Aquaculture of finfish and shellfish is an important source of protein for human food. However, cultivated fish are to a large extent fed with fish meal from wild fish which is now a limited resource. The present invention presents as an alternative source of protein a zygomycetes strains which has been cultivated on paper pulp waste water e.g. spent sulphite liquor (SSL) which is an environmental problem for many paper pulp mills. The biomass is rich in protein with an unusually favorable composition of amino acids. It also contains vitamins unsaturated 18 carbon fatty acids, and chitosan. When the standard fish meal component of fish feed was replaced by zygomycetes biomass on an isoproteic basis and fed to young rainbow trout, it was eaten well and the specific growth rate of the fish eating zygomycetes feed exceeded that of fish given standard feed.
