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: 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: 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.

Description: Highlights: • Juvenile fish somatic-, biochemical- and otolith-based condition indices are compared. • RNA/DNA, otolith increments, and somatic condition explained 〉 70% growth variability. • Response times of proxies differ after food deprivation and re-feeding. • RNA/DNA most rapid response, otolith increments explain length but not mass growth. • Caution suggested if condition indices are applied to fish in patchy prey environments. Abstract: Reliable estimates of short- and longer-term in situ growth and condition of organisms are critical if one hopes to understand how the environment regulates survival. This study reports the first comparison of somatic- (K), biochemical- (RNA–DNA ratio, RD) and otolith- (increment widths, OIW) based indices of condition of a young juvenile fish. Measurements were made on European sprat (Sprattus sprattus) that had i) known differences in somatic growth rate caused by providing different, constant prey ration levels, ii) been fed ad libitum at 7, 11, 15, 18 and 22 °C, and iii) been deprived of prey for either 4, 8 or 12 days and re-fed for 8 days. All three proxies explained significant amounts (70 to 90%) of the variability in measured growth rate. In fish experiencing a change in their feeding level and concomitant change in mass-at-length (K), RD tracked changes in both length and mass while OIW only tracked changes in length. Values of OIW and RD were highest at 18 °C suggesting that this is the optimal temperature for growth in these juveniles. During food deprivation, RD and OIW rapidly decreased and reached their lowest values within ~ 4 days. Upon re-feeding, RD increased most rapidly, K was most variable and the response time in OIW was slowest (two-times slower than RD). These patterns reflected preferential allocation of food energy to restore body mass in recently re-fed fish prior to fish increasing both mass and length. These results indicate that the sensitivity and applicability of growth proxies depend on the recent feeding history, that proxies have different response times, and that caution be taken when inferring growth and condition in early life stages of fishes that forage in patchy prey environments.

Description: We employed coupled 3-D biophysical models to better understand the effects of physical forcing conditions as well as differences in vertical distribution and growth performance on the spatial distribution of larval sprat (Sprattus sprattus) in the North and the Baltic Sea. Our model simulations analysed the influence of abiotic and biotic forcing variability on larval transport and the seasonal and inter-annual variability in spatial distribution of larvae originating from different spawning areas in each of the two systems. Due to strong spatial and temporal differences in temperature, drift durations differed between the two ecosystems. During cold spring and warm summer periods, drift durations in the Baltic were ∼35 and 15 days, respectively, but were somewhat shorter (30 and 10 d) in the North Sea. Changes in larval feeding rates markedly impacted larval growth rate and stage duration, and, hence, environmental histories experienced by larvae as well as their final distribution. Generally, specific spawning sites were relatively well connected to specific juvenile nursery areas in the Baltic. However, in the North Sea, considerable mixing of sprat populations occurred with frontal areas acted as convergence zones for older larvae originating from different spawning sites. The mixing and/or co-occurrence of 18-mm larvae from different source regions were greatest (least) in the early spring (summer) for larvae at colder (warmer) temperatures having longer (shorter) drift durations. Generally, such high mixing probability would not promote small- or medium-scale population distinctness of North Sea sprat. The results of our coupled hydrodynamic/trophodynamic model simulations provide a baseline in quantifying and understanding larval sprat transport in these different ecosystems and exemplify the extent to which environmental variability (e.g., differences in temperature as well as prey availability) can influence spatial distributions of larval fish.

Description: We review and compare four broad categories of spatially-explicit modelling approaches currently used to understand and project changes in the distribution and productivity of living marine resources including: 1) statistical species distribution models, 2) physiology-based, biophysical models of single life stages or the whole life cycle of species, 3) food web models, and 4) end-to-end models. Single pressures are rare and, in the future, models must be able to examine multiple factors affecting living marine resources such as interactions between: i) climate-driven changes in temperature regimes and acidification, ii) reductions in water quality due to eutrophication, iii) the introduction of alien invasive species, and/or iv) (over-)exploitation by fisheries. Statistical (correlative) approaches can be used to detect historical patterns which may not be relevant in the future. Advancing predictive capacity of changes in distribution and productivity of living marine resources requires explicit modelling of biological and physical mechanisms. New formulations are needed which (depending on the question) will need to strive for more realism in ecophysiology and behaviour of individuals, life history strategies of species, as well as trophodynamic interactions occurring at different spatial scales. Coupling existing models (e.g. physical, biological, economic) is one avenue that has proven successful. However, fundamental advancements are needed to address key issues such as the adaptive capacity of species/groups and ecosystems. The continued development of end-to-end models (e.g., physics to fish to human sectors) will be critical if we hope to assess how multiple pressures may interact to cause changes in living marine resources including the ecological and economic costs and trade-offs of different spatial management strategies. Given the strengths and weaknesses of the various types of models reviewed here, confidence in projections of changes in the distribution and productivity of living marine resources will be increased by assessing model structural uncertainty through biological ensemble modelling.