Preserving an intact population of weed-eating fish may be vital to saving the world's coral reefs from being engulfed by weed as human and climate impacts grow.
A new study by researchers at the ARC Centre of Excellence for Coral Reef Studies has found weed-eaters like parrotfish and surgeonfish can only keep coral reefs clear of weed up to a point. After the weeds reach a certain density, they take over entirely and the coral is lost.
For some years researchers have pinned their hopes on the ability of weed-eating fish to keep the weeds at bay while the corals recover following a major setback like bleaching, a dump of sediment from the land, or a violent cyclone.
However the latest work by Dr Andrew Hoey and Professor David Bellwood at CoECRS and James Cook University shows that once the weeds reach a certain density, the fish no longer control them, and prefer to graze less weedy areas. "As a result, the whole system tips from being coral-dominated to weed-dominated," Andrew says.
"And our work shows that it doesn't take a very high density of the fleshy seaweeds like Sargassum to discourage the fish, a patch of weed the size of a back garden could be enough to trigger a change. The fishes show a clear preference for grazing more open areas."
Coral reefs are in decline worldwide, with many of them especially in the Asia-Pacific region showing 'phase shifts' from being coral-dominated to degraded states dominated by large fleshy seaweeds.
"In countries where people harvest the weed-eating fishes with spearguns, nets and so on, like Fiji, we are seeing a fundamental change in the nature of reefs from coral to weeds," Andrew says. "In Australia where there is much less harvesting of herbivorous fishes, the corals are in better shape and bounce back more readily from setbacks."
The new insight into how well or poorly fish control weeds was gained by transplanting different densities of sargassum w
|Contact: Andrew Hoey|
ARC Centre of Excellence in Coral Reef Studies