Issue No 10: August 2005
Three years ago the City University of Hong Kong researchers began a study to determine how effective the shellfish would be in disposing of debris such as feed waste and fish excreta produced at fish farms.
Nutrients from the debris, mainly carbon, nitrogen and phosphorus, can cause significant marine pollution.
In the research, artificial reefs, with a total surface area of 4,000 square metres for the shellfish to grow on, were constructed and introduced into a fish culture zone off Kau Sai Chau, near Sai Kung.
With growth on the biofilter well established, it was calculated that its filtering creatures removed an average of 6.4 kg of carbon, 1.7 kg of nitrogen, and 0.3 kg of phosphorus a day from the fish farm. For the nitrogen count, it was the equivalent of that produced by two tonnes of fish a year.
We couldnt clean up Victoria Harbour with this method, said Principal Investigator Prof Paul K S Shin, but, as a localised biofiltering system, it has proved very effective.
The most efficient filtering shellfish were found to be barnacles and edible green-lipped mussels.
Part of the research looked at whether it would be commercially viable to harvest green-lipped mussels from the biofiltering reef, reaping a double-edged benefit by combining the mussels culture and filtering capacity.
Because the mussels are relatively inexpensive, its unlikely that growing them this way would be attractive to fishermen. We may try oysters and scallops which sell at a higher price, said Dr Shin.
In all, about 200 strands of rope with mussel colonies were hung on the outside of the biofiltering reef blocks for easy access.
The reef comprised 16 blocks of a honeycomb structure made from cement and glass fibre.
A large tank at the fish farm and a small tank at the biofilter were set up to measure the filtration rate.
Water from the large tank of unfiltered fish farm waste was pumped continuously to the small tank containing a 10 cm by 10 cm plate encrusted with the filter feeders.
The concentration of nutrient particulates was compared. By measuring the depletion of particulates in the small tank, we knew the filtration rate of the filter feeders, said Dr Shin, because the depletion could only be caused by animals on the plate.
One precaution taken in the experiment was to determine the source of the raw nutrients being measured as they could have come from outside the fish farm.
For this, a stable isotope technique was used and confirmed that most came from the fish farm.
Analysis showed that 68 percent of nutrients came from phytoplankton, 28 percent from trash fish, and 4 percent from excrement.
The filter feeders solve the pollution problem in two ways, explained Dr Shin.
As well as digesting particles of feed waste and fish excreta they also consume phytoplankton which thrives on inorganic nutrients such as phosphorus and nitrogen.
With more nutrients, phytoplankton can form algal bloom or red tides, he said, some of which can become toxic.
Research into the biofilter system was funded by the Agriculture, Fisheries and Conservation Department.
The filtering artificial reef system has high application potential in the Asia-Pacific region, said Dr Shin.