Description: Pelagic and demersal juvenile Baltic cod Gadus morhua L. were collected on the slope and the top of Rønne bank in the Baltic Sea during 2 cruises in November and December 1998. The objective of this study was to evaluate distinct changes in otolith increment width observed in demersal juveniles by comparison with laboratory-reared individuals, and to investigate the factors determining variation in these increments. The different increment-width patterns were identified with a method based on the widths of consecutive increments. Otolith increment widths of juvenile cod were found to be highly variable within and between individuals, in both the experimental and the field samples. The first change in increment pattern observed in the field samples was related to settling. The formation periodicity of increments within the different pattern intervals was confirmed with a growth model based on otolith growth rates of juvenile cod reared in the laboratory under different conditions. In this model, otolith growth rate was expressed as a function of rearing temperature and fish dry weight. Otolith growth of the field samples was calculated using ambient temperatures obtained from a 3D-circulation model. The best fit to observed otolith growth rates was obtained under the assumption that fish on the slope performed daily vertical migrations between the warm surface layer and the cold bottom layer. The data suggested that fish stayed in the surface layer during the first increment-pattern interval, performed vertical migrations during the second interval, and stayed in association with the seafloor in the subsequent interval, corresponding to the time after the breakdown of the thermocline.

Description: From numerical model simulations, fluxes of volume, heat and salt have been calculated for different hydrographical sections in areas which are important for the deep water exchange in the Baltic Sea. The calculated deep water flow in the Arkona basin is in accordance with independent estimations obtained from profile data. Model results reveal strong seasonal and inter-annual variability in the calculated fluxes. The variability is governed by the prevailing atmospheric conditions. It is found that the strength of the upper layer low saline flow in the Arkona Basin which on average is directed to the west, opposite to the mean wind direction, is compensated by a high saline flow in deeper layers. The upper layer flow is a combination of a flow forced by the fresh water surplus directed to the west, and a wind-driven part. In dependence on the prevailing wind conditions the resulting flow is either increased or decreased. Furthermore, increasing upper layer flow results in an increased lower layer flow in opposite direction. The annual mean flow is weakly correlated with the annual mean runoff to the Baltic Sea. In accordance with the mean circulation, the flow through the Bornholm Channel is on average directed to the east, and south of Bornholm to the west indicating an import of heat and salt to the Bornholm Basin through the Bornholm Channel and an export south of Bornholm. Flux characteristics change further downstream in the Stolpe Channel. The volume flow in the upper layer shows a strong seasonal signal. During autumn to spring the flow is mainly directed to the east, in summer, the flow direction is reversed. Flow in westerly directions is related to increased lower layer flow in easterly directions. On average, the net flow through the Stolpe channel is directed to the east which is in accordance with the mean circulation. Calculated fluxes show high intra- and inter-annual variability with no obvious trend during the simulation period. The variability of the deep water stratification in the deep basins of the Baltic Sea is directly controlled by the changing flux characteristics.

Description: Spatial and temporal variability in environmental factors can exert major influences on survival and growth of living organisms. However, in many key areas of fisheries science (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogeneity impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 32), we evaluated spatial and temporal differences in conditions affecting the survival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identify spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatching success and possibly recruitment in the Baltic stock.