Abstract Voss, R., Hinrichsen, H-H., Stepputtis, D., Bernreuther, M., Huwer, B., Neumann, V., and Schmidt, J. O. 2011. Egg mortality: predation and hydrography in the central Baltic. – ICES Journal of Marine Science, 68: 1379–1390. Cod and sprat are the dominant fish species in the Baltic pelagic ecosystem, both of great economic importance and ecologically strongly interlinked. Management of both species is challenged by highly variable recruitment success. Recent studies have identified predation and hydrographic conditions during the egg phase to be of critical importance. Two years of extensive field investigations in the Bornholm Basin, central Baltic Sea, were undertaken. In 2002, a typical stagnation situation characterized by low salinity and poor oxygen conditions was investigated, and in early 2003, a major inflow of North Sea water completely changed the hydrographic conditions by increasing salinity and oxygen content, thereby altering ecological conditions. The goal was to quantify egg mortality caused by predation and hydrography, and to compare these estimates with independent estimates based on cohort analysis. Results indicated high intra-annual variability in egg mortality. Cod and sprat egg mortality responded differently to the major Baltic inflow: mortality related to hydrographic conditions increased for sprat and decreased for cod. On the other hand, predation mortality during peak spawning decreased for sprat and increased for cod.
In the Western Baltic, the invasive ctenophore Mnemiopsis leidyi was recorded for the first time in autumn 2006. An eastward propagation of the ctenophore into the central Baltic, and thus into important spawning grounds of major Baltic fish stocks, was observed in 2007. The focus of the present study was to investigate the seasonal phenology of this introduced species and whether it is able to form a self-sustaining population in this area. Therefore, the variability of temporal and spatial distribution of M. leidyi in the Bornholm Basin was analyzed over the first 4 years following the invasion and related to ambient hydrographic parameters. Results show a clear seasonal pattern. In contrast to the majority of other native and exotic habitats, the seasonal phenology showed highest abundances in spring and autumn months and only sporadic or even no appearance during summer. Vertical distribution was mostly confined to water layers below the permanent halocline and significantly influenced by ambient temperature. Our results indicate that there is no self-sustaining population of M. leidyi in the central Baltic Sea. Instead, the species is most likely re-introduced into the Bornholm Basin every year via lateral advection from source populations in the Western Baltic. These findings are important not only to further assess the potential impact of M. leidyi on the pelagic ecosystem of the central Baltic Sea, but also for a better understanding of the mechanisms of its invasion into other marine areas.
We employed a coupled three‐dimensional biophysical model to explore long‐term inter‐ and intra‐annual variability in the survival of sprat larvae in the Bornholm Basin, a major sprat spawning area in the Baltic Sea. Model scenarios incorporated observed decadal changes in larval diel vertical distribution and climate‐driven abiotic and biotic environmental factors including variability in the abundance of different, key prey species (calanoid copepods) as well as seasonal changes, long‐term trends, and spatial differences in water temperature. Climate forcing affected Baltic sprat larval survival both directly (via changes in temperature) and indirectly (via changes in prey populations). By incorporating observed changes in larval diel vertical migration, decadal changes in modeled and observed survival of Baltic sprat agreed well. Higher larval survival (spawning stock biomass) was predicted in the 1990s compared to the 1980s. After changing their foraging strategy by shifting from mid‐depth, low prey environment to near‐surface waters, first‐feeding larvae encountered much higher rates of prey encounter and almost optimal feeding conditions and had a much higher growth potential. Consequently, larvae were predicted to experience optimal conditions to ensure higher survival throughout the later larval and early juvenile stages. However, this behavioral shift also increased the susceptibility of larvae to unfavorable winddriven surface currents, contributing to the marked increase in interannual variability in recruitment observed during the past decade.
