Healthy state of coral attests to water quality

PUBLISHED : Thursday, 16 August, 2007, 12:00am
UPDATED : Thursday, 16 August, 2007, 12:00am

Water quality in the northeast of Hong Kong is good enough to host 84 hard-coral species - about 10 per cent of the world's total - a marine conservation expert said yesterday.

However, the sea's rising temperature is turning the coral white.

'There are five sites out of 33 in the northeast of Hong Kong that recorded coral bleaching in the Hong Kong Reef Check 2006,' said Khaki Chan King, a marine conservation officer with the Agriculture, Fisheries and Conservation Department.

'They are Lai Chi Wo, Hoi Ha Wan pier, Port Island, Pak Ma Tsui and Siu Long Ke. It may be caused by the extended period of elevated water temperature during the summer.'

Dr Chan said the impact of the bleaching was localised.

Coral bleaching refers to the loss of a coral's colour due to the stress-induced expulsion of algae, namely zooxanthellae.

Normally, less than 0.1 per cent of the zooxanthellae are released from healthy coral. However, when experiencing stress, such as an increase in sea temperature, coral expels zooxanthellae in greater quantities, causing it to turn white.

Dr Chan said coral at all 33 sites was generally in a healthy condition, but depending on the degree of bleaching and how long the stress lasted, bleached coral could eventually die.

Sea temperatures in the tropics have risen by 1 to 2 degrees Celsius each century.

The fisheries department's report, 'The Hong Kong Reef-Building Corals', said that at less than 10 per cent of total coverage, the average area of dead coral in Hong Kong was low, while the average area of injured coral was less than 25 per cent.

'Coral is very sensitive to the external environment, such as a change in sea temperature and salinity. Its abundance testifies that the water quality is good,' Dr Chan said.

She said the sites with high coral coverage also recorded high coral diversity and an abundance of fish.

'Nineteen of a total of 20 pre-determined indicator species [of fish] were recorded in the survey last year,' she said.