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SELECTED HUMAN THREATS |
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Overfishing
This type of grouper may be overfished in some parts of the Great Barrier—at least partly due to its popularity in Asia, where diners often order it live from restaurant aquariums.
Concerns for the coral trout aside, fishing is strictly regulated on the Great Barrier, but countless other reefs aren’t so lucky. Elsewhere many are obliterated by blast fishing—in which reefs are often blown to bits in hopes of an easy catch—and cyanide poisoning, the dark secret of the aquarium trade.
In cyanide fishing, divers generally squirt the toxin onto coral reefs, then search the newly dead reef for live-but-stunned fish—or so they hope. More than half of all fish caught this way die in the poisoning process or in shipping.
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Coastal development
Croplands (as on this farm near the town of Cairns) and development along northeastern Australia’s coastal plain have replaced many seaside wetlands, the natural filters for fresh water coming from the continent. Coupled with deforestation, overgrazing by livestock, and runoff from towns, farms, and industries upstream, this sends more sediments and nutrients flowing out toward the Great Barrier. The total has quadrupled since colonial times.
Corals can persist in surprisingly murky water as long as tides and currents periodically sweep the sediments off. It’s the nutrients that wreck a reef. Anything beyond moderate levels of nitrogen hurts growth and reproduction in corals while fertilizing free-floating algae that can smother their neighbors.
—Text adapted from "Kingdom of Coral" (NATIONAL GEOGRAPHIC Magazine, January 2001)
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SELECTED NATURAL THREATS |
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Crown-of-thorns sea star
Armed with poison spikes and an appetite for hard coral, adult crown-of-thorns sea stars can kill a reef. By projecting their stomachs out of their mouths and wrapping them around coral, they slowly devour it—or not so slowly if they’re out in force, which is increasingly the case.
No one’s quite sure why crown-of-thorns epidemics are on the rise. Some believe they’re part of a natural cycle, like wildfires. Others think humans inspire the outbreaks, possibly by overfishing species that cull juvenile crown-of-thorns sea stars.
Divers are sometimes trained to kill sea stars at popular dive sites, but there doesn’t seem to be a large-scale, responsible weapon against crown-of-thorns epidemics. For now, conservationists watch and wait—and hope afflicted areas bounce back.
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Warming
When stressed, corals expel the algae that live in their transparent outer tissues, exposing the corals’ limestone skeletons. The most common cause of this bleaching is abnormally high ocean temperatures. Other culprits: inflows of fresh water, high ultraviolet radiation, and changes in salinity.
Bleached coral isn’t necessarily dead coral, though, and it often regains its healthy color when conditions return to normal. But prolonged bleaching can lead to coral death—one more way global warming and periodic climate shifts such as El Niño can change the face of the Earth.
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