Does bivalve mollusc polyculture reduce marine fin fish farming environmental impact?

The ability of bivalve culture in the proximity of an open water fin fish farm to reduce the environmental impact caused by organic wastes was tested. The experiment involved floating net cages containing cultured gilthead sea bream (Sparus aurata) and European sea bass (Dicentrarchus labrax) in the western Mediterranean. Two bivalve species, oyster (Ostrea edulis) and mussel (Mytilus galloprovincialis), were deployed for 3 months along a distance transect running from 0 to 1800 m from the fish cages. Shell growth, flesh dry weight, the concentration of stable isotopes of carbon and nitrogen, and metal accumulation (Cd, Pb, Cu and Zn) in the bivalves were analyzed. Bivalves showed significant growth compared with their respective starting sizes, although closeness to the fish farm did not enhance such growth. The stable isotopes content indicated that there was no relationship between the main input of organic matter from the fish farm (the feed) and the trophic behavior of the bivalves. Neither did metal accumulation show a trend along the distance gradient from the fish farm. All the results were consistent in indicating that neither oysters nor mussels fed on fin fish farming wastes. This work demonstrates that the polyculture of fin fish and bivalves does not represent an appropriate tool for reducing the environmental impact of fin fish aquaculture in open water.