Population Dynamics of Flounders in the Baltic Sea: – Declines, Cryptic Species AND Environmental Drivers

Research output: Types of ThesisDoctoral ThesisCollection of Articles


Initiated by signs of drastic declines of once abundant flounders in the northern Baltic Sea, this thesis characterised/quantified these declines and studied their potential reasons. Multiple approaches were used to describe population trends, reveal intricacies in stock structure, and assess links to key environmental drivers.

The results from the thesis verified clear negative trends in both adult and juvenile flounders in the northern Baltic Sea over the last 2–4 decades. Genetics revealed well-defined genetic structure and evidence for far gone speciation among Baltic Sea flounders, leading to the recognition of two instead of one flounder species, one of which was recently described as the only endemic fish species known to the region. Contrary to previous belief, flounders on the coast of Finland in the northern Baltic Sea were shown to be a mixed assemblage of this cryptic species pair. The thesis further showed that temporal variability in local species composition of the flounder assemblage explained some of the observed stock decline and was related to changing local and regional environmental conditions, of which reproductive volume, salinity, temperature and eutrophication were identified as potentially important factors. Finally, the thesis presented a new method for modelling environmental suitability for long-term population maintenance of the newly described Baltic flounder.

The knowledge obtained has great value for how we understand and investigate stock composition and population dynamics of Baltic Sea flounders, and relates to issues of source-sink mechanisms, population connectivity, biological traits, resilience to exploitation and environmental change, among others. The results are likely important for future management and conservation of these fishes in the changing environment of the Baltic Sea.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • University of Helsinki
  • Norkko, Alf, Supervisor, External person
  • Wennhage, Håkan, Supervisor, External person
Award date12 Jun 2020
Print ISBNs978-951-51-6024-9
Electronic ISBNs978-951-51-6025-6
Publication statusPublished - 2020
Externally publishedYes
MoE publication typeG5 Doctoral dissertation (article)


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