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Doctoral thesis, 2019

Understanding the spatiotemporal dynamics of demersal fish species in the Baltic Sea

Orio, Alessandro


Species are not homogeneously distributed and the interdependencies between trophic interactions, environmental factors and anthropogenic forcing determine how their distribution changes over time. Hence taking into account both the spatial and temporal components of the dynamics of potentially interacting species is essential in management and conservation. Cod and flounder are two key fish species of the Baltic Sea, both ecologically and commercially. However, their dynamics and interactions in the offshore demersal habitat have been largely neglected in the past. Therefore, the objective of this thesis was to obtain further knowledge on how their dynamics have changed in space and time in this region. This was achieved by investigating the long-term changes in the horizontal and vertical distribution of cod and flounder, and by quantifying their spatial overlap and potential interactions. The results show that large changes have occurred in the demersal fish community of the Baltic Sea during the past four decades. Cod collapsed and contracted to the south, while flounder increased both in abundance and extent of distribution in the central Baltic. A contraction in the vertical distribution have been shown for both species from the early 1990s, possibly due to a combination of expanded areas of hypoxia in deep waters and an increase in predation risk in shallow waters. These changes have increased the spatial overlap between life-stages and species, which may have amplified the interaction strength between cod and flounder. The changes in predator-prey and competitive interactions between cod and flounder are hypothesised to have contributed to the low abundances of flounder during the “cod outburst” in the early 1980s, and to the low condition and feeding level of juvenile cod in the last decades. These results are highly relevant both for fisheries management and marine spatial planning as they can be implemented in multispecies models or directly used to protect important areas and habitats. Moreover, the results could be used in stock assessments and management to take into account more realistically the dynamics of cod and flounder in the Baltic Sea.


Baltic Sea; cod; flounder; spatial distribution; , species interaction

Published in

Acta Universitatis Agriculturae Sueciae
2019, number: 2019:7
ISBN: 978-91-7760-332-0, eISBN: 978-91-7760-333-7
Publisher: Department of Aquatic Resources, Swedish University of Agricultural Sciences

Authors' information

Swedish University of Agricultural Sciences, Department of Aquatic Resources

UKÄ Subject classification

Fish and Aquacultural Science

URI (permanent link to this page)