsy divers kill them; and a slight change in the water temperature kills them. But what Simpson and his colleagues have now documented for the first time on Ningaloo Reef, where roughly half the corals died in 1989, is a much stranger scourge: mass self-inflicted death by spawning.

The cause of the die-off, Simpson explains, was quite simple. The Ningaloo corals spawn after the March full moon, at neap tide, when tidal motion is at a minimum. The stillness traps the eggs and sperm in the lagoon, allowing them to meet--typically within a couple of hours of spawning. Over the next few days, if all goes well, wind-driven waves then sweep the coral larvae out to sea through channels in the reef.

In 1989 all did not go well. There was no wind at all when the corals spawned, and the waves were unusually low. As a result there was simply too little circulation in the lagoon to break up the coral slicks and move the larvae out. Instead the slicks coalesced into a single huge slick--it covered more than one square mile--that gradually drifted into the shallow part of the lagoon.

There the rapidly growing coral larvae proceeded to suck most of the oxygen out of the shallow water. That is why the fish went belly-up-- they were asphyxiated; why the coral larvae themselves died, in milky patches that dotted the lagoon; and why their parents on the reef died, too. Simpson estimates it may take Ningaloo Reef and its lagoon between 10 and 20 years to recover.

Now that the phenomenon of mass coral suicide has been documented at Ningaloo, he says, it may turn out to have been important to the evolution of other coral reefs as well. About 2,300 miles west of Darwin, Australia, are the Cocos Islands, two coral atolls that Darwin visited in the Beagle. Simpson says that historical records kept by the Malay islanders describe major fish kills that sound suspiciously like what he observed at Ningaloo Reef. The islanders call such events air busuk, which is Malay for bad water.

If you go to the Cocos Islands now, says Simpson, what you find is that compared with a lot of other reefs in the Indian and Pacific oceans, they have a very impoverished fauna. There are not as many fish varieties; there's not as much coral. Yet when Darwin was in the Cocos in the 1830s--when he formed all his opinions on coral atolls on one visit there--it was very luxuriant.
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