This data set comprises all fish biomass data collected during the RV ALKOR cruise AL551 conducted in March 2021 (6th to 13th March) in the Western Baltic Sea by the University of Hamburg. The cruise was part of the 5-year MARSYS teaching cruise program of the University of Hamburg GFP20-1_047. The fish biomass data were collected directly on board. Fish biomass was taken after sorting through a complete fish haul into the single species and weighing them on a scale on board.