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

Abstract

Stable carbon isotope (δ13C) analysis was used in a fish-farming impacted Mediterranean area (the Gulf of Gaeta, Central Tyrrhenian) to determine the predominant carbon sources available to bivalve molluscs cultivated around fish cages. Wether the organic matter generated by fish farming was taken up by the bivalve molluscs was also investigated. Stable carbon isotope values were measured in the particulate organic carbon (POC) of samples from potential organic matter sources such as fish-pelleted feed, mollusc faecal 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 mainly to fish-farming and bivalve mollusc activities. The POC was dominated by organic waste isotopic signatures, while the bivalve mixed diet was composed of organic matter with different isotopic signatures (phytoplankton, waste material from the bivalves themselves and surplus uneaten pelletted feed). Organic waste appears to be the dominant trophic resource in the deeper-cultivated clam diet, while phytoplankton organic carbon plays a more important role in the diet of the mussel. We propose that bivalve organic matter uptake may play an effective role in reducing the environmental impact of fish organic waste. The organic matter produced by bivalves (faecal material) under these hydrodynamic conditions (low current velocities) can be recycled through the filtration activities of the bivalves themselves, together with most of the organic matter produced by fish-farming activities (uneaten 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. ©2001 Elsevier Science B.V.