Coral reefs healthy but under pressure

PUBLISHED : Sunday, 16 December, 2007, 12:00am
UPDATED : Sunday, 16 December, 2007, 12:00am

Global warming may affect HK sea gardens

Hong Kong's 33 coral reefs are generally healthy and stable, but there are indications they are under pressure from global warming.

The Reef Check 2007, released yesterday on the 10th anniversary of the first of the annual checks, recorded high coral coverage - equal to or more than 50 per cent - in 22 of the 33 sites, but six of the sites suffered 'bleaching' due to warmer waters.

Last year, five sites recorded bleaching, and 23 recorded high coral coverage.

The coral health indexes ranged from 3.33 to 4.93 this year, a slight deterioration from last year's 3.43 to 5.18. The scale maximum is 6.0.

Alan Chan Lai-koon, senior marine conservation officer of the Agriculture, Fisheries and Conservation Department, said the reef report card was positive. 'The general coral cover, species diversity and health conditions are generally stable.'

The indicator species of fish and invertebrates that inhabit the areas are 'quite rich and diverse', he said.

The department held the annual exercise with the Reef Check Foundation. It involved 36 teams totalling 360 divers recording and monitoring from July to September.

The checks covered the 33 'best coral-growing sites' in the eastern part of Hong Kong waters extending from Tung Ping Chau in the north to the Ninepin islands in the south, Mr Chan said.

Foundation co-ordinator Terence Fong Ching-wai said although the health of the coral was 'still OK, if we try to predict in a few years we are quite worried. If the temperature increases by 2 to 3 degrees, then it will have a bigger impact'.

His group would keep monitoring the water temperature, as it would increase the risk of coral bleaching.

Local coral contains the symbiotic algae Zooxanthellae, which give coral its colour and provide food.

'If these algae leave the coral body, the coral cannot survive after 10, 12 or 30 days,' Mr Fong said. 'The algae leave the coral body with an increase in temperature and increase in salinity and turbidity.'

Mr Chan said bleaching was 'very minor and localised', involving less than 5 per cent of each site.

He did not think global warming would affect local coral in the short term because it is 'quite tough'.

'Most of the corals are in the northeastern part of Hong Kong where we have very good flushing from the ocean current and also the temperature variations of seawater is not very high, ranging from 15 to 30 degrees Celsius. It is very stable so the impact on coral is not as high.'

The coral was in 'good shape' based on the health indexes which were all above 3, Mr Chan said. There might be index fluctuations due to 'random sampling and because we sampled 20 sites [whereas] last year we did 24'.

The department had commissioned Chinese University to study the distribution and diversity of soft coral, octocoral and black coral, Mr Chan said.

'The information will be useful in terms of planning, managing and conservation.'

Results are due next December.