Duckweed, the family Lemnaceae, comprises a group of around 40 different aquatic flowering plant species. They can grow very fast, producing higher biomasses compared to terrestrial plants (up to 79 t DM ha-1 a-1, Leng et al. 1995). Furthermore, they are very efficient in uptake of nitrogen and phosphorous and are producing protein of high quality (Stadtlander et al. 2019). Protein contents usually range between 25 to 35% of DM (dry matter) but have been reported to be as high as 45% of DM. Duckweed (Spirodela polyrhiza) has been successfully grown on diluted cow slurry and been fed to rainbow trout fry (Stadtlander et al. 2019). Common carp (Cyprinus carpio) is among the most important cultured fish species in the world, with a production of 4.13 mio t in 2017, and cyprinids (carp-like fish) in general contribute around 50% of global aquaculture fish production. In this study, we tested two different duckweed (S. polyrhiza) meals, one dried (DWD) and one fermented (DWF), in three different concentrations in the diet of carp fry and compared results to a duckweed free control (C)
