Study: Weed-eating fish keep reefs healthy

CANBERRA, Australia, March 10 (UPI) -- Preserving populations of weed-eating reef fish is vital to saving coral reefs from increasing human and climate impacts, Australian researchers say.

Researchers at the ARC Center of Excellence for Coral Reef Studies say weed-eaters like parrot fish and surgeon fish can only keep coral reefs clear of weed up to a point, and if the weeds reach a certain density, they take over entirely and the coral is lost, an ARC release said Thursday.

Once the marine weeds reach a certain growth extent, the fish no longer control them and prefer to graze less weedy areas, the researchers said.

"As a result, the whole system tips from being coral-dominated to weed-dominated," Andrew Hoey of ARC and James Cook University said.

"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," he said. "The fishes show a clear preference for grazing more open areas."

Coral reefs are in decline worldwide, especially in the Asia-Pacific region, with many exhibiting "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," Hoey 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."