ALBERT

Description: Fecundity of marine fish species is highly variable, but trade-offs between fecundity and egg quality have rarely been observed at the individual level. We investigated spatial differences in reproductive investment of individual European sprat Sprattus sprattus (Linnaeus 1758) females by determining batch fecundity, condition indices (somatic condition index and gonadosomatic index) as well as oocyte dry weight, protein content, lipid content, spawning batch energy content, and fatty acid composition. Sampling was conducted in five different spawning areas within the Baltic Sea between March and May 2012. Spawning sprat from the Kiel Bight were in a better nutritional condition compared to sprat from the Arkona Basin, Bornholm Basin, and the Gdansk Deep. These females were also producing up to twice as many oocytes, and invested significantly more energy in reproduction, than their counterparts sampled in the eastern part of the Baltic Sea. Still, oocytes produced by Kiel Bight sprat contained significantly lower fractions of the essential fatty acids 20:4 (n-6) and 22:6 (n-3). A seasonal trade-off between oocyte weight/lipid content and fecundity was found for Baltic sprat, albeit the gross energy invested into spawning remained constant. Observed spatial and seasonal differences in sprat reproductive investment may be linked to hydrographic conditions and food availability and will impact the survival probability of yolk-sac and first feeding larvae. These findings indicate that Baltic sprat is able to adapt its reproductive tactics to the highly variable pelagic habitat of the Baltic Sea.

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: Seasonal variability in fecundity has been observed in a number of marine fishes and is important towards understanding the reproductive potential of a fish stock. However, the seasonal dynamics of egg production of Baltic sprat (Sprattus sprattus balticus S) have not been well described to date. We present data on the timing of spawning, and the seasonal variability in batch fecundity, number of developing oocytes, oocyte dry weight, spawning fraction, fish condition and atresia for this species in the Bornholm Basin, Baltic Sea. Histological techniques in combination with image analysis were applied to investigate those variables based upon material sampled in 2005 and 2008. Sprat were reported to be in spawning condition from January to June in each year plus in 2008 signs of ovarian maturation were also observed in November. Relative batch fecundity was found to vary two-fold with 85 eggs g−1 ovary free body weight observed early in the spawning season (January 2005) and 165 eggs g−1 ovary free body weight late in the spawning season (June 2008). Variability in batch fecundity during peak spawning was rather low. A seasonal decrease in oocyte dry weight was related to an increase in batch fecundity. Spawning fraction varied over the course of the spawning period with values ranging from 0.29 in March to 0.18 in June. Stereometric analyses confirmed the indeterminate fecundity of Baltic sprat. Prevalence of atresia was low during peak spawning in April to June (1.0–4.0%) but considerably higher during the early spawning period in March (16.4%) and highest in November (38.5%). Female sprat condition was low during the spawning period and increased sharply after spawning ceased. Our study provides a better understanding of fecundity and spawning dynamics of Baltic sprat which will aid to improve the assessment of reproductive potential of this ecologically and economically important fish species.

Description: Absolute and relative batch fecundity of Baltic sprat (Sprattus sprattus balticus S.) during peak spawning time was investigated for several years over the last two decades by applying the hydrated oocyte method. Batch fecundity was analysed for three important spawning areas of sprat in the central Baltic Sea, namely the Bornholm Basin, Gdansk Deep and Southern Gotland Basin. Environmental parameters such as hydrography, fish condition and stock density were tested in order to investigate the observed previous termvariabilitynext term in sprat fecundity. Absolute batch fecundity was found to be positively related to fish length and weight. Significant differences in absolute and relative batch fecundity of Baltic sprat among areas and years were detected, and could partly be explained by hydrographic features of the investigated areas. A non-linear multiple regression model taking into account fish length and ambient temperature explained 70% of previous termvariabilitynext term in absolute batch fecundity. Oxygen content and fish condition were not related to sprat batch fecundity. Additionally, a negative effect of stock size on sprat batch fecundity in the Bornholm Basin was revealed. The obtained data and results are important to assess the stock reproductive potential of this important Baltic fish stock. Highlights ► Batch fecundity in Baltic sprat varies among areas and years. ► previous termVariabilitynext term in batch fecundity is related to ambient temperature and stock size. ► Valuable data to evaluate the stock reproductive potential of an important species are provided.

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: Ovaries of Baltic sprat (Sprattus sprattus balticus S.) were analysed histologically to identify stages of postovulatory follicles (POF) and to assess the oocyte development pattern. Samples were taken every 3 h during a 24 h trawl survey conducted in the Bornholm Basin in April 2007. Gonad histology revealed spawning of sprat throughout the day which hampered the exact ageing of POFs by the postovulatory follicle method and therefore did not allow direct estimation of spawning frequency. However, it was possible to define four stages of POFs, according to their histological features. The occurrence of these POF stages (I to IV) corresponded clearly to the development of the leading oocyte cohort. Further, the oocyte recruitment pattern revealed that the spawning batch can be identified prior to hydration. The POF stages I and II were present almost exclusively in vitellogenic ovaries, POF III were found in ovaries in the germinal vesicle stage, and the most deteriorated POF stage IV was found in actively spawning fish with hydrated oocytes. Since POF were absent only in very few ovaries (5%), and in each ovary in general only one POF stage was present, the duration of POF degeneration approximately equals the average batch interval, i.e. the time lag between subsequent spawning events. The results of the present study will serve as basis for future studies on Baltic sprat oocyte recruitment and daily spawning fraction. Highlights: ► Histological classification of sprat gonad development stages. ► Histological classification of POF in sprat in relation to gonad developmental stages. ► Daily spawning pattern of Baltic sprat.