Mussels do not directly assimilate fish farm wastes: Shifting the rationale of integrated multi-trophic aquaculture to a broader scale.

Pollution is one of the most significant issues that is currently impeding the development of fish farming. Integrated multi-trophic aquaculture (IMTA) has the potential to reduce the accumulation of organic wastes in the environment by using taxa of lower trophic levels such as filter feeders. However, the capacity of filter feeders to assimilate significant quantities of fish farm wastes has not yet been fully tested in situ. We analyzed the stable isotopes δC and δN in mussels from six fish farms and from six other areas that were not influenced by fish farming, at two water strata (surface and mid-water) across a marked gradient of eutrophication along more than 900 km of coastline in the Western Mediterranean. We found that the mussels did not directly assimilate fish farming wastes. Consequently, fish farming wastes did not constitute a major component of mussel diet, irrespective of local productivity and depth in the water column. These outcomes do not necessarily mean that IMTA is not suitable in other cases, but rather that there should be a shift in the rationale of IMTA by modifying the concept of direct assimilation of wastes to a more general approach of IMTA based on regional budgets of nutrients.