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Baltic cod

Gadus morhua

Baltic cod is one of the major cod stocks globally. It has a key role in the Baltic Sea with impact on the whole ecosystem, from the main pelagic fish  herring to the zooplankton and phytoplankton.

Conservation genetic management advice 

  • Manage the eastern and western Baltic cod as separate units.
  • Monitor mixing proportions of eastern and western cod in the Arkona basin.
  • The western Baltic cod should potentially be divided into more than one management unit, since it is unclear whether there are several spawning populations within this area. 

Threats to genetic biodiversity

The eastern population is adapted to spawning in low salinity and may not be replaced by cod from elsewhere if depleted. 

Immigration of eastern cod into the western Baltic management unit may mask a poor state of the populations in the western unit. 

Cod is subjected to large scale fishing, which is expected to affect the genetic composition in the populations.

Knowledge of genetics of Baltic Sea cod (2018)

One large cod population spawns east of Bornholm, and one or several populations spawn west of Bornholm, see map. Eastern Baltic cod is genetically distinct and adapted to the brackish environment. Adaptations include differences in hemoglobin type, osmoregulatory capacity, egg bouyancy, sperm swimming characteristics and spawning period.

In the western Baltic, there are spawning aggregations in Öresund, Kiel and Mecklenburg Bay which are genetically distinct from both the eastern Baltic cod, and from the North Sea cod. In the Arkona basin, spawning and migrating cod from the eastern and western Baltic cod stocks intermingle in proportions that vary seasonally.

Individuals in spawning condition have been observed in the Åland deep, but it is unclear if fertilisation actually takes place and if the offspring survive. Historically spawning occured also in the Gdansk deep and off Gotland, but these populations seem to have gone extinct.

Population structure Baltic Cod. Illustration by Azote.Spawning areas and relative abundance of the eastern and western cod stocks in the Baltic Sea. Abundance is approximated from catches and surveys in ICES Subdivisions in 2016, and indicated by colour saturation. Illustration: Elsa Wikander, Azote.

References

CONTRIBUTOR

Carl André
carl.andre@gu.se

Additional information

Analyses of ancient cod DNA indicate that the eastern cod population has been isolated for a long time, possibly already from the time of the Baltic colonisation 6 000 to 8 000 years BP.

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