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: The Baltic is a semi-enclosed sea characterised by decreasing salinity in the eastern and northern direction with only the deeper parts of the southern Baltic suitable as spawning grounds for marine species like cod. Baltic cod exhibits various adaptations to brackish water conditions, yet the inflow of salty North Sea water near the bottom remains an influence on the spawning success of the Baltic cod. The eastern Baltic population has been very weakly studied in comparison with the western population. The aim of this study is to demonstrate for the first time genetic differentiation by the use of a large number of SNPs between eastern and western Baltic populations existing in differentiated salinity conditions. Two cod samples were collected from the Bay of Gdańsk, Poland and one from the Kiel Bight, Germany. Samples were genotyped using a cod derived SNP-array (Illumina) with 10 913 SNPs. A selection of diagnostic SNPs was performed. A set of 7944 validated SNPs were analysed to assess the differentiation of three samples of cod. Results indicated a clear distinctness of the Kiel Bight from the populations of the eastern Baltic. FST comparison between both eastern samples was non-significant. Clustering analysis, principal coordinates analysis and assignment test clearly indicated that the eastern samples should be considered as one subpopulation, well differentiated from the western subpopulation. With the SNP approach, no differentiation between groups containing ‘healthy’ and ‘non-healthy’ cod individuals was observed.

Description: The European sprat (Sprattus sprattus) was a main target species of the German GLOBEC program that investigated the trophodynamic structure and function of the Baltic and North Seas under the influence of physical forcing. This review summarizes literature on the ecophysiology of sprat with an emphasis on describing how environmental factors influence the life-history strategy of this small pelagic fish. Ontogenetic changes in feeding and growth, and the impacts of abiotic and biotic factors on vital rates are discussed with particular emphasis on the role of temperature as a constraint to life-history scheduling of this species in the Baltic Sea. A combination of field and laboratory data suggests that optimal thermal windows for growth and survival change during early life and are wider for eggs (5–17 °C) than in young (8- to 12-mm) early feeding larvae (5–12 °C). As larvae become able to successfully capture larger prey, thermal windows expand to include warmer waters. For example, 12- to 16-mm larvae can grow well at 16 °C and larger, transitional-larvae and early juveniles display the highest rates of feeding and growth at ~18–22 °C. Gaps in knowledge are identified including the need for additional laboratory studies on the physiology and behavior of larvae (studies that will be particularly critical for biophysical modeling activities) and research addressing the role of overwinter survival as a factor shaping phenology and setting limits on the productivity of this species in areas located at the northern limits of its latitudinal range (such as the Baltic Sea). Based on stage- and temperature-specific mortality and growth potential of early life stages, our analysis suggests that young-of-the year sprat would benefit from inhabiting warmer, near-shore environments rather than the deeper-water spawning grounds such as the Bornholm Basin (central Baltic Sea). Utilization of warmer, nearshore waters (or a general increase in Baltic Sea temperatures) is expected to accelerate growth rates but also enhance the possibility for density-dependent regulation of recruitment (e.g., top-down control of zooplankton resources) acting during the late-larval and juvenile stages, particularly when sprat stocks are at high levels.

Description: Gaining reliable estimates of how long fish early life stages can survive without feeding and how starvation rate and time until death are influenced by body size, temperature and species is critical to understanding processes controlling mortality in the sea. The present study is an across-species analysis of starvation-induced changes in biochemical condition in early life stages of nine marine and freshwater fishes. Data were compiled on changes in body size (dry weight, DW) and biochemical condition (standardized RNA–DNA ratio, sRD) throughout the course of starvation of yolk-sac and feeding larvae and juveniles in the laboratory. In all cases, the mean biochemical condition of groups decreased exponentially with starvation time, regardless of initial condition and endogenous yolk reserves. A starvation rate for individuals was estimated from discrete 75th percentiles of sampled populations versus time (degree-days, Dd). The 10th percentile of sRD successfully approximated the lowest, life-stage-specific biochemical condition (the edge of death). Temperature could explain 59% of the variability in time to death whereas DW had no effect. Species and life-stage-specific differences in starvation parameters suggest selective adaptation to food deprivation. Previously published, interspecific functions predicting the relationship between growth rate and sRD in feeding fish larvae do not apply to individuals experiencing prolonged food deprivation. Starvation rate, edge of death, and time to death are viable proxies for the physiological processes under food deprivation of individual fish pre-recruits in the laboratory and provide useful metrics for research on the role of starvation in the sea. Highlights ► Biochemical condition (RNA–DNA ratio) decreases exponentially during starvation. ► Starvation parameters of individuals can be derived from data collected on groups. ► Physiological rates of starvation compare well across a broad range of temperatures. ► Species and life stages specific starvation parameters indicate selective adaptation.

Description: Throughout the last decades there has been a world-wide, general warming trend. In this study, we use the example of the Baltic Sea to resolve the overall estimated temperature trend into smaller, meso-scale spatial units. Afterwards, we investigate the spatially resolved potential impact of the temperature trend on larval survival for two important fish species, cod and sprat. We used two different sets of hydrographic data: (i) long-term temporally and depth-resolved data measured in situ originating from one geographic position and (ii) long-term horizontally resolved data, originating from a circulation model. In contrast to basin-wide integrated results, our modelling approach revealed different results related to smaller spatial scales. In shallow and coastal areas non-significant long-term temperature trends were observed. In some cases even decreasing temperature trends were found. Average distribution maps (1973–2010) of cod and sprat eggs and larvae confirmed the higher importance of central, deep basins as nursery grounds. Applying the temperature trends when calculating cod larval window of opportunity values, resulted in decreased durations of 1–3 days (~ 3–13%) in most areas. Sprat larval window of opportunity values mainly increased up to 4 days (~ 45%), indicating a potential reproduction advantage of sprat over cod under anticipated future temperature increase. Highlights ► We resolve the overall positive temperature trend in the Baltic into meso-scale spatial units and investigate the impact on larval survival for two important fish species, cod and sprat. ► In shallow and coastal areas non-significant or even negative temperature trends occurred. ► Cod larval window of opportunity values decreased by 1–3 days (~ 3–13%). ► Sprat larval window of opportunity values increased up to 4 days (~ 45%). ► Sprat will have a reproduction advantage over cod under anticipated future temperature change.

Description: The European sprat (Sprattus sprattus) was a main target species of the German GLOBEC program that investigated the trophodynamic structure and function of the Baltic and North Seas under the influence of physical forcing. This review summarizes literature on the ecophysiology of sprat with an emphasis on describing how environmental factors influence the life-history strategy of this small pelagic fish. Ontogenetic changes in feeding and growth, and the impacts of abiotic and biotic factors on vital rates are discussed with particular emphasis on the role of temperature as a constraint to life-history scheduling of this species in the Baltic Sea. A combination of field and laboratory data suggests that optimal thermal windows for growth and survival change during early life and are wider for eggs (5–17 °C) than in young (8- to 12-mm) early feeding larvae (5–12 °C). As larvae become able to successfully capture larger prey, thermal windows expand to include warmer waters. For example, 12- to 16-mm larvae can grow well at 16 °C and larger, transitional-larvae and early juveniles display the highest rates of feeding and growth at ∼18–22 °C. Gaps in knowledge are identified including the need for additional laboratory studies on the physiology and behavior of larvae (studies that will be particularly critical for biophysical modeling activities) and research addressing the role of overwinter survival as a factor shaping phenology and setting limits on the productivity of this species in areas located at the northern limits of its latitudinal range (such as the Baltic Sea). Based on stage- and temperature-specific mortality and growth potential of early life stages, our analysis suggests that young-of-the year sprat would benefit from inhabiting warmer, near-shore environments rather than the deeper-water spawning grounds such as the Bornholm Basin (central Baltic Sea). Utilization of warmer, nearshore waters (or a general increase in Baltic Sea temperatures) is expected to accelerate growth rates but also enhance the possibility for density-dependent regulation of recruitment (e.g., top-down control of zooplankton resources) acting during the late-larval and juvenile stages, particularly when sprat stocks are at high levels. Highlights ► Field, laboratory and modeling research on the ecophysiology of all sprat life stages is summarized. ► Environmental factors influencing growth and survival are revealed. ► Ontogenetic changes in thermal tolerance and prey requirements constrain life cycle scheduling. ► Gaps in knowledge are identified and future research efforts recommended on sprat recruitment dynamics. ► Exploring seasonal energy allocation will allow a mechanistic understanding of climate impacts.

Description: Highlights: • The stress response to salinity in subpopulations of the Baltic cod was examined. • Two different profiles of response to salinity were observed. • Changes in response profiles may be a functional adaptation to variable salinity. • Adaptation protects cod against stress during vertical and long-distance migrations. • Salinity is a barrier maintaining the genetic and physiological separations of cod. Abstract: Atlantic cod (Gadus morhua) occur in marine water of different salinities: from oceanic waters at salinity of 35 to Baltic Sea waters where the lowest level of salinity reaches 5–6. The stress response to different salinities in the eastern and western Baltic cod populations was examined. Two genes of Na +, K + -ATPase 1a (atp1a) and heat shock protein 70 (hsp70) expression, plasma cortisol and osmolality were used as markers of osmotic stress to characterize the reaction profiles of two populations of G. morhua from the western and eastern parts of the Baltic Sea. Atlantic cod were sampled in November 2012 from western Kiel Bight (KIEL, salinity of 18) and eastern Gdańsk Bay (GDA, salinity of 8). Live fish were transported to the Marine Station of the University of Gdańsk in Hel and were settled in tanks (3500 L). Cod were kept at 10 °C in recirculated water, which simulated the natural salinities of the geographic source region of the fish. Results showed that in the reduced and elevated salinity water of the KIEL group, we observed no change in expression of atp1a and slightly increased expression of hsp70. In the GDA group, there were no significant changes of hsp70 expression but the level of atp1a was significantly increased in both salinities. In both groups, concentration of cortisol increased after exposure to elevated salinity, while in fish exposed to reduced salinity, a significantly higher concentration of cortisol was observed after 72 h. The high expression of atp1a that observed in the eastern group (GDA) supports the thesis of a genetic background to the adaptation to variable salinity. This adaptation may protect this species against an osmotic stress caused by daily vertical migrations and long-distance migration to spawning areas. At the same life-time, salinity is a barrier maintaining the genetic and physiological separations between G. morhua stocks and affecting the structure of this fish subpopulation in the Baltic Sea.

Description: Highlights: • Egg specific gravity vary between areas/subpopulations as an adaptation to salinity. • Egg diameter differ between areas/subpopulations whereas egg dry weight does not. • Habitat suitability for egg survival vary depending on salinity and oxygen conditions. • Egg survival probabilities increased following a major saline water inflow event. Abstract: Vertical distribution of eggs as determined by the egg buoyancy, i.e. the difference in specific gravity between the egg and the ambient water, have profound implications for the reproductive success and hence recruitment in fish. Here variability in egg specific gravity of flounder, Platichthys flesus, was studied along a salinity gradient and by comparing two reproductive strategies, spawning pelagic or demersal eggs. Egg characteristics of 209 egg batches (covering ICES subdivisions (SD) 22–29 in the brackish water Baltic Sea) was used to reveal the significance of egg diameter and egg dry weight for egg specific gravity (ESG), subpopulations, and egg survival probabilities of pelagic eggs following a major saline water inflow event. As an adaptation to salinity, ESG (at 7 °C) differed (p 〈 0.001) between areas; three subpopulations of flounder with pelagic eggs: 1.0152 ± 0.0021 (mean ± sd) g cm−3 in SD 22, 1.0116 ± 0.0013 g cm−3 in SD 24 and 25, and 1.0096 ± 0.0007 g cm−3 in SD 26 and 28, contrasting to flounder with demersal eggs, 1.0161 ± 0.0008 g cm−3. Egg diameter differed (p 〈 0.001) between subpopulations; from 1.08 ± 0.06 mm (SD 22) to 1.26 ± 0.06 mm (SD 26 and 28) for pelagic eggs and 1.02 ± 0.04 mm for demersal eggs, whereas egg dry weight was similar; 37.9 ± 5.0 μg (SD 22) and 37.2 ± 3.9 μg (SD 28) for pelagic, and 36.5 ± 6.5 μg for demersal eggs. Both egg diameter and egg dry weight were identified as explanatory variables, explaining 87% of the variation in ESG. ESG changed during ontogeny; a slight decrease initially but an increase prior to hatching. Egg survival probabilities judged by combining ESG and hydrographic data suggested higher egg survival in SD 25 (26 vs 100%) and SD 26 (32 vs 99%) but not in SD 28 (0 and 3%) after the inflow event, i.e. highly fluctuating habitat suitability. The results confirm the significance of ESG for egg survival and show that variability in ESG as and adaptation to salinity is determined mainly by water content manifested as differences in egg diameter; increase in diameter with decreasing salinity for pelagic eggs, and decreased diameter resulting in demersal eggs.

Description: The period of larval drift into a suitable nursery area is considered to be of great significance for recruitment variability in flatfish. Here, a hydrodynamic model coupled with a Lagrangian particle tracking technique was utilized to study the drift from the first feeding larval stage until time of settlement of Central and Eastern Baltic flounder (Platichthys flesus), originating from spawning in the Baltic Sea deep basins, the Arkona- and Bornholm basin (central Baltic Sea), and the Gdansk deep and Gotland basin (eastern Baltic Sea). We examined the spatio-temporal dynamics of the probability to settle in preferred nursery habitat by detailed drift model simulations. The study suggests that the majority of larvae (89% and 74% for Central- and Eastern Baltic flounder, respectively) drift towards coastal areas and settle at metamorphosis ≤20 km from a sandy habitat enabling further migration to a preferred nursery area, i.e. larval drift seems not to be a major bottleneck in recruitment of flounder spawning in the Baltic Sea deep basins. The drift model results suggest that Central Baltic flounder utilize nursery areas mainly in the central and western Baltic, and in the Kattegat, whereas Eastern Baltic flounder mainly utilize the coast in the central and eastern Baltic. Thus, the two stock components seem to use different nursery areas following settlement. Further, in accordance with the “nursery size hypothesis”, the model demonstrates that larvae from the Bornholm basin, utilizing areas with extensive distribution of preferred nursery habitat, display the highest relative successful transport to nursery grounds until settling (72% of successfully settled larvae), suggesting that spawning in the Bornholm Basin is of great importance for stock recruitment of deep basin spawning Baltic flounder.

Description: Highlights: • Recruitment indicator: egg survival probability vs. “reproductive volume concept”. • Traditional sampling methodology unable to resolve spatial egg distributions. • Predominance of self-sustaining stock components and low connectivity. • Egg buoyancy and topographic features: limitation of cod egg transport. • Sedimentation as a new mortality source of eastern Baltic cod eggs. Abstract: In the highly variable environment of the Baltic Sea two genetically distinct cod stocks exist, one west of the island of Bornholm, which is referred to as the western stock, and one to the east of Bornholm, the eastern stock. A hydrodynamic model combined with a Lagrangian particle tracking technique was utilised to provide spatially and temporally resolved long-term information on environmentally-related (i) spawning habitat size, (ii) egg/yolk-sac larval survival, (iii) separation of causes of mortality, and (iv) connectivity between spawning areas of eastern Baltic cod. Simulations were performed to quantify processes generating heterogeneity in spatial distribution of cod eggs and yolk sac larvae up to the first-feeding stage. The spatial extent of cod eggs represented as virtual drifters is primarily determined by oxygen and salinity conditions at spawning, which define the habitat requirement to which cod’s physiology is suited for egg development. The highest habitat suitability occurred in the Bornholm Basin, followed by the Gdansk Deep, while relatively low habitat suitability was obtained for the Arkona and the Gotland Basin. During drift egg and yolk sac larval survival is to a large extent affected by sedimentation. Eggs initially released in the western spawning grounds (Arkona and Bornholm Basin) were more affected by sedimentation than those released in the eastern spawning grounds (Gdansk Deep and Gotland Basin). Highest relative survival of eastern Baltic cod eggs occurred in the Bornholm Basin, with a pronounced decrease towards the Gdansk Deep and the Gotland Basin. Relatively low survival rates in the Gdansk Deep and in the Gotland Basin were attributable to oxygen-dependent mortality. Low oxygen content had almost no impact on survival in the Arkona Basin. For all spawning areas temperature dependent mortality was only evident after severe winters. Egg buoyancy in relation to topographic features like bottom sills and strong bottom slopes could appear as a barrier for the transport of Baltic cod eggs and yolk sac larvae and could potentially limit the connectivity of Baltic cod early life stages between the different basins in the western and eastern Baltic Sea. The possibility of an eastward directed transport up to the first-feeding larval stage exists only for eggs and yolk sac larvae at high buoyancy levels, suggesting that dispersal of early life stages between these spawning areas is limited.