Half of US coral reefs are in poor or fair condition, threatened by climate change and human activities like sports fishing, shipping and the release of untreated sewage, a US government report said.
Reefs in the Caribbean, in particular, are under severe assault and coral in the US Virgin Islands and off Puerto Rico had not recovered from 2005, when unusually warm waters that led to massive bleaching and disease killed up to 90 per cent of the marine organisms on some reefs.
"The evidence is warning us that many of our coral reef ecosystems are imperiled and we as a community must act now," said Kacky Andrews, programme manager of the Coral Reef Conservation programme at the National Oceanic and Atmospheric Administration.
The new NOAA report on the state of coral reefs in the US and Pacific territories, including Palau and Guam, was presented at a meeting of coral reef scientists in Fort Lauderdale, Florida.
It was the third such report and the second to be based on actual monitoring of reefs. The reefs were classified as excellent, good, fair or poor based on such things as water quality, fish population and the threats they faced.
The last report was issued in 2005 when warm Atlantic waters killed off large swaths of coral through bleaching, a condition that occurs when environmental stresses, like heat, break down the symbiotic relationship between coral polyps and unicellular algae that give them colour.
Half the coral reefs off the US Virgin Islands and Puerto Rico were killed that year, said Jenny Waddell, a marine biologist at NOAA's Centre for Coastal Monitoring and Assessment. On some reefs, the fatality rate reached 90 per cent, she said.
A series of powerful hurricanes also devastated coral reefs off the Florida Keys in 2005.
But scientists at NOAA said coral reefs had been suffering for much longer due to a warming climate and other "stressors," many due to human activity, such as overfishing and damage caused by ship anchors.
"It is important to note that these declines did not happen overnight, they did not happen during the last three years," said Dr Andrews.