Seaweed biomass is a promising, abundant but underutilized resource for industrial bioconversions. The aim of the project is to develop microbes and anaerobic microbial processes applying marine microalgae feedstock for production of health promoting bioactive ingredients in feed. In previous and ongoing projects, different types of seaweed were processed and a range of fermenting bacteria tested. Several combinations of processed seaweed fractions and fermentation bacteria were selected for further analysis and development. The objective of the current proposal is to up-scale the fermentation process and examine the benefit of using the resulting symbiotic feed formulation (both prebiotic and probiotic) as additives or supplements in fish feed. The project will have and economic and environmental impact by supplying feed formulations promoting gut health in fish, enhancing productivity and animal welfare and reducing utilization of antibiotic agents.