Aquaculture likely plays an important role in future diets, yet the potential of aquaculture to upcycle biomass and supply nutrients in circular food systems remains largely unstudied. Under the circularity paradigm, animal production is constrained by the availability of so called low-opportunity-cost feed (LCF). Aquaculture species differ in their ability to convert LCF into food, and the implication for nutrient supply in food systems are unclear. In this paper we explore the specific role of aquaculture to upcycle LCF into valuable food for humans in Europe using the resource allocation model FEEDSOM (FEED Systems Optimization Model). We provide insight into what nutrients aquaculture can supply to the human diet, how much aquatic food can be produced when animals are fed exclusively with LCF, and what LCF can be recycled into fish feed. We selected Atlantic salmon, European seabass and common carp, produced in multiple productivity levels, to represent European aquaculture. Carnivorous aquaculture species can play an important role in circular food systems by upcycling eicosapentaenoic (EPA) and docosahexaenoic (DHA) from fisheries by-products. However, their dependence on fisheries by-products to supply EPA/DHA also limits their capacity to expand. Omnivorous aquaculture species do not rely on fisheries to supply EPA/DHA, but their overall contribution to EPA/DHA supply is relatively low. We found that under current aquaculture production and consumption in Europe, we cannot supply enough EPA/DHA for the European population. To reduce the nutrient gap of EPA/DHA, we can either increase aquaculture or eat more edible parts of the fish (both fish from aquaculture and fisheries). However, expanding aquaculture should not be the priority, as this requires more LCF, production area, and puts pressure on the environment. Eating more edible parts of the fish can provide enough nutrients, including EPA/DHA, to meet the demand of the European population and help feed a