Örey, Serra (2018) Thinner females - fewer eggs? Temporal trends in Eastern Baltic cod fecundity (2005-2016). (Master thesis), Christian-Albrechts-Universität Kiel, Kiel, Germany, 59 pp.

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Abstract

Baltic cod (Gadus morhua) is ecologically important as a top predator, and commercially essential as a fisheries species in the Baltic Sea. Better understanding of the species’ reproductive potential is important for explaining variations in recruitment and to improve future stock assessments. Studies of individual potential fecundity (i.e., the potential number of eggs to be released per female in a given spawning season) and relative fecundity (i.e., the number of oocytes relative to the total body weight of individuals) are important since these parameters vary over time and in relation to nutritional condition of the fish. This is particularly crucial for the eastern Baltic cod stock, which has shown drastic declines in individual nutritional condition since 2005 until present (“the starving cod problem”) raising the question whether this has negatively affected resources invested in reproduction and thus fecundity. Since previous studies providing fecundity data covered the period from 1987 to 2000, an update for the most recent period is of particular interest. Here, I use an archive of pre-spawning female cod ovaries collected as part of the GEOMAR’s Bornholm Basin integrative long-term data series by annual multidisciplinary research cruises since 1987. Females from a range of nutritional conditions and several size classes were selected for the study. The focus area was the Bornholm Basin, presently the primary spawning ground of eastern Baltic cod. Individual fecundity measurements were obtained by applying the autodiametric fecundity method, which I successfully adapted for the first time to frozen ovary samples compared to the original approach on formalin preserved samples. Contrary to expectations, and despite the significant decrease in nutritional condition, mean potential fecundity of the eastern Baltic cod stayed stable over time, corresponding to an increase in relative fecundity due to the decline in weight at length. At the same time, the gonadosomatic index (i.e., the gonad weight relative to the gutted body weight of individuals) remained stable, which means that total resource allocation into gonads declined proportionally with condition. Jointly, this would suggest a decline in the energy invested per oocyte. Ultimately, my results suggest limited consequences of declining condition for overall fecundity of Baltic cod, but potentially important implications for the quality of reproductive output (i.e., decreased size or poor composition of oocytes). This raises questions regarding egg properties and survival of early life stages in the brackish water system of the Baltic after spawning, and should be investigated further. Fecundity data provided by this study represent long-term information that can support assessment of reproductive capacity and fisheries independent stock assessments based on egg production, and thus contribute to better management strategies for Baltic cod. This study also demonstrated the value of long-term sample archives to address questions related to temporal (either anthropogenic or natural) changes, which is particularly relevant in the highly dynamic and changing environment of the Baltic Sea, and in the context of global change.

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