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Doctoral thesis2021Open access

Increasing the biological knowledge of Baltic Sea cod: growth, movements and reproductive potential from historical and contemporary data

Mion, Monica


Knowledge about life-history traits of commercially exploited fish stocks and their possible changes over time is essential for implementing a sustainable management. Biological parameters such as growth rate, fecundity and movement patterns are in fact, underlying determinants for stock responses to environmental forcing and fishing exploitation.

Historically, the Eastern Baltic cod (EBC) has been one of the most important commercial stocks in the Baltic Sea but currently is one of the most severely threatened fish stocks in Europe. During the past two decades, a number of changes in biology and ecological conditions has affected the EBC stock, raising concerns among fisheries scientists and managers. One of the main biological changes has been the contraction in the size structure of the stock towards smaller fish. However, due to the large uncertainties in age estimations, it was unclear whether this change was the result of reduced growth or increased mortality of older individuals, or a combination of both. This has led to the failure of the analytical stock assessment between 2014 and 2018. The contracted size distribution of the stock could have important implications also for its potential fecundity, affecting recruitment, and movement patters.

The aim of this thesis was to increase the knowledge on key biological parameters of EBC, including growth, fecundity and movement patterns. To this end, I collated data from historical and contemporary tagging experiments, to estimate EBC individual growth using length-based methods. The results revealed that the current growth of cod is the lowest observed in the past 7 decades indicating very low productivity. These estimations have contributed to the re-establishment of the EBC analytical stock assessment since 2019. In addition, the thesis showed that the currently low growth that lead to smaller fish sizes, together with the observed decline in condition, is expected to negatively affect the fecundity and thus the reproductive output of the stock.

The re-analyses of historical data confirmed the presence of different movement behaviours, stationary and migratory, with larger distances covered by cod released in the northern and central Baltic areas compared to cod released in the southern Baltic. In addition, larger fish seemed to move over larger distances than smaller fish, underlying the importance of having larger fish with higher potential of dispersion in the stock. Furthermore, data from the recent tagging experiment indicate enduring resident strategy in the southern Baltic area.

This thesis presents methods and results that increased the understanding of the EBC biology, relevant for its management and that could be applied for future monitoring.


Baltic cod; mark-recapture; historical data; time-series; growth modelling; potential fecundity; movement patterns

Published in

Acta Universitatis Agriculturae Sueciae
2021, number: 2021:20ISBN: 978-91-7760-718-2, eISBN: 978-91-7760-719-9Publisher: Swedish University of Agricultural Sciences, Department of Aquatic Resources