Coral reefs are trying to recover

Coral cover has shown small signs of recovery on two thirds of reefs surveyed along the Great Barrier Reef – but it’s a bittersweet victory amongst continued onslaught by bleaching, crown-of-thorn starfish outbreaks and tropical cyclones.

And the full impact of last year’s mass coral bleaching event is yet to be revealed, according to an annual report from the Australian Institute of Marine Science (AIMS), which has been monitoring the vulnerable World Heritage-listed site for more than three decades.

“The third mass coral bleaching in five years unfolded in late summer, and this event may well have reversed the gains in hard coral cover,” says project lead Mike Emslie from AIMS, based in Townsville, Queensland.

The impact of bleaching, which results from prolonged high sea temperatures, continues weeks to months afterwards so it’s not fully covered by the report, which is based on surveys conducted between September 2019 and June 2020.

The Great Barrier Reef is the largest living structure on the planet. For now, its revival is encouraging, says Emslie, but there’s a long way to go and it needs more time to recover between damaging events, which are occurring more frequently.

“Full recovery of coral reefs takes years to decades and requires many years without subsequent disturbances”, he explains, and although the Great Barrier Reef has shown an ability to start coming back, “such resilience clearly has limits”.

At the time of the survey, the percentage of live hard coral cover along the seafloor – a proxy for reef health – varied considerably among the 86 reefs surveyed.

The dynamic southern region, extending down to Rockhampton, had the most average cover at 24%, an increase of 1% and just over half of what it was in 1988. This was a slight setback from the reef’s remarkable recovery from 9% in 2011 to 32% in 2017 after crown-of-thorns starfish outbreaks.

In the central region, between Hinchinbrook Island and Mackay, hard coral cover increased on average from 12% in 2019 to 14% in 2020. This area also had low numbers of crown-of-thorns starfish, likely due to a control program.

Coral cover in the Northern Great Barrier Reef, north of Cooktown, remained stable at 15% after increasing from 12% since 2017.

The team made their estimates using manta tow surveys, a standard technique that involves visual categorisation of percentage cover as low (0-10%) to extremely high (75-100%), averaged from a series of two-minute tows, each covering around 2000 square metres. AIMS deems coral as healthy with as low as 30% to 50% cover.

Encouragingly, the survey also found that coral trout have continued to grow larger in size and number in a marine park rezoned in 2004 compared to reefs where fishing is still allowed. These Green Zones had nearly twice as many fish than the latter Blue Zones.

This suggests “well-managed and resourced networks of no-take marine reserves have the desired conservation benefits,” says Emslie, and further, “larger trout inside no-take marine reserves produce more babies, which can help re-seed populations in areas that are open to fishing”.

Some of the larvae from the trout, one of the most valuable species for recreational anglers and commercial fishers, are moved to the Blue Zones.