This fattening project of bluefin tuna aims at giving a first assessment of the technical feasibility of fattening wild bluefin tuna caught by the French and Spanish bait boats in the south of the Bay of Biscay. In a context of more and more restrictive fishing for this species (Total Allowable Catch and quotas, individual weight limits, drifting gill-net prohibition, PPS and licenses, etc.), this system can constitute a way of increasing the value of the catches. The working hypothesis rests on the choice of exploitation including strong involvement of the fishermen, both for the supply of tunas and fish food, and for the maintenance of the cages. This first study (Caill-Milly et al., 2001a) has been built primarily on a bibliographic research and on visits to existing Mediterranean sites (Spain and Malta). The constraints and the advantages of the south of the Bay of Biscay are pointed out.