Description: The files contain the data according to the figures in the publication.

Description: Highlights: • Recruitment indicator: egg survival probability vs. “reproductive volume concept”. • Traditional sampling methodology unable to resolve spatial egg distributions. • Predominance of self-sustaining stock components and low connectivity. • Egg buoyancy and topographic features: limitation of cod egg transport. • Sedimentation as a new mortality source of eastern Baltic cod eggs. Abstract: In the highly variable environment of the Baltic Sea two genetically distinct cod stocks exist, one west of the island of Bornholm, which is referred to as the western stock, and one to the east of Bornholm, the eastern stock. A hydrodynamic model combined with a Lagrangian particle tracking technique was utilised to provide spatially and temporally resolved long-term information on environmentally-related (i) spawning habitat size, (ii) egg/yolk-sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of eastern Baltic cod. Simulations were performed to quantify processes generating heterogeneity in spatial distribution of cod eggs and yolk sac larvae up to the first-feeding stage. The spatial extent of cod eggs represented as virtual drifters is primarily determined by oxygen and salinity conditions at spawning, which define the habitat requirement to which cod’s physiology is suited for egg development. The highest habitat suitability occurred in the Bornholm Basin, followed by the Gdansk Deep, while relatively low habitat suitability was obtained for the Arkona and the Gotland Basin. During drift egg and yolk sac larval survival is to a large extent affected by sedimentation. Eggs initially released in the western spawning grounds (Arkona and Bornholm Basin) were more affected by sedimentation than those released in the eastern spawning grounds (Gdansk Deep and Gotland Basin). Highest relative survival of eastern Baltic cod eggs occurred in the Bornholm Basin, with a pronounced decrease towards the Gdansk Deep and the Gotland Basin. Relatively low survival rates in the Gdansk Deep and in the Gotland Basin were attributable to oxygen-dependent mortality. Low oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas temperature dependent mortality was only evident after severe winters. Egg buoyancy in relation to topographic features like bottom sills and strong bottom slopes could appear as a barrier for the transport of Baltic cod eggs and yolk sac larvae and could potentially limit the connectivity of Baltic cod early life stages between the different basins in the western and eastern Baltic Sea. The possibility of an eastward directed transport up to the first-feeding larval stage exists only for eggs and yolk sac larvae at high buoyancy levels, suggesting that dispersal of early life stages between these spawning areas is limited.

Description: In the Baltic Sea, two genetically distinct cod populations occur, the eastern and the western Baltic cod. Since 2006, cod abundance has increased substantially in the Arkona Basin (SD 24), the potential mixing area between the two stocks management areas, presumably due to spill-over from the eastern stock. In this study, the spatio-temporal dynamics of stock mixing were analysed using shape analysis of archived otoliths. Further, the impact of eastern cod immigration on recruitment in the western Baltic Sea was investigated using hydrographic drift modelling. The percentage of eastern Baltic cod in the Arkona Basin increased from ca. 30% before 2005 to 〉80% in recent years. Geographic patterns in stock mixing with a pronounced east–west trend suggest that immigration occurs north of Bornholm, but propagates throughout the Arkona Basin. The immigration cannot be attributed to spawning migration, as no seasonal trend in stock mixing was observed. Based on environmental threshold levels for egg survival and time-series of hydrography data, the habitat suitable for successful spawning of eastern cod was estimated to range between 20 and 50% of the maximum possible habitat size, limited by primarily low salinity. Best conditions occurred irregularly in May–end June, interspersed with years where successful spawning was virtually impossible. Using a coupled hydrodynamic modelling and particle-tracking approach, the drift and survival of drifters representing eastern cod eggs was estimated. On average, 19% of the drifters in the Arkona Basin survive to the end of the yolk-sac stage, with mortality primarily after bottom contact due to low salinity. The general drift direction of the surviving larvae was towards the east. Therefore, it is the immigration of eastern cod, rather than larval transport, that contributes to cod recruitment in the western Baltic Sea.