Variability in reproductive investment is an important factor behind recruitment and population dynamics of fish. We investigated absolute individual fecundity (AF) of the currently depleted population of autumn spawning herring in the northern Baltic Sea over three sequential years (2008–2010). Fecundity of fish in relation to changes in individual body mass, individual body length, body condition factor, age, and gonadosomatic index was investigated. AF varied between 11 838 and 108 093 oocytes per fish and, as an average, varied insignificantly between years. AF was positively significantly related to fish length and weight, but not to age. In general, individuals with a higher condition factor (K) had a higher AF. However, the correlation with K was weak or non-significant in some cases. Relative fecundity of the fish was found to vary 24–33% between years, being significantly different in all three years. In addition, the obtained results on AF were compared with historical findings from 1959–1970, when the population was at a high level. Notable differences were found in the AF between the two time periods by age groups, while by weights AF remained the same. The results of the present study, together with ongoing additional biological and ecological investigations, will help to identify mechanisms that configure the fecundity and recruitment processes.

Because of the high management relevance, commercial fish related aspects have often been central in marine ecosystem investigations. The iterative shiftogram method was applied to detect occurrence, type and timing of shifts in the single and multivariate time series linked to the spring spawning herring larvae in the Gulf of Riga (Baltic Sea). Altogether nineteen larval herring and related environmental variables were utilized during the period of 1957–2010. All the time series investigated, either single or multivariate, exhibited one or more shifts with variable type and timing. Multivariate shiftogram based on all time series identified two distinct states (1957–1983 and 1992–2010) in studied variables, separated by a smooth transition period lasting almost ten years. The observed shift was mainly related to hydroclimate and not to phenology or biota. Significantly increased variability was found in larval herring and recruitment abundances after the shift. While the shift in hydroclimate (1985–1991) was followed by the shift in phenology (1991–1997), the shift in biota occurred remarkably later (2003). It is likely that the dynamics in biota were affected by other drivers than those investigated in the current paper.

Baltic herring (Clupea harengus membras) is a total spawner with a group-synchronous ovarian organization. Age polymodality in total spawners is considered an important factor in assuring that a strong population is sustainable under an intensive harvesting regime and different climatic conditions. In the present study, we investigated the seasonal and inter-annual variation in spawner age structure and the effect of preceding winter thermal conditions on the start of the herring spawning and larvae retention period. Herring spawning season in the Gulf of Riga starts up to six weeks later after colder winters compared to milder winters. Significantly older individuals dominated at the beginning of the spawning season, and thus herring mean age gradually decreased towards the end of the spawning season from 1999–2015. On an annual scale, this pattern was obvious after cold winters, while after mild winters the pattern did not continue, indicating a more homogenous maturation cycle and spawning period, despite the age and size of the herring population in mild winters. Further, herring condition factor was studied in relation to age and spawning season following different winter thermal conditions. Young, 2- and 3- year old first-spawning herring experienced significantly lower conditions after cold winters compared to older ages, indicating an age-dependent effect of preceding winter on herring maturation cycle, condition and spawning time.

Age of fish at maturation depends on the species and environmental factors but, in general, investment in growth is prioritized until the first sexual maturity, after which a considerable and increasing proportion of resources are used for reproduction. The present study summarizes for the first the key elements of the maturation of European smelt (Osmerus eperlanus) young of the year (YoY) in the North-eastern Gulf of Riga (the Baltic Sea). Prior to the changes in climatic conditions and collapse of smelt fishery in the 1990s in the Gulf of Riga, smelt attained sexual maturity at the age of 3–4 years. We found a substantial share (22%) of YoY smelt with maturing gonads after the collapse of the smelt fisheries. Maturing individuals had a significantly higher weight, length and condition factor than immature YOY, indicating the importance of individual growth rates in the maturation process. The proportion of maturing YoY individuals increased with fish size. We discuss the factors behind prioritizing reproduction overgrowth in early life and its implications for the smelt population dynamics.