Field studies were carried out to determine and compare the impact of marine fish farming activities on the water quality and bottom sediment at four fish culture sites with different hydrographic and culture conditions in a sub-tropical environment where trash fish is used as feed. The major impact identified was on the sea bottom, resulting in the development of reducing and anoxic sediments, high sediment oxygen demand, production of hydrogen sulphide and elimination/decrease in benthos. 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 poor tidal flushing and high stocking density. However, no significant changes in total suspended solids, light extinction coefficient, chlorophyll a, phaeopigment and E. coli were found near the fish rafts at any sites. Environmental impacts vary considerably between sites, and were significantly reduced at sites with good water circulation and low stocking density. Despite the high organic and nutrient loadings generated by marine fish farming activities, the impacts on water quality and sediments at all sites were localised and did not appear to extend beyond a distance of 1-1.5 km from the fish rafts. Results of the present study also do not support the suggestion that marine fish farming activities have caused eutrophication on a large scale. Copyright © 1994 Published by Elsevier Ltd.