IMPACT OF MARINE FISH FARMING ON WATER-QUALITY AND BOTTOM SEDIMENT - A CASE-STUDY IN THE SUBTROPICAL ENVIRONMENT

Field studies were carried out to determine and compare the impact of marine fish farming activities on the water quality and bottom sedimen t at four fish culture sites with different hydrographic and culture c onditions in a sub-tropical environment where trash fish is used as fe ed. The major impact identified was on the sea bottom, resulting in th e development of reducing and anoxic sediments, high sediment oxygen d emand, production of hydrogen sulphide and elimination/decrease in ben thos. The impact on water quality was less conspicuous. A decrease in dissolved oxygen was observed at all sites while increases in ammonia, inorganic P, nitrate and nitrite were observed only at sites with poo r tidal flushing and high stocking density. However, no significant ch anges in total suspended solids, light extinction coefficient, chlorop hyll a, phaeopigment and E. coli were found near the fish rafts at any sites. Environmental impacts vary considerably between sites, and wer e significantly reduced at sites with good water circulation and low s tocking density. Despite the high organic and nutrient loadings genera ted by marine fish farming activities, the impacts on water quality an d sediments at all sites were localised and did not appear to extend b eyond a distance of 1-1.5 km from the fish rafts. Results of the prese nt study also do not support the suggestion that marine fish farming a ctivities have caused eutrophication on a large scale.