Ecological assessment of aquaculture impact in the Mediterranean Sea

The exponential growth of off-shore mariculture that has occurred worldwide over the last 10 years has raised concern about the impact of the waste produced by this industry on the ecological integrity of the sea bottom. Investigations into this potential source of impact on the biochemistry of the sea floor have provided contrasting results, and no compelling explanations for these discrepancies have been provided to date. To quantify the impact of fish-farm activities on the biochemistry of sediments, we have investigated the quantity and biochemical composition of sediment organic matter in four different regions in the temperate-warm Mediterranean Sea: Akrotiri Bay (Cyprus), Sounion Bay (Greece), Pachino Bay (Italy) and Gulf of Alicante (Spain). In these four study regions, the concentrations of biopolymeric C in the sediments were measured, comparing locations receiving wastes from fish farms to control locations in two different habitats: seagrass beds (Posidonia oceanica) and soft non-vegetated bottoms. The biopolymeric C concentrations in the sediment suggest idiosyncratic effects of fish-farm waste on the biochemistry of sediments. These are possibly related to differences in the local physico-chemical variables that could explain a significant proportion of the differences seen between the control and fish-farm locations. Results indicate that quantitative and qualitative changes in the organic loads of the sediments that arise from intensive aquaculture are dependent upon the ecological context and are not predictable only on the basis of fish-farm attributes and hydrodynamic regimes. Therefore, the siting of fish farms should only be allowed after a case-by-case assessment of the ecological context of the region, especially in terms of the organic matter load and its biochemical composition.