Preserving a species of weed-eating fish may be the key to saving the world’s coral reefs from being engulfed by weed as human and climate impacts grow.

A new study by the Centre of Excellence for Coral Reef Studies (CoECRS) has found that weed-eaters like parrotfish and surgeonfish can keep coral reefs clear of weed up to a point.

For some years, researchers have pinned their hopes on the ability of such 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, the journal Ecology Letters reports.

However, the latest work by Andrew Hoey and 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, according to a CoECRS statement.

“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 fish, the corals are in better shape and bounce back more readily from setbacks.”