Does flat oyster (Ostrea edulis) rearing improve under open-sea integrated multi-trophic conditions?

Flat oysters (Ostrea edulis) were reared for a year both in integrated experimental conditions—close to a gilthead seabream (Sparus aurata) and seabass (Dicentrarchus labrax) farm—and in an open-sea reference location in the SW Mediterranean to ascertain whether their development could be enhanced in the vicinity of a fish farm as a consequence of the use of associated wastes. Some biometric data including size and weight measures, condition index (CI), lipid content, 15N signature, and fatty acid (FA) composition were compared in oysters raised in both conditions. Some water column descriptors were measured: temperature, salinity, dissolved oxygen, chlorophyll-a content, total particulate matter flux, particulate organic matter, total nitrogen content, and δ15N signaling. FA and δ15N in fish feed were also analyzed. Development of integrated oysters was significantly improved, induced by the greater availability of potential feed sources, and the CI and lipid content of oysters were also higher. Some specific FA of terrestrial origin such as oleic and linoleic acids were slightly higher in integrated oysters, while typical FA of the marine food web, such as eicosapentaenoic and docosahexaeonic acids, showed similar levels in both groups of oysters. The oyster FA profile and its correlation with environmental conditions revealed that a direct contribution on the part of fish farm-derived wastes occurred, but of minor relevance. Nevertheless, the enrichment in 15N observed in integrated oysters suggests that they might use some fish farm-derived wastes indirectly, probably through the phytoplankton which would have used the dissolved wastes.