ALBERT

Description: This data set contains results produced by particle drift modeling exercises with the Kiel Baltic Sea Ice-Ocean Model (BSIOM) to analyze the variability of large scale drift patterns within the Baltic Sea. The provided parameters are: - Mean geographical distance [km], divided into north-south and east-west component - stability of particle drift distance - relative dispersal [%] The parameters were separated and averaged for the following time and space intervals: - drift duration [days] in 5 days increments from 5 to 50 days drift - vertical particle drift depth [m] with the following values: 2.5; 7.5; 12.5; 17.5; 25.0; 35.0; 45.0; 55.0; 65.0; 75.0; 85.0 - horizontal resolution: 54.0°N to 66.0°N in 0.5° increments and 10°E to 30°E in 1° increments. The analyses based on this data set are described in more detail and results are discussed in the article. Connectivity between different populations of a species is a central parameter in the fields of ecology and evolutionary biology. We here provided decadally, regionally, and depth layer resolved information on connectivity and dispersal patterns for the entire Baltic Sea as a tool for supporting population genetic and ecological studies. The general method to assess dispersal used was bio-physical modelling, which is suitable for biological dispersal that is highly influenced by the physical water transport in ocean circulation. The results were assessed from Lagrangian particle tracking using ocean circulation model outputs. Generally, for the whole Baltic Sea as well as for all subareas, we observed persistent patterns of dispersal that reflected the basin-like structure of the Baltic Sea, with less transport between the basins. At the same time, dispersal distance and in extension, local retention versus dispersal of particles to other sub-areas, varied considerably over four decades (1970–2010) and among regions within the Baltic Sea, corresponding to a range from high connectivity to partial dispersal barriers. Based on the example of Eastern Baltic cod we then investigated how our dispersal distance datasets can serve as a tool to assess dispersal and the expected connectivity among different populations of a species, as long as some biological information is available. For example, our finding of high dispersal of particles from the Bornholm Basin to the other Eastern Baltic basins could help to explain recent results indicating lack of genetic differentiation of cod across the eastern Baltic Sea. Our results also indicate that the shift in spawning time observed in cod over the past decades and the resulting exposure of eggs and larvae in the water column to a time of the year with a different current regime has likely affected egg and larval export. Finally, our case study also demonstrates how inter-annual variability of ocean current speed and direction at the time of peak reproduction is likely to affect the connectivity among the subareas in the Baltic. To conclude, connectivity datasets from this study are freely available, and can represent a powerful tool to apply in evolutionary and ecological studies of a variety of species in the Baltic Sea.

Description: This data set contains several tables with modeled and measured data and also correlations coefficient data used in the publication. The model data are aggregations of areas in the Arkona Basin (AB) and Bornholm Basin (BB) from the Kiel Baltic Sea Ice and Ocean model (BSIOM) for the period 1971 to 2015. See the puplication for details about horizontal aggregation. Raw Data of the Model is not included in this data deposition, but are given by the authors upon request and when credited in any use of the data. Monthly Means of the AB and BB are used for detailed correlation analysis with time lags (raw data is given in the files "ModelData_O2_BornholmBasin_MM_ModelDepths.." and "ModelData_S_ArkonaBasin_MM_modelDepths.."). Resulting data of all combinations for depth layers in either Basin, and Monthly lags up to a full year, is given in the "ModelData_correlation_ABMM_vs_BBMM.txt" file. Monthly profiles of the Bornholm Basin were also used to produce monthly means of S,T,O2 and cod egg survival chances on certain levels of water density (1009, 1010, 1011, 1012 and 1013 kg/m^3) given in the file "ModelData_O2_BornholmBasin_MM_onDensLayers.txt" Model data for the Bornholm Basin were also used to produce a time series of daily resolution of the mean oxygen content below the halocline. Data is given in the file "ModelData_O2_BornholmBasin_DM_rough_layers.txt" Measured salinity at the 33m depth level in a daily resolution is taken from the Arkona Basin platform (permanent measuring buoy) operated by the Federal Maritime and Hydrography Agency, Hamburg, Germany. Given in file "Measured_S_ArkonaBasin_DM_33m_corrected.txt"

Description: In this study the drift of eastern Baltic cod larvae and juveniles spawned within the historical eastern Baltic cod spawning grounds was investigated by detailed drift model simulations for the years 1971 to 2010, to examine the spatio-temporal dynamics of environmental suitability in the nursery areas of juvenile cod settlement. The results of the long-term model scenario runs, where juvenile cod were treated as simulated passively drifting particles, enabled us to find strong indications for long-term variations of settlement and potentially the reproduction success of the historically important eastern Baltic cod nursery grounds. Only low proportions of juveniles hatched in the Arkona Basin and in the Gotland Basin were able to settle in their respective spawning ground. Ocean currents were either unfavorable for the juveniles to reach suitable habitats or transported the juveniles to nursery grounds of neighboring subdivisions. Juveniles which hatched in the Bornholm Basin were most widely dispersed and showed the highest settlement probability, while the second highest settlement probability and horizontal dispersal was observed for juveniles originating from the Gdansk Deep. In a long-term perspective, wind-driven transport of larvae/juveniles positively affected the settlement success predominately in the Bornholm Basin and in the Bay of Gdansk. The Bornholm Basin has the potential to contribute on average 54 % and the Bay of Gdansk 11% to the production of juveniles in the Baltic Sea. Furthermore, transport of juveniles surviving to the age of settlement with origin in the Bornholm Basin contributed on average 13 and 11% to the total settlement in the Arkona Basin and in the Gdansk Deep, respectively. The time-series of the simulated occupied juvenile cod habitat in the Bornholm Basin and in the Gdansk Deep showed a similar declining trend as the Fulton's K condition factor of demersal 1-group cod, which may confirm the importance of oxygen-dependent habitat availability and its effect on density dependence as a process relevant for recruitment success.

Description: The Eastern Baltic cod abundance started rapidly to increase in the mid-2000s as evidenced by analytical stock assessments, due to increased recruitment and declining fishing mortality. Since 2014, the analytical stock assessment is not available, leaving the present stock status unclear and casting doubts about the magnitude of the recent increase in recruitment. Earlier studies identified main factors impacting on cod reproductive success to be related to the loss of two out of three spawning areas in the 1980s caused by lack of major Baltic inflows with a concurrent reduction in salinity and oxygen. Other important factors include prey availability for first-feeding larvae, egg predation by sprat and herring and cannibalism on juveniles, all in one way or the other related to the prevailing hydrographic conditions. These factors cannot explain increased reproductive success in the last decade, as the period was characterized by an absence of large-scale Baltic inflows since 2003 and persistent anoxic conditions in the bottom water of the deep Baltic basins. This questions the perception of the increased recruitment in later years and challenges our present understanding of cod recruitment dynamics in the Baltic Sea. In this contribution, we review evidence from the recent literature supplemented by information from latest research cruises to elucidate whether cod reproductive success indeed has increased during the last decade, and we suggest the key processes responsible for the recent dynamics in cod recruitment and outline directions for future research.

Description: Dates of Cruise: 16.05. – 28.05.2016 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Klaipeda, Lithuania, 21.05. – 23.05.2016

Description: Dates of Cruise: 13.05. – 29.05.2017 Areas of Research: Physical, chemical, biological and fishery oceanography Port Calls: Visby, Sweden, 19.05. – 21.05.2016

Description: The disappearance of larger individuals and the decrease in individual body condition suffered by Atlantic cod Gadus morhua in the eastern Baltic during the past two decades can be expected to affect the stock reproductive output. To investigate this, female G. morhua were collected during the spawning and pre-spawning period in 2015-2016. The current individual potential fecundity (FP) of eastern Baltic G. morhua was estimated and analysed in relation to total length (LT) and indices of nutritional status such as body condition (K) and hepato-somatic index (IH) using generalized linear models. In addition, the current prevalence of atresia and its potential relation to K were investigated. Moreover, a calibration curve to estimate FP from oocyte diameter, based on the autodiametric oocyte counting method, was established for the first time for eastern Baltic G. morhua and can be used for future fecundity studies on this stock. The results showed that FP was mainly positively related to fish length, but K and IH also contributed significantly to the variation in FP. The model predicted that fish with K=1·2 have a FP 51% higher than fish of the same LT with K=0·8. The prevalence of fecundity regulation by atresia was 5·8%, but it was found only in fish in the pre-spawning maturity stage and with low K. Temporal changes in biological features such as the length composition and individual body condition of eastern Baltic G. morhua, should be accounted for when estimating stock reproductive potential.