This data set comprises CTD data of the RV ALKOR cruise AL549 conducted in January 2021 (January 25th – February 1st) 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.

Note that the CTD data presented here is already processed data containing the deletion of the upcast (i.e. measurements in the data are obtained from the downcast of the CTD haul) and a rough quality check for outliers (used reliability thresholds: salinity should range no more than from 37 PSU: Pressure should be always 15 mg; Albeit temperature no negative values should occur). Measurements exceeding these reliability thresholds were flagged as outliers and removed. Note that additional outliers in the data may occur.