Fisheries chiefs plan to extend a scheme in which a type of artificial reef is sunk near fish farms to clean the water by attracting organisms that eat waste.
The 'bio-filters' - specially designed containers - have been used mainly in deep water until now but a new design is to be tested by the Agriculture, Fisheries and Conservation Department to see if it works well in shallower waters.
Shellfish and scavengers that take up residence in the containers consume bodily waste from the farmed fish as well as excess feed that drifts away, helping clean the water and prevent disease in the fish stocks.
'If the water is filtered and is less rich in organic nutrients, red tide may also be prevented,' senior fisheries officer Edward Wong Cheuk-kee said yesterday, referring to the sometimes toxic algal bloom seen in Hong Kong waters in summer.
As part of an artificial reef project launched by the department in 1996, the first 16 bio-filters, built with wooden frames, were sunk in fish culture zones at Kau Sai Chau, Sai Kung.
More were sunk in Shum Wan, Aberdeen, and Lo Tik Wan, Lamma Island, mainly in deep-water areas, in 2007 and 2008.
The department now expects to be allocated HK$3 million to extend the scheme over the next two years.
The latest bio-filters, measuring up to six cubic metres, will be made of stainless steel and designed to maximise the surface area with cone shapes and rough surfaces, providing living space for the largest possible number of organisms.
'We are planning to put more [bio-filters] in shallower waters such as Sam Mun Tsai, Tai Po and Sha Tau Kok,' Wong said. 'But we have to conduct some trials first, as we are not yet sure whether they will work well in shallow waters or not.'
Since 1996, more than 660 artificial reefs made from derelict vessels or old tyres have been sunk to the seabed around Hong Kong.