Drivers and stressors of Atlantic herring (Clupea harengus) recruitment in inshore Baltic Sea spawning areas

For fishery assessments in Northern Europe it is a major challenge to quantify the role of small- and meso scale drivers and stressors for overall recruitment strength of Atlantic herring populations. The “Ruegen” herring is considered a significant component of the spring spawning herring stock in the Western Baltic Sea. Since reproduction and rearing of larvae takes place in estuaries and lagoons, local hazards to early life stages can potentially introduce survival bottlenecks transported to higher spatial scales and affect entire population dynamics. To increase mechanistic understanding on the effects of local stressors, top down as well as bottom up drivers were investigated as well as consequences of highly fluctuating environment in the shallow transitional waters. A combination of long term monitoring data on larval abundance and field experiments on egg mortality on littoral spawning beds revealed two major survival bottlenecks occurring at different life stages in different periods along the reproductive phase. The first hatching cohort in March is rather subjected to limitations in suitable plankton food whereas the second hatching cohort in mid-April is mostly structured by egg mortality induced by cascading consequences of warmer spring temperatures in synthesis with high eutrophication levels. Related factors increasing egg mortality are e.g. stickleback (Gasterosteus aculeatus) predation, storm events and fungal infestations.