Description: The overall aim of my thesis was to improve our understanding of environmental drivers causing the dynamics of the Eastern Baltic cod reproduction habitat and to assess their relevance for a possible application in the sustainable stock management. A novel approach to map the reproductive habitat of Eastern Baltic cod, the Buoyancy Depending Reproductive Layers (BDRL), was developed and used to propose an alternative stock indicator, the effective Spawning Stock Biomass (eSSB). The eSSB was found to improve the fit of a recruitment model compared to the model using the conventional Spawning Stock Biomass. Oxygen depletion was also negatively impacting the available size of nursery areas for juveniles. The mechanism was able to partly explain the observed decline of the condition of juveniles in the nursery areas, because a high population density in the remaining habitat could increase the impact of density depending effects. Furthermore, by the application of the novel approach of BDRLs the spawning habitat was shown to be sensitive to eutrophication and that this sensitivity is strongly depending on the size of the female spawner using the habitat. As predicted, the BDRL approach was superior to the “classic” approach, the Reproductive Volume (RV) because it was more sensitive to environmental change, able to incorporate stock structure, was not overestimating the spawning habitat in the eastern spawning areas and could provide estimates of other stressors depending on the female spawner size. Due to a permanently installed measurement platform in the Arkona Basin, the new methodology could be used to establish a new environmental indicator on the spawning habitat conditions. It was recommended to be used in future stock assessments.

Description: Wochenbericht der Forschungsreise mit R.V. „Alkor“ AL 493 13.05.2017 – 20.05.2017

Description: The Baltic Sea is a dynamic environment responding to various drivers operating at different temporal and spatial scales. In response to climate change, the Baltic Sea is warming and the frequency of extreme climatic events is increasing (Lima & Wethey 2012, BACC 2008, Poloczanska et al. 2007). Coastal development, human population growth and globalization intensify stressors associated with human activities, such as nutrient loading, fisheries and proliferation of invasive and bloom-forming species. Such abrupt changes have unforeseen consequences for the biodiversity and the function of food webs and may result in loss of ecological key species, alteration and fragmentation of habitats. To mitigate undesired effects on the Baltic ecosystem, an efficient marine management will depend on the understanding of historical and current drivers, i.e. physical and chemical environmental conditions and human activities that precipitate pressures on the natural environment. This task examined a set of key interactions of selected natural and anthropogenic drivers in space and time, identified in Task 3.1 as well as WP1 and WP2 (e.g. physico-chemical features vs climate forcing; eutrophication vs oxygen deficiency vs bio-invasions; fisheries vs climate change impacts) by using overlay-mapping and sensitivity analyses. The benthic ecosystem models developed under Task 2.1 were used to investigate interactions between sea temperature and eutrophication for various depth strata in coastal (P9) and offshore areas (P1) of the Baltic Sea. This also included investigation on how the frequency and magnitude of deep-water inflow events determines volume and variance of salinity and temperature under the halocline, deep-water oxygen levels and sediment fluxes of nutrients, using observations and model results from 1850 to present (P1, P2, P6, P9, P12). The resulting synthesis on the nature and magnitude of different driver interactions will feed into all other tasks of this WP3 and WP2/WP4. Moreover, the results presented in this report improve the process-based and mechanistic understanding of environmental change in the Baltic Sea ecosystem, thereby fostering the implementation of the Marine Strategy Framework Directive.

Description: Oxygen minimum zones are increasing, yet the effects of these zones on the genetic composition of marine fish stocks has been neglected. We assessed the combined effects of stock size and structure, and the prevailing oxygen situation, on Eastern Baltic cod (ICES SD25) genetic diversity. For this purpose, we used an integrative long-term otolith sample and data series (1995-2013) to (1) calculate the approximate number of females with surviving eggs in a given year, i.e., contributing to reproduction (n F), and (2) the annually resolved cohort mean allelic richness as proxy of genetic diversity, based on 12 microsatellite markers. Cohort mean allelic richness showed strong year-to-year fluctuations though no permanent decline. Importantly, it was highly correlated with n F, but with an unexpected 1.5 year time lag that may be an artefact of Eastern Baltic cod ageing problems. Our findings indicated that environmental pressure can effect rapid alterations in exploited fish stock genetic composition, and pointed to the importance of large females for Eastern Baltic cod reproduction during stagnation periods. Considering the importance of standing genetic variation for the evolutionary potential of populations, this is relevant for projections of the future state of cod stocks under global change.

Description: 12.05. – 27.05.2014 Port Calls: Karlskrona, Sweden, 18.05. – 20.05.2014; Rönne, Danmark, 24.05. – 25.05.2014

Description: 16.05.2015 - 30.05.2015 Physical, chemical, biological research and fishery oceanography This multidisciplinary cruise extended a long-term data series on (eco-)system composition and functioning of the Baltic Sea, with a focus on the deeper basins. The series has been collected in similar form since 1986. A key characteristic of the cruise is the integration of oceanographic and biological information to enhance understanding of environmental and (fish) population fluctuations, and evolutionary processes in this system. The resulting datasets and samples are an essential component of the international EU project BONUS BIO-C3 coordinated by GEOMAR. The spatial focus lay on the Bornholm Basin (the most important spawning area of Baltic cod), but also included the Western Baltic Sea, Arkona and Gotland Basin, Gdansk Deep, and Stolpe Trench. Specific investigations included a detailed hydrographic survey (oxygen, salinity, temperature) of the Bornholm Basin, plankton surveys (zoo- and ichthyplankton, with the goal to determine the composition and the abundance and vertical and horizontal distribution of species, and to take samples for later measurements of nutritional condition), and pelagic fishery hauls for clupeid and gadoid fish. The latter served to determine stock structure, gonadal maturation, stomach contents, and egg production of sprat and cod, and to sample tissue for later genetic analyses and otoliths for age estimatation of individual cod. The abundance and distribution of fish species in the investigated area was also assessed with hydroacoustic methods. Additional cruise components were: (i) cod gonad and liver sampling for the determination of lipid content and histological studies; (ii) vertically resolved phytoplankton and zooplankton sampling for studies of plankton phenology.