The degradation of fish-cage waste in sediments during fallowing

The composition and rate of degradation of organic waste deposited in sediment underneath and adjacent to fish cages in the Huon Estuary, Tasmania, Australia, were investigated. Sediment samples from two near-adjacent sites, but with different sediment types and depths, were analyzed for total organic matter, lipids (fatty acids and sterols), %C, δ13C, %N, δ15N, and redox potential during a 12-month fallowing period. Additionally, representative samples of fish food and fish faeces were analyzed. It was found that most of the accumulation of organic matter was confined to an area directly underneath the fish cages, but at 30 m from the center of the cage, indicators of fish cage waste (faeces and fish food) were still elevated compared with reference sites. As both fish food and faeces have distinctive fatty acid profiles, the relative proportion of food and/or faeces deposited on the sediment could be determined. After 12 months fallowing, fish-farm-derived organic matter in surface sediment at the center of the cage remained greater than at 30 m distance, even though redox potentials indicated that normal oxic conditions had returned. (C) 2000 Elsevier Science B.V.