Save the gardeners, save the reef

The fate of the world’s coral reefs may hang on a group of weed-eating fish, an Australian scientist has warned.

Worldwide, coral reefs are being smothered beneath a green tide of weed which is flourishing as humans pour nutrients into the oceans from erosion, agriculture, sewage and development.

But a major factor in the reefs’ decline may also be the uncontrolled slaughter and removal of parrot fish and surgeon fish, the ‘gardeners of the reef’, says Professor Dave Bellwood of the Australian Research Council Centre of Excellence for Coral Reef Studies and James Cook University.

“We are only just beginning to realize what a vital role these groups of fish play in keeping reefs clean, healthy and free of weed.

“Remove them, and you as good as remove the reef itself,” Prof. Bellwood says. “Without the fish to mow the weed, it soon takes over completely from the coral.”

Worldwide the larger herbivorous fish are now the targets of a mass plunder by fish trappers, spear fishermen and gill netting, as the highly-prized carnivores – like coral trout and snappers – are hunted out.

In Australia, the picture is less bleak, with traps illegal, no-fish zones and an ethos in spear-fishing clubs not to hunt herbivores – but the situation still needs watching closely, he cautions.

“People are generally unaware how essential these groups of fishes are to the upkeep and protection of the reef. And, as scientists, we still do not understand clearly which are the critical functional groups – the most important ones for preserving the health of the coral ecosystem.

“Australia faces the loss of many of the big weed-removers – the dugongs and green turtles. On reefs, the job is almost entirely down to the fishes. If we lost them too, it would spell tragedy.”

For instance, he says, some herbivores in turn depend on mangroves as nurseries for their young. The removal of mangroves may thus lead to loss of herbivores and, ultimately, to loss of coral.

Experiments have shown that the removal of grazing fishes from an area of reef leads to the coral becoming completely smothered by weed in a short time.

So far no-one in the world has yet succeeded in reversing the process when a coral reef turns to weed, but the scientists at the Coral Reef Centre remain optimistic.

“I’d say we simply don’t know enough about the role of the different species, and which ones will be most effective in ‘mowing’ a reef that has been engulfed by weed. That is what we are exploring now,” Prof. Bellwood says.

In the past 50 million years, coral reefs have shown extraordinary resilience, coping with massive swings in climate and ocean level.

“But somehow, it seems, they have built up a dependency on this group of weed-eating fish – and maybe on other groups that perform important functions such as the holothurians which sweep the seabed. It looks as if coral cannot survive for long without its constant gardeners.”

Professor Bellwood says that if the Centre’s research can identify the key species and work out a strategy to get them to remove the weed from an infested reef it would bring new hope for many of the world’s reefs which are now in dire condition.

More information:
Professor David Bellwood, CoECRS, +61 7 4781 4447, David.Bellwood@jcu.edu.au
Professor Terry Hughes, Director, ARC Centre of Excellence for Coral Reef Studies, +61 7 4781 4000
James Cook University Media Office,+61 7 4781 4586