Commercially viable aquaculture of noble crayfish Astacus astacus (Linnaeus, 1758), a once plentiful food species in Europe, requires production of suitable artificial diets for optimal growth in recirculating aquaculture systems (RAS). This technique provides the opportunity to culture this species in an enclosed ecosystem which minimizes the risk of infection— resulting in high-value food products as well as high-quality juveniles for restocking purposes. In the current study, noble crayfish were fed in RAS with a commercial diet and relied on natural forage in an open pond system (OPS) for two months. Energy, lipid, and fatty acid (FA) content of available and alternative diets [Cyprinin K2 (CK2), Mytilus edulis Rafinesque, 1815, Elodea spp. Michx.) and crayfish tissue were determined. Crayfish from the pond system had significantly (P < 0.01 OPS versus all RAS treatments; Pairwise Wilcoxon) higher lipid content (8.51%) and more diverse FA composition than RAS crayfish. At least 15 partly combined FAs were present in crayfish whereas seven were provided by the diet CK2 and 10 by M. edulis and Elodea spp. In addition to the recommended lipid (<10%) and protein (>30%) proportions for commercial dietary formulation, analyses indicate that FA composition and amount play a key role in A. astacus growth, particularly in RAS. At least 15 measured FAs are required in an appropriate crayfish diet in sufficient amounts to achieve high growth rates while not losing energy and growth potential for FA synthesis. The FA profile of Elodea spp. and M. edulis indicate suitability as a feed or at least as a viable supplementary dietary component.