THE ENVIRONMENTAL-IMPACT OF MARINE FISH CULTURE - TOWARDS A SUSTAINABLE FUTURE

The environmental impact of marine fish-farming depends very much on s pecies, culture method, stocking density, feed type, hydrography of th e site and husbandry practices. In general, some 85% of phosphorus, 80 -88% of carbon and 52-95% of nitrogen input into a marine fish culture system as feed 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 periodical ly. Problems caused by high organic and nutrient loadings conflict wit h other uses of the coastal zone. The use of chemicals (therapeutants, vitamins and antifoulants) and the introduction of pathogens and new genetic strains have also raised environmental concerns. Despite the h igh pollution loadings, results from various studies show that some 23 % of C, 21% of N and 53% of P of feed input into the culture system is being accumulated in the bottom sediments and the significant impact is normally confined to within 1 km of the farm. The major impact is o n the sea bottom, where high sediment oxygen demand, anoxic sediments, production of toxic gases and a decrease in benthic diversity may res ult. Decreases in dissolved oxygen and increases in nutrient levels in the water are also evident but are normally confined to the vicinity of the farm. Tributyltin (TBT) contamination and the development of an tibiotic-resistant bacteria have been reported near fish farms. The st imulating effects of vitamins/fish wastes on growth of red tide specie s have been demonstrated in a number of laboratory studies. Neverthele ss, there is no evidence to support the suggestion that the present us e of therapeutants, vitamins and antibiotics and the introduction of p athogens and new genetic strains would pose a significant threat to th e environment. Marine fish culture can be a sustainable development, p rovided pollution loadings generated by fish farms are kept well below the carrying capacity of the water body. Effects can be significantly reduced by careful site selection, control of stock density, improved feed formulation and integrated culture (with macroalgae, filter-feed ers and deposit-feeders). An example of the application of computer mo delling in mariculture management is demonstrated. Environmental impac t assessment and monitoring should also be carried out to ensure cultu re activities are environmentally sustainable.