Description: Ten female (mean 179 g wet mass (WM) and 25.3 cm standard length (SL) and ten male (182 g WM, 24.5 cm SL) Atlantic herring (Clupea harengus L.) were obtained from Kiel Bight (54°22'N, 010°09'E) and strip-spawned. The eggs were strip-spawned onto polyethylene plates, fertilized and incubated within aerated 250-L tanks containing 16 (±0.2) psu, 10 (±0.12)°C water at a light regime of 14 h (light):10 h (dark). After hatch on April 17, 2007, larvae were reared in semi-static (30% water exchange day−1) 100-L tanks (Ø 60 cm) at 17 (±0.5) psu and either 13.2 (±0.4), 10 (±0.4) or 7.2 (±0.3)°C. Tanks were “greened” with R. baltica (50,000 cells mL−1) and larvae were fed ad libitum rations of newly-hatched A. tonsa nauplii which corresponded to 5 prey mL−1 until a larval age of 13 days post hatch (dph) and then 2 prey mL−1 of early and late naupliar and copepodite stages. During rearing, larval SL- and dry mass (DM)-at-age was monitored every 2 to 3 days.

Description: Ongoing climate change is leading to warmer and more acidic oceans. The future distribution of fish within the oceans depends on their capacity to adapt to these new environments. Only few studies have examined the effects of ocean acidification (OA) and warming (OW) on the metabolism of long-lived fish over successive generations. We therefore aimed to investigate the effect of OA on larval and juvenile growth and metabolism on two successive generations of European sea bass (Dicentrarchus labrax L.) as well as the effect of OAW on larval and juvenile growth and metabolism of the second generation. European sea bass is a large economically important fish species with a long generation time. F0 larvae were produced at the aquaculture facility Aquastream (Ploemeur-Lorient, France) and obtained at 2 days post-hatch (dph). From 2 dph F0 larvae were reared in the laboratory in two PCO2 conditions (ambient and Δ1000). Larval rearing was performed in a temperature controlled room and water temperatures were fixed to 19°C in F0. In juveniles and adults, water temperatures of F0 sea bass were adjusted to ambient temperature in the Bay of Brest during summer (up to 19°C), but were kept constant at 15 and 12°C for juveniles and adults, respectively, when ambient temperature decreased below these values. F1 embryos were obtained by artificial reproduction of F0 broodstock fish. Fertilized eggs were incubated at 15°C and at the same PCO2 conditions as respective F0. Division of F1 larvae from egg rearing tanks into experimental tanks took place at 2 dph. F1 larvae were reared in four OAW conditions: two temperatures (cold and warm life condition, C and W) and two PCO2 conditions (ambient and Δ1000). Larval rearing was performed in a temperature controlled room and water temperatures were fixed to 15 and 20°C for C and W larvae, respectively. In juveniles, water temperatures of F1 sea bass were adjusted to ambient temperature in the Bay of Brest during summer (up to 19°C), but were kept constant at 15°C when ambient temperature decreased below these values. F1-W was always 5°C warmer than the F1-C treatment. OAW conditions for F0 and F1 rearing were chosen to follow the predictions of the IPCC for the next 130 years: ΔT = 5°C and ΔPCO2 = 1000 µatm, following RCP 8.5. We analysed larval and juvenile growth in F0 and F1. Larval routine metabolic rates (RMR, in F1), juvenile standard metabolic rates (SMR, in F0 and F1) and juvenile critical oxygen concentrations (PO2crit, in F0 and F1) were obtained on individuals via intermittent flow-respirometry. Measurements were conducted at the rearing conditions of the respective larva or juvenile. Fish were fasted for 3h and 48-72h for larvae and juveniles, respectively. After the respirometry trial, larvae were photographed to measure there body length and frozen until measurement of dry mass. Juveniles body length and wet mass was directly determined with calipers and a balance.

Description: Ongoing climate change is leading to warmer and more acidic oceans. The future distribution of fish within the oceans depends on their capacity to adapt to these new environments. Only few studies have examined the effects of ocean acidification (OA) and warming (OW) on the metabolism of long-lived fish over successive generations. We therefore aimed to investigate the effect of OA on larval and juvenile growth and metabolism on two successive generations of European sea bass (Dicentrarchus labrax L.) as well as the effect of OAW on larval and juvenile growth and metabolism of the second generation. European sea bass is a large economically important fish species with a long generation time. F0 larvae were produced at the aquaculture facility Aquastream (Ploemeur-Lorient, France) and obtained at 2 days post-hatch (dph). From 2 dph F0 larvae were reared in the laboratory in two PCO2 conditions (ambient and Δ1000). Larval rearing was performed in a temperature controlled room and water temperatures were fixed to 19°C in F0. In juveniles and adults, water temperatures of F0 sea bass were adjusted to ambient temperature in the Bay of Brest during summer (up to 19°C), but were kept constant at 15 and 12°C for juveniles and adults, respectively, when ambient temperature decreased below these values. F1 embryos were obtained by artificial reproduction of F0 broodstock fish. Fertilized eggs were incubated at 15°C and at the same PCO2 conditions as respective F0. Division of F1 larvae from egg rearing tanks into experimental tanks took place at 2 dph. F1 larvae were reared in four OAW conditions: two temperatures (cold and warm life condition, C and W) and two PCO2 conditions (ambient and Δ1000). Larval rearing was performed in a temperature controlled room and water temperatures were fixed to 15 and 20°C for C and W larvae, respectively. In juveniles, water temperatures of F1 sea bass were adjusted to ambient temperature in the Bay of Brest during summer (up to 19°C), but were kept constant at 15°C when ambient temperature decreased below these values. F1-W was always 5°C warmer than the F1-C treatment. OAW conditions for F0 and F1 rearing were chosen to follow the predictions of the IPCC for the next 130 years: ΔT = 5°C and ΔPCO2 = 1000 µatm, following RCP 8.5. We analysed larval and juvenile growth in F0 and F1. Larval routine metabolic rates (RMR, in F1), juvenile standard metabolic rates (SMR, in F0 and F1) and juvenile critical oxygen concentrations (PO2crit, in F0 and F1) were obtained on individuals via intermittent flow-respirometry. Measurements were conducted at the rearing conditions of the respective larva or juvenile. Fish were fasted for 3h and 48-72h for larvae and juveniles, respectively. After the respirometry trial, larvae were photographed to measure there body length and frozen until measurement of dry mass. Juveniles body length and wet mass was directly determined with calipers and a balance.

Description: European sea bass (Dicentrarchus labrax) is a large, economically important fish species with a long generation time whose long-term resilience to ocean acidification (OA) and warming (OW) is not clear. We incubated sea bass from Brittany (France) for two generations (〉5 years in total) under ambient and predicted OA conditions (PCO2: 650 and 1700 µatm) crossed with ambient and predicted ocean OW conditions in F1 (temperature: 15-18°C and 20-23°C) to investigate the effects of climate change on larval and juvenile growth and metabolic rate. We found that in F1, OA as single stressor at ambient temperature did not affect larval or juvenile growth and OW increased developmental time and growth rates, but OAW decreased larval size at metamorphosis. Larval routine and juvenile standard metabolic rates were significantly lower in cold compared to warm conditioned fish and also lower in F0 compared to F1 fish. We did not find any effect of OA as a single stressor on metabolic rates. Juvenile PO2crit was not affected by OA or OAW in both generations. We discuss the potential underlying mechanisms resulting in the resilience of F0 and F1 larvae and juveniles to OA and in the beneficial effects of OW on F1 larval growth and metabolic rate, but on the other hand in the vulnerability of F1, but not F0 larvae to OAW. With regard to the ecological perspective, we conclude that recruitment of larvae and early juveniles to nursery areas might decrease under OAW conditions but individuals reaching juvenile phase might benefit from increased performance at higher temperatures.

Description: In fishery science, early life-stage survival and development are regarded as major factors driving the population dynamics of marine fishes. During the last century, the main research focus has been on the spatio-temporal match of larval fish and appropriate food (bottom-up processes). However, these field studies are often criticised for their limited capability to disentangle their results from mortality caused by predation since these top-down mechanisms are rarely studied. We examined the predation on herring (Clupea harengus) larvae in a Baltic inshore lagoon by investigating the spatio-temporal overlap of larval herring and their potential predators such as the dominant threespine stickleback (Gasterosteus aculeatus) in distinct habitats (sublittoral and littoral areas) using a set of different gears and sampling techniques. Despite significant spatial and temporal predator-prey overlap, stomach analyses suggested that very few larvae were consumed by sticklebacks, even if projected to the entire study area and season. Other well-known predators of clupeid larvae such as gelatinous plankton occur later in the year after young herring have migrated out of the system. The observed predation on herring larvae was much less than expected and appears being a minor factor in determining herring reproduction success in our study area, particularly if compared to other causes of mortality such as egg predation. Providing a relatively good shelter from predation might be a key element making transitional waters valuable nursery grounds for the offspring of migrating marine fish species.

Description: Atlantic herring (Clupea harengus) migrates from offshore to coastal areas to spawn and their eggs and larvae may substantially increase prey resources for resident predators. We combined an in situ predator exclusion experiment using eggs naturally spawned on submerged aquatic vegetation and field observations of predator abundance to estimate the magnitude of predation mortality of herring eggs. During our predator exclusion experiment, performed in an important spawning ground in the southwest Baltic Sea, 20% of the herring eggs were consumed resulting in an extrapolated predation of 42% of all eggs between spawning and hatch. Abundance and stomach content analyses indicated that one predator (threespine stickleback, Gasterosteus aculeatus) was responsible for the majority of the predation impact. Predation mortality estimates from this in situ study were more than 10-fold higher than those of an empirical egg predation model for the same predator in the same region. Our findings highlight the potential of resident predators to regulate the survival of early life stages of ocean-going fishes that rely on the nursery functions of inshore transitional waters.

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: Baltic sprat (Sprattus sprattus balticus S.) is a key species in the pelagic ecosystem of the Baltic Sea. Most stocks of small pelagic species are characterized by natural, fishery-independent fluctuations, which make it difficult to predict stock development. Baltic sprat recruitment is highly variable, which can partly be related to climate-driven variability in hydrographic conditions. Results from experimental studies and field observations demonstrate that a number of important life history traits of sprat are affected by temperature, especially the survival and growth of early life stages. Projected climate-driven warming may impact important processes affecting various life stages of sprat, from survival and development during the egg and larval phases to the reproductive output of adults. This study presents a stage-based matrix model approach to simulate sprat population dynamics in relation to different climate change scenarios. Data obtained from experimental studies and field observations were used to estimate and incorporate stage-specific growth and survival rates into the model. Model-based estimates of population growth rate were affected most by changes in the transition probability of the feeding larval stage at all temperatures (+0, +2, +4, +6 °C). The maximum increase in population growth rate was expected when ambient temperature was elevated by 4 °C. Coupling our stage-based model and more complex, biophysical individual-based models may reveal the processes driving these expected climate-driven changes in Baltic Sea sprat population dynamics.

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: The GLOBEC Germany program (2002–2007) had the ambitious goal to resolve the processes impacting the recruitment dynamics of Baltic sprat (Sprattus sprattus L.) by examining various factors affecting early life history stages. At the start of the research program, a number of general recruitment hypotheses were formulated, i.e. focusing on (1) predation, (2) food availability, (3) physical parameters, (4) the impact of current systems, and finally (5) the importance of top-down vs bottom-up effects. The present study synthesizes the results of field sampling (2002 and 2003), laboratory experiments, and modeling studies to re-evaluate these hypotheses for the Baltic sprat stock. Recruitment success was quite different in the 2 years investigated. Despite a lower spawning stock biomass in 2003, the total number of recruits was almost 2-fold higher that year compared to 2002. The higher recruitment success in 2003 could be attributed to enhanced survival success during the post-larval/juvenile stage, a life phase that appears to be critical for recruitment dynamics. In the state of the Baltic ecosystem during the period of investigation, we consider bottom-up control (e.g. temperature, prey abundance) to be more important than top-down control (predation mortality). This ranking in importance does not vary seasonally. Prevailing water circulation patterns and the transport dynamics of larval cohorts have a strong influence on sprat recruitment success. Pronounced transport to coastal areas is detrimental for year-class strength particularly at high sprat stock sizes. A suggested mechanism is density-dependant regulation of survival via intra- and inter-specific competition for prey in coastal areas. A documented change in larval vertical migration behavior between the early 1990s and early 2000s increased the transport potential to the coast, strengthening the coupling between inter-annual differences in the magnitude and direction of wind-driven surface currents and year-to-year changes in reproductive success. However, due to the strong linkages and feed-back loops in the Baltic Sea food web, the most robust projections of the future strength of the Baltic sprat stock will need to take into account climate-driven changes in both abiotic (e.g., drift trajectories) and biotic (trophodynamic) factors. Although our understanding of processes affecting pre-recruit (larval) growth and survival has been advanced by the integrated research conducted within the GLOBEC Germany program, key mechanisms potentially affecting life stages outside of the spawning basins remain to be explored including the dynamics of coastal habitats of juveniles and the feeding and overwintering grounds of adults. Highlights: ► Food limitation may contribute to the formation of seasonal ‘windows of survival’. ► Change in larval migration exalted the importance of transport. ► Temperature is the most important physical factor influencing sprat recruitment. ► Bottom-up control is more important than top-down control. ► Projected Baltic water temperature increase suggests higher sprat recruitment potential.