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Biogeosciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/bg-2020-74
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/bg-2020-74
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 13 Mar 2020

Submitted as: research article | 13 Mar 2020

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This preprint is currently under review for the journal BG.

Changes in population depth distribution and oxygen stratification explain the current low condition of the Eastern Baltic Sea cod (Gadus morhua)

Michele Casini1,2, Martin Hansson3, Alessandro Orio1, and Karin Limburg1,4 Michele Casini et al.
  • 1Swedish University of Agricultural Sciences, Department of Aquatic Resources, Institute of Marine Research, Lysekil, Sweden
  • 2University of Bologna, Department of Biological, Geological and Environmental Sciences, Bologna, Italy
  • 3Swedish Meteorological and Hydrological Institute, Gothenburg, Sweden
  • 4State University of New York College of Environmental Science and Forestry, Department of Environmental and Forest Biology, Syracuse, New York, USA

Abstract. During the past twenty years, hypoxic areas have expanded exponentially in the Baltic Sea, which has become one of the largest marine dead zones in the world. At the same time, the most important commercial fish population of the region, the Eastern Baltic cod, has experienced a drastic reduction in mean body condition, but the processes relating hypoxia to condition remain elusive. Here we use extensive long-term monitoring data on cod biology and distribution as well as on hydrological variations, to investigate the processes that relate deoxygenation and cod condition during the autumn season. Our results show that the depth distribution of cod has increased during the past four decades at the same time of the expansion, and shallowing, of the waters with an oxygen concentration known to be detrimental for cod performance. This has resulted in a spatial overlap between the cod population and low-oxygenated waters after the mid-1990s, which relates with the observed decline in cod mean body condition. Complementary analyses on fish otolith microchemistry also revealed that since the mid-1990s, cod individuals with low condition were indeed exposed to low-oxygen waters during their life. This study helps to shed light on the processes that have led to a decline of the Eastern Baltic cod body condition, which can aid the management of this population currently in distress. Further studies should focus on understanding why the cod population has moved to deeper waters in autumn and on analysing the overlap with low-oxygen waters in other seasons to quantify the potential effects of the variations in physical properties on cod biology throughout the year.

Michele Casini et al.

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Michele Casini et al.

Michele Casini et al.

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Short summary
In the past twenty years the condition of the Eastern Baltic cod has dropped with large implications for the fishery. Our results show that during the same time, the cod population has moved deeper, while low-oxygenated waters detrimental for cod growth have shallowed. Cod has thus dwelled more in detrimental waters, which relates to the drop in its condition. This study, using long-term fish and hydrological monitoring data, evidences the impact of deoxygenation on fish biology and fishing.
In the past twenty years the condition of the Eastern Baltic cod has dropped with large...
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