* Intact mangroves worth more than fish farms
* Mangroves support sealife, slow climate change
* Around a fifth of mangroves lost since 1980
By Alister Doyle
OSLO, Nov 14 (Reuters) - Valuable mangrove forests that protect coastlines, sustain sealife and help slow climate change are being wrecked by the spread of shrimp and fish farms, a U.N.-backed study showed on Wednesday.
About a fifth of mangroves worldwide have been lost since 1980, mostly because of clearance to make way for the farms which often get choked with waste, antibiotics and fertilisers, according to the study.
Intact mangroves were almost always more valuable than shrimp farms, said its authors, who drew on forestry and conservation expertise from several U.N. organisations.
Mangroves - trees and shrubs that grow in salty coastal sediment - can be found in 123 nations in the tropics and sub-tropics and cover an area slightly larger than Nepal. They are nurseries for wild fish stocks, sources of wood for building and serve as buffers to storm surges.
They absorb carbon dioxide, the main greenhouse gas from burning of fossil fuels, and store it in their roots. And their growth can help counteract the effects of rising sea levels as it elevates coastlines.
"There is an opportunity for many countries to go for restoration of mangroves," Hanneke Van Lavieren, lead author of the study at the U.N. University's Institute for Water, Environment and Health (UNWEH), told Reuters.
"Mangroves can be seen as a key ecosystem for food security in the world," she said.
Many of the shrimp farms are in southeast Asian nations. World production surged to about 2.8 million tonnes in 2008 from about 500,000 two decades earlier, mostly in China, Thailand and Indonesia.
The fish farmers are often encouraged by subsidies to expand, even though other lucrative businesses depend on mangroves for their own survival.
Wild prawns caught off Australia's Northern Territories and Queensland, for instance, rely on mangroves to grow and are one of the country's most valuable fisheries, earning almost $72 million a year, the report said.
Protecting almost 12,000 hectares (30,000 acres) of mangroves in Vietnam cost about $1 million but saved more than $7 million on dyke maintenance, it said.
Countries such as Australia and Brazil had been good at preserving their mangroves while nations including Indonesia, China and Vietnam had lost big tracts and projects to restore them needed more support.
Zafar Adeel, head of UNWEH, suggested that people could also choose to avoid buying shrimps raised in farms.
"We as consumers internationally play a big role," he said. "For the first time in human history about half the global population is living in coastal areas. The stresses are going to be higher." (Reporting By Alister Doyle, Environment Correspondent; Editing by Tom Pfeiffer)