Text and photo: Henrik Hamrén, Baltic Eye, Baltic Sea Centre

– We try to identify possible “bottle necks” in body growth during the cods entire lifetime, that might help explain the stocks development and ability to recover, says Anna Gårdmark, associate professor at Department of Aquatic Resources, SLU.

Together with her colleague, associate professor Michele Casini, she leads a part of the DEMO-project concerned with analysing the Baltic cod growth.

The studies, which use a new method recently described in the scientific journal Philosophical Transactions of the Royal Society of London, was initiated during a DEMO-workshop in Lysekil the 10-12 of November. During the next couple of months, the scientists will connect environmental factors like salinity, temperature and areas of dead zones to data on cod growth, gastric content and food availability.

– The cod eats different prey during different life stages: first zooplankton, then benthic crustaceans, and finally, when it has grown large enough, mainly sprat and herring. That’s why it’s important to connect the growth analysis to specific life stages, says Anna Gårdmark.

Searching for general mechanisms

The goal is to identify possible critical stages during the life of the cod, when it is particularly sensitive to environmental effects and food availability.
The analysis contains a comparative study of two different cod stocks: one in the Baltic Sea and one off the Atlantic coast of Canada.

– By comparing the two stocks we can identify if there are general mechanisms driving growth and recovery, and if any critical life stages of the cod are the same in different stocks, says Michele Casini.

Research to match current needs

Studies of cod growth are topical – especially due to the recent dramatic changes in the Baltic Sea cod stocks. Nowadays the Baltic stocks contain quite a lot of small cods – but very few larger individuals. And in many cases the cods are unusually slim and in bad condition.

– It’s not enough to just note that the Baltic Sea cod is growing poorly. We want to understand weather it’s growing poorly all the time or just in specific life stages – and if so, why, says Anna Gårdmark.

– Such knowledge makes it easier to assess which management measures are needed for the cod population to recover

Interview with some of the workshop participants