A team of scientists studied 123 sites along 83 kilometres of Upolu's coast in 2016, and their findings have been published in the journal Marine Pollution Bulletin.
In half the sites surveyed, live corals populated less than one percent of a given reef.
At 80 percent of sites, live corals were below 10 percent.
The scientists estimate these reefs may have previously had 60 to 80 percent live coral.
