The effect of fish farming organic waste on food availability for bivalve molluscs (Gaeta Gulf, Central Tyrrhenian, MED): stable carbon isotopic analysis

Stable carbon isotope (delta C-13) analysis was used in a fish-farming impa cted Mediterranean area (the Gulf of Gaeta, Central Tyrrhenian) to determin e the predominant carbon sources available to bivalve molluscs cultivated a round fish cages. Whether the organic matter generated by fish fanning was taken up by the bivalve molluscs was also investigated. Stable carbon isoto pe values were measured in the particulate organic carbon (POC) of samples from potential organic matter sources such as fish-pelleted feed, mollusc f aecal waste and bivalve flesh. The sources of organic matter affecting the study area water column and benthic communities appeared to be terrigenous- continental, autochthonous (phytoplankton) and anthropogenic inputs due mai nly to fish-fanning and bivalve mollusc activities. The POC was dominated b y organic waste isotopic signatures, while the bivalve mixed diet was compo sed of organic matter with different isotopic signatures (phytoplankton, wa ste material from the bivalves themselves and surplus uneaten pelletted fee d). Organic waste appears to be the dominant trophic resource in the deeper -cultivated clam diet, while phytoplankton organic carbon plays a more impo rtant role in the diet of the mussel. We propose that bivalve organic matte r uptake may play an effective role in reducing the environmental impact of fish organic waste. The organic matter produced by bivalves (faecal materi al) under these hydrodynamic conditions (low current velocities) can be rec ycled through the filtration activities of the bivalves themselves, togethe r with most of the organic matter produced by fish-farming activities (unea ten feed and faecal material). Bivalve cultivation around cages may reduce the environmental impact of organic waste from fish-farming activities and increase the profitability of fish culture activities. (C) 2001 Elsevier Sc ience B.V. All rights reserved.