One of the most frequently cited problems associated with aquaculture is pollution of the local environment, through the release of wastes comprising uneaten food, faecal and urinary products, chemicals and chemotherapeutants. For instance, Wu (1995) calculated that in general some 85% of phosphorus, 80-88% of carbon and 52-95% of nitrogen input into a marine fish culture system may be lost into the environment through feed wastage, fish excretion, faeces production and respiration. Cleaning of fouled cages may also add an organic loading to the water, albeit periodically, and the use of chemicals (therapeutants, vitamins and antifoulants) has also raised environmental concerns. The major impact is on the sea bottom, where high organic loading can result in high sediment oxygen demand, anoxic sediments, production of toxic gases elimination of benthic organisms (Wu 1995; Lu and Wu 1998). TBT contamination and the development of antibiotic-resistant bacteria have also been reported near farms. Nonetheless, Wu (1995) argued that the significant impact is normally confined to within 1 km of the farm. In addition, a study (Lu and Wu 1998) on the recolonisation and succession of marine macrobenthos in organic-enriched sediment deposited from fish farms suggested it was unlikely there would be a long-term impact once farming activities ceased. Recolonisation occurred rapidly and temporal changes over subsequent months resembled the spatial changes seen in benthic communities along a decreasing pollution gradient (e.g., Pearson and Rosenberg 1978).

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Department of Environment and Heritage