In this study, the production of polyhydroxyalkanoated PHA-rich microbial biomass as a novel feed additive in aquaculture was investigated at a lab-scale. Bio-based volatile fatty acids (VFAs), obtained from the acidogenic fermentation of agricultural residues in existing anaerobic digestion plants, were used as carbon and energy to cultivate the PHA-rich microbial biomass. The experimental activities were carried out using Thauera sp. Sel9 as pure strain, which was grown in a continuous stirred-tank reactor (CSTR) operated at three different hydraulic retention times (HRT). The highest productivity obtained of biomass cells was 0.69 g/L day, operating at one day HRT while the observed PHAs production yield was 0.14 gPHA/g soluble COD removed. At these conditions, the PHA concentration in the microbial cells was 41%. Although the sulfur amino acids were available at high concentrations and above the typical concentration found in fishmeal, the amino acids profile of the obtained biomass revealed a lack of histidine and threonine. A preliminary economic analysis showed that the production of a novel source of feed additive from the conversion of agro-residues could give higher benefits in terms of revenues compared to the production of biogas production through anaerobic digestion.