<P>Macro algal seaweeds are a promising feedstock for biofuels production. Yet, their relatively low fermentable carbohydrate content and the inefficient methods used for their conversion hamper their utilization. The optimized production of <I>Ulva rigida</I> co-cultured with fed-fish in an offshore mariculture (fish cages) system is reported. Enhanced production of biomass with elevated content of desired carbohydrates is achieved. The farmed biomass was further converted to bioethanol by a one-step sonication assisted SSF process. An ethanol yield of 16 wt% (based on the dry weight of algae) is obtained.</P>