Hard coral cover on the Great Barrier Reef is near record lows in its northern stretch and in decline in the south, surveys by government scientists have found.
A report card by the government’s Australian Institute of Marine Science says hard coral cover in the northern region above Cooktown is at just 14% – a slight increase on last year but close to the lowest since monitoring began in 1985.
A series of “disturbances” – coral bleaching linked to rising water temperatures, crown-of-thorns starfish outbreaks and tropical cyclones – have caused hard coral cover to shrink across much of the world heritage landmark over the past five years. Depending on the location, coral coverage is between 10% and 30%Mike Emslie, the institute’s acting head of long-term monitoring, said the report included glimmers of hope: individual reefs, including those on the outer shelf in the Whitsunday Islands, were found to have lively communities and tiny juvenile corals were discovered across the 2,300km reef system. The density of juvenile coral suggested recovery was possible if there were not further disturbances.He said it indicated there was some resilience in the system but added: “The important thing is the absence of further disturbances. If we have more coral bleaching events all bets are off.”The northern and central sections of the reef were hit by back-to-back mass bleaching events as ocean heating increased in early 2016 and 2017, killing vast areas of coral. A study led by Terry Hughes, the director of the ARC Centre of Excellence for Coral Reef Studies, found 30% of coral died after the 2016 heatwave alone. Intergovernmental Panel on Climate Change scientists estimated 99% of corals across the globe are likely to be lost if the climate crisis is not addressed and global heating reaches 2C.The Institute of Marine Science report warns the extent of hard coral in the north may be even lower than the 14% estimated due to skewed surveying – the greatest bleaching damage was on inshore reefs and they were under-represented in surveys due to safety concerns. The highest level of coral reef cover recorded in the north was 30% in 1988.Emslie said the reef’s southern section had escaped the worst effects of coral bleaching and cyclones since 2009 but has been affected by a severe outbreak of crown-of-thorns starfish since 2017. Crown-of-thorns feed on coral and spawns so rapidly it is difficult to tackle once it takes hold. Its spread has been linked to nitrogen from fertiliser and pesticides in agricultural run-off.He said the starfish had a particularly devastating impact in the Swain Reefs national park, more than 100km off the coast between Rockhampton and Mackay. Across the south, hard coral cover is 24%, down from a high of 43% three decades ago.The central region, from Airlie Beach to north of Cairns, has been significantly damaged by Tropical Cyclone Debbie in 2017 and the crown-of-thorns’ southward spread. Hard coral cover fell from 14% to 12% last year. It was 22% just three years ago.