Description: Highlights: • Belt Sea cod, plaice and flounder differentiate in their specific egg densities. • Ontogenetic egg density increase in stage IV cod eggs elevates modeled egg mortality. • Drift model indicates retention in western Baltic in cod and flatfish yolk sac larvae. • No eastward transport to Arkona Sea or Bornholm Sea until end of yolk sac stage. • Ambiguity in flounder egg density could reflect more complex population structure. Abstract: Vertical distribution is an important feature of pelagic fish eggs and yolk sac larvae impacting their survival and dispersal, especially in heterogeneous and highly variable estuarine environments like the Baltic Sea. Egg densities determining the vertical distribution pattern were experimentally ascertained for cod (Gadus morhua), plaice (Pleuronectes platessa) and flounder (Platichthys flesus) from the western Baltic Sea. Plaice eggs floated at lower mean (±standard deviation) density range (1.0136 ± 0.0007 g cm−3) compared to cod (1.0146 ± 0.0009 g cm−3) and flounder eggs (1.0160 ± 0.0015 g cm−3), which floated on the highest density level. In flounder egg diameter was significantly related to egg density and in cod a weak correlation could be found between egg dry weight and density. All other relationships between female size, egg size, egg dry weight and egg density were not significant for any of the species. Available egg density data for Baltic Sea cod, plaice and flounder are summarized considering ICES subdivisions and stock management units. A hydrodynamic drift modeling approach was applied releasing drifters in the Belt Sea continuously from December to May, covering the species’ spawning seasons. The model implemented experimentally derived egg density ranges and included ontogenetic egg density modifications for cod eggs, increasing egg density from a late egg development stage to first hatch. A drifter was removed from the model, i.e. considered dead, when its initially prescribed density value exceeded the density range available at the temporally resolved geographical positions along the drift trajectories. Highest survival occurred during releases in April and May but no cohorts survived if they were drifted east into the central Arkona Basin or the central Baltic Sea, irrespective of whether a major Baltic Inflow (1992/1993) or a stagnation-year (1987/1988) was simulated. The dispersal characteristics of the surviving yolk sac larvae of all three species reflected retention within the Belt Sea or northwards transport through the Great Belt into the Kattegat and partly into the Skagerrak. There was no successful transport to more eastern Baltic areas past a hypothetical line from the island of Moen (Denmark) to Kap Arkona on Rügen Island (Germany).

Description: Die 160. Reise von FS "Poseidon" vom 6. Juli bis 4. August 1989 bildet den Auftakt einer Serie fischereibiologischer Fahrten zur Untersuchungen der Rekrutierungs- und Interaktionsmechanismen bei grönländischen Seefischbeständen. Das Programm unter der Leitung von Prof. D. Schnack wird in enger Zusammenarbeit mit dem Grönländischen Fischereiforschungsinstitut und dem Institut für Seefischerei der BFA für Fischerei durchgeführt. Es soll wissenschaftliche Grundlagen für ein ökosystem-orientiertes Fischereimanagement im Seegebiet vor Grönland liefern.

Description: Recruitment patterns of Baltic Sea sprat (Sprattus sprattus) were correlated to time series of (i) month- and depth-specific temperature conditions and (ii) larval drift patterns inferred from long-term Lagrangian particle simulations. From the latter, we derived an index that likely reflected the variable degree of annual larval transport from the central, deep spawning basins to the shallow coastal areas of the Baltic Sea. The drift index was significantly (P 〈 0.001) correlated to sprat recruitment success and explained, together with sprat spawning stock biomass, 82% of the overall variability between 1979 and 2003. Years of strong larval displacement towards southern and eastern Baltic coasts corresponded to relative recruitment failure, while years of retention within the deep basins were associated with relative recruitment success. The strongest correlation between temperature and recruitment occurred during August in surface waters, explaining 73% of the overall variability. Together, the two approaches advocate that new year classes of Baltic sprat are predominantly composed of individuals born late in the season and are determined in strength mainly by processes acting during the late larval and early juvenile stages. However, prior to be included in recruitment predictions, the biological mechanisms underlying these strong correlations may need to be better resolved

Description: The Baltic Sea is characterised by a heterogeneous oceanographic environment. The deep water layers forming the habitat of Baltic cod (Gadus morhua callarias L.) are subjected to frequently occurring pronounced anoxic conditions. Adverse oxygen conditions result in physiological stress for organisms living under these conditions. For cod e.g. a direct relationship between oxygen availability and food intake with a decreasing ingestion rate at hypoxia could be revealed. In the present study, the effects of oxygen deficiency on consumption rates were investigated and how these translate to stock size estimates in multi-species models. Based on results from laboratory experiments, a model was fitted to evacuation rates at different oxygen levels and integrated into the existing consumption model for Baltic cod. Individual mean oxygen corrected consumption rates were 0.1–10.9% lower than the uncorrected ones. At the currently low predator stock size, however, the effect of oxygen-reduced consumption on the total amount of eaten prey biomass and thus predation mortalities was only marginal. But should successful management lead to higher cod stock sizes in the future, then total predation mortalities will greatly increase and thus improved precision of these estimates would be valuable for the assessment of prey stocks.

Description: To study inter-annual differences in drift patterns of larval sprat from the Bornholm Basin, we used a 3D, eddy-resolving circulation model of the Baltic Sea and simulated the drift of Lagrangian particles for each of 24 years (1979-2002) of available forcing data. We observed that in some years particles were transported almost completely out of the basin, whereas circulation in other years retained the majority of drifters within the seeding area (Bornholm Basin). A new retention index was derived that is associated to age 0 sprat recruitment in ICES Subdivision 25, estimated from area-disaggregated MSVPA runs. The significant linear relationship (P〈0.018) between retention index and sprat recruitment explained 24% of the overall variability between 1979-2001. However, the correspondence has not been apparent during the 1980s, when both spawning stock biomasses and recruitment levels were consistently low in the central Baltic Sea. A strong positive coupling of recruitment success to basin retention was only seen during the last decade, characterized by relatively high levels of spawning stock biomass and a tremendous variability in sprat recruitment success. Retention indices corresponding to sprat larvae born late in the year (i.e. June) were much better correlated to recruitment success (R² 〉 80%) than those derived from particles released earlier into the model domain. This intra-annual pattern may suggest that recruitment success in central Baltic sprat was – at least during the last decade - predominantly determined by the relative survival of larval cohorts emerging late in the spawning season. The index is significantly cross-correlated to other environmental time series, most importantly annual surface temperature in August, which makes it difficult to conclude on the processes crucial for central Baltic sprat recruitment success.

Description: Pseudocalanus elongatus is a key species in the pelagic zone of the deep basins of the Central Baltic Sea. The copepod serves as a major food organism for larval as well as for adult, pelagic planktivorous fish. Large interannual fluctuations in the standing stock of P. elongatus have been attributed to significant changes in the hydrographic environment over the last two decades. In particular, the decreasing salinity in the Baltic deep basins, a result of a change in atmospheric forcing leading to an increase in rainfall since the 1980s and of a lack of pulses of saline water intrusions from the North Sea, was found to affect reproduction and maturation of the copepod. In parallel, dramatic changes in the weight-at-age of herring, one of the most important commercial fishes of the Baltic Sea, have been observed since the late 1980s. Using time-series on herring stomach contents, as well as length and weight, we provide evidence for a chain of events relating variability in climate, salinity and P. elongatus abundance to changes in diet and condition of herring in the Central Baltic Sea.