Hong Kong scientists have warned local coral reefs will face more severe and frequent bleaching, with sea temperatures recently rising to as high as 30.7 degrees Celsius (87.3 Fahrenheit) and resulting in the situation becoming widespread across key marine reserves.
The city is known to be close to the northern limit of where corals can survive, as anywhere in a latitude higher than Hong Kong is generally considered inhospitable for the animals.
Frequent coral bleaching can threaten their survival, with the disappearance of reefs, which occupy just 1 per cent of the ocean floor, considered a global concern and a threat to the survival of the world’s most biodiverse habitats.
Local wildlife authorities earlier reported “different levels of coral bleaching” had been discovered in Hong Kong’s waters, citing how the average water temperature at a site in Tung Ping Chau Marine Park was 2 degrees higher in the past month compared with the same period last year.
The city’s marine biologists also discovered mass coral bleaching in other protected waters, with some sites recording levels of bleaching at more than 70 per cent of its “coral communities”.
Dickson Wong Chi-chun, a team scientist at global NGO Reef Check Foundation, said this year’s bleaching event was “very alarming” because of the higher frequency of recurrence and the greater scale of damage.
Coral bleaching events occur when the animals are subject to environmental stresses, such as shifts in temperature and salinity in their surrounding waters, and expel the algae that live symbiotically within them.
As the algae, which are called zooxanthellae, supply up to 95 per cent of the nutrients needed by the coral, the latter can starve to death if conditions fail to improve and ensure the algae can return to the coral structure in time.
Wong began studying corals in Hong Kong waters in 2003, when he said bleaching only occurred to individual animals within a community. It was not until 2017 that he noticed bleaching exceeded 50 per cent of certain coral communities.
On July 16, he and other researchers embarked on an expedition in the waters around Sharp Island in the Hong Kong Unesco Global Geopark and discovered that 73 per cent of the corals had turned pale.
Wong said the last major bleaching event happened in 2022 and it was only last summer that the corals had managed to regain their strength.
“But bleaching has recurred so frequently and I’m concerned that if the corals cannot recover quickly enough, the whole community may no longer be sustainable and disappear for good,” he said.
In a 2022 survey conducted by the Agriculture, Fisheries and Conservation Department, Baptist University and the Chinese University of Hong Kong, coral bleaching was observed at 18 of 19 study sites.
Among these, 13 and 33 per cent of corals had been bleached at reefs around the A Ma Wan bay on Tung Ping Chau island and northern Sharp Island respectively.
Wong said this year’s situation was “a lot more shocking”.
In a separate survey conducted on July 30, international conservation group WWF Hong Kong also discovered more than half of the corals had bleached at a site in Hoi Ha Wan Marine Park.
Similarly, at another site in Tolo Harbour, more than 50 per cent of the 300 corals the NGO had planted since June last year had turned white, the group’s oceans conservation manager, Kelvin So Jun-yin, added.
He called this year’s bleaching event “one of the worst ordeals” faced by corals in Hong Kong in recent years because some species known to be heat resilient had also lost their colourful hue.
“During the survey, we also saw some corals turn fluorescent, which is a sign that they were under immense stress. These corals are likely to fall ill or die if conditions do not improve soon,” So explained.
He added that, apart from rising sea temperatures, the prolonged rainfall that started in March this year was also an aggravating factor, as run-offs had decreased the salinity of Hong Kong sea waters.
The high amounts of rainfall had made things worse for local corals since they thrived in shallow waters at depths of between 50cm and three metres due to freshwater generally remaining at levels above the denser seawater, So said.
According to the Observatory’s monthly weather summary, Hong Kong has become wetter than usual since April, with over 700mm of rainfall recorded over parts of Sai Kung just a month later.
Responding to questions from the Post, the Agriculture, Fisheries and Conservation Department said it recorded an average water temperature of 30.7 degrees in a coral community in Tung Ping Chau Marine Park from mid-June to mid-July, up from 28.6 degrees during the same period last year.
Most corals in Hong Kong grow optimally in water temperatures between 26 and 28 degrees.
A spokesman added that the department had recorded four cases of coral bleaching as of July 18 this year. It logged six cases last year, 22 in 2022 and nine in 2021.
“Corals in Hong Kong are relatively resilient and will normally regain health and continue to grow when water temperatures in Hong Kong waters resume the normal level,” he said.
He appealed to the public to submit reports to the department if they discovered coral bleaching, saying authorities would continue to closely monitor the state of coral health in waters around the city and “follow up as necessary”.
In April, the United States’ National Oceanic and Atmospheric Administration warned that the earth was experiencing the fourth global coral bleaching event in history, with such phenomenon occurring in at least 54 countries or regions since February 2023.
Australian authorities also warned in April that the Great Barrier Reef was experiencing its worst bleaching event ever, with 73 per cent of corals turning white.
The average surface temperature of the world’s oceans also broke monthly records from March to December last year, hitting a new high of 21.1 degrees on August 23 and 24.
To prevent further damage to local coral communities, WWF Hong Kong’s So suggested that residents who went swimming in the sea limit their use of sunblock or opt for those containing coral-friendly chemicals.
Wong from Reef Check Foundation also urged the public not to go near bleached corals to avoid causing further disturbance or stresses within the local habitat.
