The pristine Ningaloo Reef is at risk of bleaching as unusually hot water in the Pilbara moves down the West Australian coast, scientists warn.
Heat stress maps show the risk of coral bleaching is at the highest alert level, due to hot water temperatures, over the next eight weeks.
The maps are based on satellite imagery from the United States-run National Oceanic and Atmospheric Administration.
It comes after this month's widespread and severe bleaching event at the Great Barrier Reef, and has left researchers, Exmouth locals and tourism operators holding their breath.
Australian Institute of Marine Science coral reef ecologist James Gilmour said the US data and Australia's Bureau of Meteorology sea surface temperature measurements showed temperatures one or two degrees above normal for this time of the year on the Ningaloo coast, a World Heritage site.
Dr Gilmour said coral experienced heat stress in water that was one degree Celsius above normal, but anything above that became far more dangerous to coral health.
"If you get to a period of two degrees above normal for a period of weeks, then that's when we see coral bleaching happening," Dr Gilmour said.
He predicted there would be moderate bleaching if the heat stress lasted four weeks and it would be severe if it lasted eight weeks, although that was unlikely.
Coral facing 'double whammy' of stress
Dr Gilmour, who works from the Indian Ocean Marine Research Centre at the University of Western Australia, said coral spawning season was adding to the potential harm.
"That's a phenomenal event when the corals reproduce but it's also a time when, as you can imagine, they are stressed and they're tired in the reproduction," he said.
"And then following that you get these rising sea surface temperatures in March and then April, so it's a double whammy almost in stress."
With the tourist season underway, local operators are watching closely.
Ningaloo Glass Bottom Boat Tours co-owner Rebecca Kitson said a healthy and beautiful coral reef was more than just central to their business and livelihood.
"For us, it's a deeper concern of what's going on," Ms Kitson said.
"There has been coral bleaching events before and there's no way to predict how severe this one will be. Coral does have an incredible way of regenerating, and it's about to spawn.
"My understanding is that as long as it's not over an extended period of time, it can recover."
Coral bleaching linked to climate change
The last time there was a major coral bleaching event at Ningaloo was about a decade ago.
In 2011, a marine heatwave combined with a tropical cyclone caused severe damage, with 80-95 per cent of the coral cover affected in the Bundegi area of the marine park, according to the WA's Department of Biodiversity, Conservation and Attractions.
Between 2012 and 2013, there was bleaching in the eastern Muirons area.
The department noted the Ningaloo Reef was generally in very good condition, with an abundance of marine life.
However, it noted changes were occurring and seawater temperatures had increased by 0.8-1.0 degrees over the last 32 years, with coral cover seemingly in decline.
It predicted this would lead to more bleaching events at Ningaloo.
Bleaching to occur annually by 2049
"The best available science predicts that by 2041, Ningaloo will experience significant bleaching twice every decade," the department predicted.
"By 2049, bleaching is expected to occur annually."
Dr Gilmour said bleaching was already occurring more often.
"Not only have we seen an increase in the scale of bleaching but also the frequency, so the window of opportunity for recovery is decreasing," he said.
Sarah Ellis from Ningaloo Discovery, which takes people swimming with whale sharks, said action was needed now.
"If you look at what's happened on the Great Barrier Reef and we're so lucky at Ningaloo Reef that it hasn't had any of the impacts of that nature yet...we've got time do something about it," she said.
"It would be really nice to see a bit more effort from everyone to try and put a stop to that as much as we can."
Scientists seek solutions
Dr Gilmour said reducing carbon emissions, which were causing the ocean warming, was essential.
Beyond that, it was vital to understand the resilience of the reef, with his work partly focusing on how different corals reacted and adapted to heat stress.
"That variation is one of the main things we're trying to understand and study now. Not all corals bleach as badly as others, not all spots on a reef bleach as badly as others," he said.
"If we can understand that natural adaptation, which usually happens over thousands of years, but we need to implement over decades, then we may be able to aid the process of adaptation."
"So we can then look at active interventions and potentially even restoration activities that may become useful until the point at which we can slow these temperature increases."
Paul Gamblin, director at the Australian Marine Conservation Society, said at the same time it was important to ensure there were no extra stresses on the reef.
"What we need to do is translate that concern into action, and part of that is to remove local pressures from the stressed environment," he said.
"To not allow industrialisation, to reduce over-exploitation, to relieve the stress, the local stressors of these incredibly fragile and important ecosystems that are facing such a challenging future."
