We analyzed recruitment -environment relationships for 5 distinct Baltic Sea herring stocks inhabiting the areas of the Western Baltic (WBH), the Main Basin (MBH), the Gulf of Riga (GRH), the Bothnian Sea (BSH) and the Bothnian Bay (BBH). A number of hydro-climatic and biological predictors were tested for their effect on recruitment. Temperature was determined to be an important predictor for 3 of the stocks (MBH, GRH and BSH). However, spawning stock biomass was the major factor explaining recruitment for GRH and weight-at-age of the spawners was an important predictor of MBH recruitment. For 2 out of 4 stocks for which complete zooplankton data were available (BSH and MBH), food supply was also a significant predictor, suggesting that changes in climate and/or food web structure may indirectly affect herring recruitment via prey availability for the recruits or spawners. Our results emphasize both similarities and differences in the main regulators of recruitment dynamics for the 5 stocks that should be taken into consideration in the development of area-specific management strategies throughout the Baltic Sea basin.

Oeberst, R., Klenz, B., Gröhsler, T., Dickey-Collas, M., Nash, R. D. M., and Zimmermann, C. 2009. When is year-class strength determined in western Baltic herring? – ICES Journal of Marine Science, 66: 1667–1672. Weekly surveys of larvae in the Strelasund and the Greifswalder Bodden were used to investigate when year-class strength is determined in western Baltic spring-spawning (WBSS) herring. An abundance metric of larvae reaching a length of 20 mm over the entire spawning season was constructed by accounting for increases in daily growth resulting from seasonal increases in temperature (5–20°C). The index was significantly correlated with the acoustic estimates of age-1 herring in the western Baltic Sea (r = 0.88) and with the estimates of year-class strength obtained from stock assessment (r = 0.65). Previous studies of herring elsewhere have reported that year-class strength is determined during the late larval stage, but we show that year-class strength can already be fixed at a larval length of only 20 mm. Although the index obtained may be used in stock assessment and predictions, the intriguing question remains, namely how can the signal of larval productivity from one single spawning component of WBSS herring reflect the year-class dynamics of the entire stock?

Dominant drivers of larval survival are considered to include oceanographic dispersal, sea temperatures, and food availability in the phase of first-feeding. However, research progress on larval herring survival dynamics indicates that multiple factors might act on differing larval developmental stages. Hypothesizing that in inshore systems of the western Baltic Sea bottlenecks of herring development occur before the point of first-feeding, we analysed an extensive time-series of weekly abundances of early stage larvae in Greifswald Bay, an important spawning area for Western Baltic herring. Additionally, we investigated whether distinct hatching cohorts contribute differently to established survival indices on the level of (i) later larval stages in Greifswald Bay and (ii) 1+ group juveniles in the overall western Baltic Sea. Results revealed that abundances of the earliest larval stage explain 62% of the variability of later stage larvae and 61% of the variability of surviving juveniles, indicating pre-hatching survival bottlenecks. Hatching cohorts occurring later during the spawning season contribute most to the surviving year class. Earlier hatching cohorts were not found to result in significant amounts of growing larvae, indicating a bottleneck phase at the critical period when larvae start feeding.

Gröhsler, T., Oeberst, R., Schaber, M., Larson, N., and Kornilovs, G. 2013. Discrimination of western Baltic spring-spawning and central Baltic herring (Clupea harengus L.) based on growth vs. natural tag information. – ICES Journal of Marine Science, 70: 1108–1117. In the Baltic Sea, several stocks of herring (Clupea harengus L.) are surveyed and managed separately. For assessment purposes, a spatial stock separation based on ICES subdivisions is implemented. However, especially in the western Baltic, the distribution areas of two stocks, the western Baltic spring-spawning herring and the central Baltic herring, overlap. Results of regularly conducted surveys for assessment purposes indicated variable degrees of mixing of both stocks in the survey area, based on conspicuous differences in weights/lengths within certain age groups, especially in an area known for overlapping distribution of both stocks. At present, varying fractions of the central Baltic herring stock have not been taken into account during regular surveys conducted in the western Baltic, leading to possible undetected biases in assessment indices derived from these surveys. Additionally, methods otherwise applied for stock separation of Baltic herring so far are based on parameters that cannot readily be derived during regular surveys. In this paper, we present a simple and quick method to reliably allocate herring to either stock based on a separation function derived from survey-based length-at-age data, thus facilitating a more precise estimate of biomass and abundance indices from regular surveys and commercial fisheries.

We examined spatial and temporal differences in growth patterns of young-of-the-year (YoY) sprat Sprattus sprattus synchronously sampled in October 2002 from 4 different regions of the Baltic Sea (western, central, eastern, northeastern Baltic). The microstructure of the sagittal otoliths of 427 individuals from 64 sampling sites were analysed to determine the day of first feeding (DFF) and the growth history of YoY survivors. DFF distributions differed markedly between Baltic areas, with a shift to later mean DFFs and narrower distributions from west to northeast. This was consistent with the shift in mean seasonal spawning effort of Baltic sprat, derived from long-term observations (1973 to 2002) of sprat egg abundance in these areas. Otolith growth trajectories (i.e. increment width-at-age) had a characteristic shape related to sampling area and, more importantly, to the time of the year at which the individual started feeding (DFF). During the larval stage, individuals from the northeastern area and those born later in the year had higher growth rates than their earlier born conspecifics, while the pattern was reversed during the juvenile stage. Weekly means of satellitebased sea-surface temperature were used to approximate the potential temperature history of YoY survivors, which significantly influenced the shape of the otolith growth trajectory. We conclude that different DFFs and therefore different temperature histories were primarily responsible for the largescale spatial growth variability between newly recruited Baltic YoY sprat in 2002.KEY WORDS: Young-of-the-year sprat · Otolith microstructure analysis · Satellite data · Sea-surface temperature · Growth patterns · Day of first feeding · Egg abundance Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 317: [225][226][227][228][229][230][231][232][233][234][235][236] 2006 throughout the larval and early juvenile stages of sprat . Because survival in the field is often coupled with growth (Houde 1989), a better understanding of the processes that influence growth patterns in larval and juvenile Baltic sprat is desirable. However, an inherent limitation of growth studies on larvae and early juveniles is that 'average' patterns do not reflect the small fraction of individuals that will eventually emerge as survivors of these stages (Sharp 1987). It is thus meaningful to obtain samples from successful sprat recruits and investigate their growth histories, which can later be compared to individuals sampled earlier from the population.Growth histories of sprat survivors are best inferred from otolith microstructure analysis of newly recruited, young-of-the-year (YoY) or 0-group sprat because, until their first winter, sprat are known to form readily discernible, daily increments that correspond in width to daily somatic growth rates and allow a direct backcalculation until the day of first feeding (DFF) (Baumann et al. in press). In sprat, DFFs comprise the best available proxy for hatch date, beca...