Marine biologists on Wednesday said Hong Kong's record-breaking summer heat left coral "stressed and starving" across the territory as they warned that water activities were potentially making the situation even worse.

Reseachers from the Agriculture, Fisheries and Conservation Department, Baptist University and the Chinese University conducted checks in August at 19 coral locations across the SAR and found bleaching at 18 of them.

Apple Chui, a research associate professor from CUHK's school of life sciences, said this was the second bleaching event with such "extensive impact" in the SAR in recent years. The previous one, in 2017, was Hong Kong's worst ever.

“I think the bleaching [that] occurred this year was due to environmental factors, like elevated sea water temperatures for sure, because this year we broke a lot of records for very hot [days in] summer in Hong Kong,” Chui said.

Bleaching occurs when warming of the water causes the coral to expel the algae that provide more than 90 percent of their nutrients and the coral's colour. It reduces coral growth and leaves it vulnerable.

Chui said human activities in areas where coral grows were likely making the situation worse, noting that 33 percent of coral was bleached at Sharp island in Sai Kung – a hotspot for divers, snorkelers and kayakers.

“For example… those sunscreen, there might be some active ingredients, chemical compounds, if they get into the water, they may cause coral bleaching,” Chui explained.

“People may not realise [coral] are living organisms, instead of rocks, so perhaps sometimes…people will step on the coral. This of course will have some physical damage directly on the coral.”

She said coral that is physically damaged may take a century to regrow, and it will only recover its colour quickly when the environmental stress factors are removed.

Even though CUHK and green group WWF are collaborating on a programme to restore coral, Chui said the pace just cannot catch up with the rate of degradation.

Kevin So from WWF called on the SAR to vastly expand its marine protected areas, where human activities such as fishing and tourism are regulated. At present, 5 percent of the SAR's waters are protected, but So said this should be expanded to 30 percent to provide a “complete solution” to reduce the impact of human actions.