Fish processing towards production of fillet gives rise to wastewater streams that are ultimately directed to biogas production

and/or wastewater treatment. However, these wastewater streams are rich in minerals, fat, and proteins that can be converted to

protein-rich feed ingredients through submerged cultivation of edible filamentous fungi. In this study, the origin of wastewater

stream, initial pH, cultivation time, and extent of washing during sieving, were found to influence the amount of recovered

material from the wastewater streams and its protein content, following cultivation with Aspergillus oryzae. Through cultivation

of the filamentous fungus in sludge, 330 kg of material per ton of COD were recovered by sieving, corresponding to

121 kg protein per ton of COD, while through its cultivation in salt brine, 210 kg of material were recovered per ton of COD,

corresponding to 128 kg protein per ton of COD. Removal ranges of 12–43%, 39–92%, and 32–66% for COD, total solids, and

nitrogen, respectively, were obtained after A. oryzae growth and harvesting in the wastewater streams. Therefore, the present

study shows the versatility that the integration of fungal cultivation provides to fish processing industries, and should be

complemented by economic, environmental, and feeding studies, in order to reveal the most promising valorization strategy.