New perspectives on Eastern Baltic cod movement patterns from historical and contemporary tagging data

Abstract

Knowledge of the movement patterns and area utilisation of commercially important fish stocks is critical to management. The Eastern Baltic cod Gadus morhua, one of the most commercially and ecologically important stocks in the Baltic Sea, is currently one of the most severely impacted fish stocks in Europe. During the last 2 decades, this stock has experienced drastic decreases in population size, distributional range, individual growth and body condition, all of which may have affected the movements between different areas of the Baltic Sea. In this study, we investigated the seasonal movement patterns of Eastern Baltic cod by re-analysing historical tagging data collected by the countries surrounding the Baltic Sea (1955-1988) and compared historical patterns with contemporary data from a recent international tagging experiment (2016-2019). Our re-analyses of historical data showed the presence of different movement behaviours, i.e. resident or seasonally migratory, with larger distances moved by cod released in the northern and central Baltic areas compared to cod released in the southern Baltic areas. Furthermore, trends from the recent tagging experiment indicate a persistent resident strategy in the southern Baltic area. These findings present additional information on general movement patterns and area utilisation of Eastern Baltic cod that could inform future management actions and aid stock recovery.

Evaluating drivers of spatiotemporal individual condition of a bottom-associated marine fish

“…We conducted a sensitivity analysis by fitting the condition model to different parts of the data. The different models were only cod above 30 cm, only cod below 30 cm, omitting subdivision 24 (the mixing zone with western Baltic cod (Mion et al ., 2022)), and including only grid-points with cod above a certain threshold when calculating median variables across the ICES rectangle. However, the model coefficients were similar across all models ( SI Appendix , Fig.…”

“…For the condition model, we included covariates at spatial scales that roughly reflect the habitats cod would have been exposed to during the seasonal build-up of energy reserves. Recent tagging studies suggest cod are either stationary or mobile over the course of a year moving between feeding and spawning habitats (Mion et al ., 2022). However, within the feeding season, cod move roughly over an area corresponding to an ICES rectangle (1° by 30’, SI Appendix Fig.…”

Diverse Transcriptome Responses to Salinity Change in Atlantic Cod Subpopulations

“…The latter is also used by EBC, which can result in mixing [20]. Former spawning grounds for the eastern stock in Gda ńsk and Gotland Deep are dysfunctional because of strongly reduced inflows of high salinity and oxygenated water from the North Sea and increased hypoxia; thus, reproduction mainly takes in the Bornholm Basin [23][24][25]. During vertical and horizontal migrations to spawning areas with a depth occupation of over 60 m and active travelling speed of up to 14 m d −1 , Baltic cod is exposed to variable salinities [26].…”

“…However, understandings of the local genetic adaptation to low salinity in the Baltic Sea are still limited. Further, in recent decades, the Baltic cod population has decreased, especially EBC, which have experienced a strong depletion manifested by decreased resistance to pathogens, length-weight factor, reproduction/productivity, and an increase in natural mortality [25,35]. The reason for this, alongside overfishing, is the change in climate and environmental conditions, including salinity, which has caused a reduction in spawning areas.…”