By Ben Hirschler

LONDON, Oct 31 (Reuters) - The delicate balance of the Caribbean's coral reefs is in jeopardy as more parrotfish end up on dinner plates, international scientists said on Wednesday.

The colourful grazing fish, named for their parrot-like beaks which are used to scrape up algae, play a vital role in stopping seaweed from smothering coral. But their numbers are now being threatened by over-fishing.

New research based on computer modelling shows parrotfish are a key defence in preventing the vulnerable Caribbean reefs from becoming a very different ecosystem -- one dominated not by living coral but by blooms of algae or seaweed.

"The future of some Caribbean reefs is in the balance and if we carry on the way we are then reefs will change forever," said Peter Mumby, a marine biologist from Exeter University, England, who led the research.

"Things have a habit of spiralling out of control and if a reef is allowed to get into an unhealthy state, covered in seaweed, it is extremely difficult to turn that around," Mumby added.

Coral reefs around the world are under threat from climate change, due to warming seas and increased ocean acidity, but the problem is particularly acute in the Caribbean following a series of disasters in the 1980s and 1990s.

The most devastating was the near-extinction of the long-spined sea urchin in 1983, due to disease. These grazing urchins had kept down seaweed levels and their disappearance leaves parrotfish as the sole grazers on many Caribbean reefs.

Compounding the problem, a number of hurricanes, starting with Hurricane Allen in 1980, have destroyed coral colonies and given invading seaweed new space to take hold.

The result, according to projections published in the journal Nature by Mumby and colleagues at the University of California, Davis, is that coral reefs could be damaged beyond repair unless management of the marine environment is changed urgently.

Top of the list should be controls over the use of fish traps -- devices similar to lobster pots that are used to catch most parrotfish.

Although not renowned for their flavour, parrotfish have become increasingly popular in restaurants around the Caribbean following over-exploitation of more prized fish species such as grouper and snapper.

"We need to think about parrotfish in the entire coastal zone. It is not enough to say we'll protect parrotfish in marine reserves and we'll fish them outside. We have to try and control the fishing everywhere," Mumby said in a telephone interview.

There are currently very few restrictions on fishing for parrotfish outside protected Caribbean marine parks, although catches are rare in Florida and Bermuda, where fish traps are not used.

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