On differentiation of cod (Gadus morhua L.) groups in Baltic Sea

Russian Journal of Genetics - Using the AGP*, PGI-1*, PGI-2*, LDH*, IDH*, and PGM* allozyme markers, the differentiation of cod groups during the spawning period in Baltic Sea was evaluated. It was...     

Separating two herring stocks from growth data: long‐term changes in survey indices for Western Baltic Spring Spawning Herring (Clupea harengus) after application of a stock separation function

In the Baltic Sea, herring stocks are surveyed and managed according to a spatial allocation based on ICES (International Council for the Exploration of the Sea) subdivisions. In the western Baltic, the distribution areas of two stocks overlap: the Western Baltic Spring Spawning Herring (WBSSH) and the Central Baltic Herring (CBH). Survey results of length‐at‐age data indicate in Subdivision (SD) 24, which is a part of the WBSSH management area, that a considerable fraction of CBH is present and correspondingly erroneously allocated to WBSSH stock indices. Accordingly, a stock Separation Function (SF) based on growth parameters was established to identify the fraction of CBH in the WBSSH area. In the present study, the SF was applied to 8 years of data from the GERman Acoustic Survey (GERAS), which is conducted annually in autumn in ICES subdivisions 21‐24. Results showed a distinct fraction of CBH in SD 24, and exclusion of the CBH greatly improved the quality of the GERAS index used in the assessment of the WBSSH stock.     

The potential fecundity of cod in the Baltic Sea from 1993 to 1999

Investigations on the reproduction biology of cod in the Baltic Sea (Gadus morhua L. and G. morhua callarias L.) were carried out beginning in 1993, with yearly sampling of gonads to estimate the potential absolute and relative individual fecundity in the Belt Sea (subdivision, SD: 22) and in the Arkona Sea (SD: 24). Additional samples were taken in the Bornholm Sea (SD: 25). The potential absolute fecundity of the total area of investigation is described by the regression models: F_a = 6.89 × TL^3.09 and F_a = ?40 586 + 977.7 × W. As a further result the mean relative fecundity was 978 oocytes per gram body weight. Furthermore, it was shown that there was a difference in development of the potential individual fecundity between the Belt Sea cod and the original eastern Baltic cod stock. In contrast to the stable fecundity of cod in the Belt Sea, the fecundity of the large females in the Bornholm Sea significantly increased from 1993 to 1999. This change is related to a decrease in the dry weight of oocytes in the ovaries. In the same period this cod stock dramatically decreased.     

First evidence of spawning of eastern Baltic cod (Gadus morhua callarias) in the Belt Sea,the main spawning area of western Baltic cod (Gadus morhua L.)

Two cod stocks (western Baltic cod, WBC, and eastern Baltic cod, EBC) are managed in the Baltic Sea which is characterized by different main spawning areas and different main spawning periods. In this study we analyse the spatial and temporal occurrence of spawning individuals of both cod stocks in the main spawning grounds of the Baltic Sea based on eight microsatellite loci. Our results suggest that EBC (Gadus morhua callarias) has formed currently temporally stable, substantially homogeneous population not only in the Bornholm Sea (ICES SD: 25) but also in the Arkona Sea (ICES SD: 24). The presented analyses proved that EBC (G. m. callarias) can temporarily also spawn in the Belt Sea.     

Seasonal maturity development of Baltic cod in different spawning areas: importance of the Arkona Sea for the summer spawning stock

We investigated the seasonal maturity development of cod in four areas of the Baltic Sea. Two different spawning peaks were identified and found to be consistent over the period 1992–2005. In the Kiel Bight and Mecklenburg Bight (ICES SD 22) a spawning peak was observed from March to April (spring spawning). In the areas of the Arkona Sea (ICES SD 24) and Bornholm Sea (ICES SD 25) the spawning peak occurred during summer. In the Bornholm Sea, the main spawning activities began in June and ended in September, with a spawning peak in June–August (summer spawning). In the Arkona Sea, which is a transition area between the Mecklenburg Bight and the central Baltic Sea, spawning began in March and lasted until July, with a spawning peak in June–July (summer spawning). Seasonal maturity development and proportions of spawning cod in June in the Arkona Sea were similar to that of the Bornholm Sea. In addition, the proportion of spawning cod in the Arkona Sea was positively correlated with the size of the spawning stock in the Bornholm Sea. Our results provide evidence of a spatial expansion of spawning activities of the summer spawning stock from the eastern Baltic Sea into the Arkona Sea. Therefore, the Arkona Sea should be considered as one of the spawning habitats of the summer spawning stock of Baltic cod.