A team of scientists have come up with a benchmark for how many fish are needed to protect the health of coral reefs.
Scientists from James Cook University, the Australian Institute of Marine Science, the University of Queensland and the Wildlife Conservation Society have studied fish populations in more than 800 reefs around the world.
They have discovered that near-pristine reefs contain one tonne of fish per hectare.
Using this figure as a benchmark, they found 83 per cent of fished reefs have lost more than half their fish populations.
JCU coral reef expert Dr Nick Graham says fish play important roles in the functioning of reef ecosystems, including controlling seaweed and invertebrates.
"By linking fisheries to ecology, we can now uncover important ecosystem functions for a given level of fish biomass," Dr Graham said.
The study, published in the journal Nature, found most fished reefs would take about 35 years of protection to recover, while the most depleted areas would take almost 60 years.
Reefs that are heavily degraded and near collapse include those in Papua New Guinea, Guam, St Lucia and Antigua.
The Great Barrier Reef, Belize's Barrier Reef, and Pitcairn and Easter Islands reefs are doing relatively well under fishing restrictions, the study found.
The researchers say the vast majority of fished reefs are missing more than half their expected fish populations.
Co-author Tim McClananhan, of the Wildlife Conservation Society in New York, said the findings would help fishers determine how much catch to take and how much to leave behind.
"These results give clear consequences for taking too much fish, and uncover the ecological benefits of different forms of fisheries management," he said.