This data contains monitoring data of the RV ALKOR cruise AL585 conducted in January 2023 (January 23rd – January 31st) in the Western Baltic Sea by the University of Hamburg. The cruise was part of the 5-year monitoring programme Winter cod 2021-25 (Monitoring winter spawning activity of Western Baltic cod (Gadus morhua) (2021-25)) of the University of Hamburg aiming at investigating the early-winter spawning activity of Western Baltic cod to identify potential climate-induced shifts in spawning phenology. The data set includes cod single fish analysis data. All single fish measurements were made directly on board. Full wet weights of cod were measured and subsequently cod were gutted and liver weights, gonad weights and sexes, gutted wet weights (i.e., gutted wet mass), gonad stages and liver nematode infestations were recorded, and Otoliths were taken.