Retention of Western Baltic herring larvae within the main spawning area

During 2004-2008, recruitment success of Western Baltic Spring-Spawning herring (WBSS) has shown an unprecedented decline. The reasons for this development are currently unknown. Among others, larval drift is one crucial factor that is known to affect larval survival and therefore could influence recruitment success. This might be of particular importance for WBSS which spawn in semi-enclosed waters that are assumed to provide more suitable conditions for larval survival than the surrounding open Baltic Sea. Within this study, the risk of herring larval dispersal from the Greifswalder Bodden (GWB), the main spawning area of WBSS to the adjacent Baltic Sea has been analyzed. Larval drift was simulated by a twodimensional Lagrangian particle tracking model which was forced by depth-integrated flow fields derived from a high resolution three-dimensional hydrodynamic model. Drift patterns under constant and real wind conditions were investigated. Focus was set on the WBSS spawning seasons in 2008 and 2009, two years with contrasting recruitment success. Finally, a retention index was defined which estimates the percentage of remained larvae during both years, based on calculated retention probabilities of virtual cohorts and larval abundances found in the GWB during the Rügen herring larvae survey in 2008 and 2009. The results impressively demonstrate that for all scenarios tested, a significant proportion of larvae remained in the GWB. Intra- and inter-annual differences in the amount of removed larvae were low. Taken together, larval drift can not explain the observed recruitment failure of WBSS in 2008. By contrast, results suggest that larval retention is a crucial and stable feature of the life strategy of WBSS. Significant changes in prevailing wind fields as a consequence of climate change could adversely affect this basic feature and thus reduce larval survival of WBSS. While many case studies deal with potential impacts of climate change on open-ocean ecosystems, the present study offers a great opportunity to address climate change scenarios to shallow water systems.