Circular Approaches in Aquaculture: Fishmeal Replacement and Algal Bioremediation

This project investigates two strategies to improve the sustainability of aquaculture: replacing part of the fishmeal in goldfish diets with protein-rich brewing by-products (brewer’s spent grain and yeast, BSG and BSY) and assessing the ability of Arthrospira platensis (Spirulina) to bioremediate aquaculture wastewater while producing valuable biomass.

Fish were assigned to three dietary treatments: a control and two diets with 10% of fishmeal protein replaced by BSG or BSY. Growth performance, specific growth rate (SGR), weight gain ratio (WGR); feed conversion ratio (FCR), condition factor (CF), and protein efficiency ratio (PER) were assessed. Fish In: Fish Out (FIFO) was calculated to quantify reliance on marine-derived ingredients.

The bioremediation capacity of A. platensis was evaluated using wastewater from the control fish tanks. Algal growth was tested under three conditions: Zarrouk medium (control, C); wastewater (WW); and bicarbonate added wastewater (WWB). Biomass production, biochemical composition, and nutrient removal efficiency were measured. A life cycle assessment (LCA) was also performed using ISO 14040 standards and the ReCiPe method.

Replacing fishmeal with BSG or BSY did not affect growth performance, SGR, or FCR compared with the control diet. Fish fed the BSG diet showed a significantly higher PER (p < 0.05). FIFO values decreased from 5.39 (control) to 2.90 (BSG) and 3.27 (BSY). Spirulina cultivated in WW and WWB achieved biomass yields and protein contents comparable to the control treatment, while phycocyanin levels increased significantly. Nutrient removal efficiencies exceeded 94% for nitrogen and 99% for phosphorus in WW-based systems. The LCA identified WW-based cultivation as the most sustainable scenario due to the elimination of synthetic fertilisers and the additional benefits associated with wastewater bioremediation.

Integrating brewery by-products into aquafeeds and growing Spirulina on aquaculture wastewater provides a circular-economy approach that reduces reliance on marine ingredients, recycles nutrients, and lowers environmental impacts, promoting sustainable aquaculture intensification.