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Institute of

OF Baltic Sea Fisheries


Tagging Baltic Sea cod (TABACOD)

[Translate to English:] Dorsch mit äußerer Markierung
© Thünen-Institut/C.Zimmermann/A.Schütz

TABACOD (Tagging of Baltic Sea Cod) - Solving the aging and stock assessment problems of eastern Baltic Sea cod with state-of-the-art tagging methods

How old is a fish, and how fast does it grow? These simple questions are almost impossible to answer for eastern Baltic cod, with massive consequences for fisheries management. A large-scale international tagging project may be the key to solving this problem.

Background and Objective

The age of a fish is an influential variable in fisheries biology. Stock assessments which form the basis of management advice use fish age as a key input parameter when calculating estimates of catch, abundance and recruitment. For the eastern Baltic cod stock, age of individual fish is currently estimated using the traditional method of visual analysis of “annual otolith rings”. The otolith (a bone inside the inner ear of the fish) grows throughout the life of a fish, forming annual rings that can be read in a similar way to tree rings. However, the annual rings of eastern Baltic cod otoliths are notoriously difficult to interpret due to irregular growth patterns and low contrast between the seasonal growth zones, and these difficulties have dramatically increased in recent years. The current method for age-reading of eastern Baltic cod is now so unreliable that in 2014 the traditional age-based stock assessment failed due to the lack of high quality input data.

Not only is reliable age information important for stock assessments, it is also key to understanding the current status and condition of the stock. Since 2013, there has been a noticeable lack of larger cod in the eastern Baltic Sea. This lack of large individuals is likely to be driven by either high mortality of older individuals, slow individual growth or some combination of these two processes. However, without accurate estimates of age it is impossible to reliably disentangle the influences of growth and mortality on the status of the stock. This uncertainty hinders the selection of management measures appropriate for responding to this worrying stock development.

There is currently an urgent need to calculate accurate estimates of growth and age of individual eastern Baltic cod, for present and future management. The objectives of the TABACOD project are to provide the data necessary to calculate reliable estimates of growth rates for this stock, and to develop tools which can be used to objectively and continuously derive growth and age information in the future.


The task of obtaining accurate growth and age estimates for eastern Baltic cod will be tackled by using two interlinked methods: mark-recapture of individual fish and analysis of the chemical composition of their otoliths.

The mark-recapture method is a widely-used and relatively accurate method for calculating the growth rate of wild fish. This method involves marking fish with easily identifiable tags, and then releasing them back into the wild. When these fish are re-captured estimates of growth rate can be calculated based on the change in length of the fish and the length of time at liberty. Three types of tag will be used to tag eastern Baltic cod in the TABACOD project. (1) External tags will be used to identify individual fish and to provide information on recapture time and position. (2) Data storage tags (DSTs) will be used to collect data on depth and temperature experienced by individual cod, important factors which can influence fish growth. (3) Fish will also be tagged with internal chemical tags which deposit a permanent, visible mark on the otolith that corresponds to the time of tagging. As otoliths grow proportionally to the fish, growth can be back-calculated by comparing the distance between the tag mark and the outer edge of the otolith with the distance between the tag mark and the centre of the otolith. Over the 4 year duration of the TABACOD project, thousands of cod will be tagged in a huge international effort between Germany, Denmark, Sweden and Poland. Combined with historical data on Baltic cod growth, this will ensure a large sample size from which to calculate reliable growth estimates.

Additionally, state-of-the-art micro-chemical analysis of otoliths will also be conducted in an attempt to develop new methods for aging this difficult-to-age stock. As certain trace elements are incorporated into otoliths throughout the life of the fish in response to physiology, they can potentially be used as “chemical calendars”. The marks incorporated into otoliths during tagging will be used to validate hypotheses relating elemental fluctuations in otoliths to seasonal cycles.


see final report (publications below) and publications

Links and Downloads

For more information please visit the TABACOD website

Information from the Thünen Institute (in German):

Projekt TABACOD zur Dorschmarkierung startet

Altersbestimmung und Wachstum - Markierte Dorsche

Information about Baltic 2020

Involved external Thünen-Partners

  • Danish Technical University (DTU)
    (Kopenhagen, Hirtshals, Charlottenlund, Dänemark)
  • Swedish University of Agricultural Science - SLU
    (Uppsala, Lysekil, Schweden)
  • National Marine Fisheries Research Institute
    (Gdynia, Polen)

Funding Body

  • BalticSea2020
    (international, öffentlich)


3.2016 - 5.2020

More Information

Project status: finished


  1. 0

    Hüssy K, Haase S, Mion M, Hilvarsson A, Radtke K, Thomsen TB, Krüger-Johnsen M, Casini M, Sturrock AM (2024) Into the wild: coupling otolith and archival tag records to test assumptions underpinning otolith chemistry applications in wild fish. Front Mar Sci 11:1365023, DOI:10.3389/fmars.2024.1365023

  2. 1

    Schuchert P, McCully Phillips S, Abecasis D, Bolland J, Ellis J, Farthing M, García Villar J, Gillson J, Gregory S, Johnston G, Jónsdóttir IG, Junge C, Krumme U, Maes J, Mann B, Maxwell H, Mayo P, McGuckin E, Musick S, Neto O, et al (2024) Workshop on mark-identification tagging (WKTAG). Copenhagen: ICES, iv, 57 p, ICES Sci Rep 6(27), DOI:10.17895/

  3. 2

    Lundgreen RBC, Nielsen A, van Deurs M, Olesen HJ, Mion M, Haase S, Casini M, Krumme U, Hüssy K (2023) Stock connectivity patterns and indications of sub-stock component structuring of cod in the Sound in the western Baltic Sea. Fish Res 261:106617, DOI:10.1016/j.fishres.2023.106617

  4. 3

    Haase S, Barz F, Casini M, Hüssy K (2023) The journeys of Wally and Wilma: How scientists reconstruct the movements of fish. Front Young Minds 11:1042452, DOI:10.3389/frym.2023.1042452

  5. 4

    Lundgreen RBC, Nielsen A, Krüger-Johnsen M, Righton D, Mion M, Radtke K, Plikshs M, Leskelä A, Raitaniemi J, Griffiths CA, Casini M, Krumme U, Hüssy K (2022) Examining fish movement in terms of advection and diffusion: a case study of northeastern Atlantic cod. Mar Ecol Progr Ser 691:115-129, DOI:10.3354/meps14065

  6. 5

    Mion M, Griffiths CA, Bartolino V, Haase S, Hilvarsson A, Hüssy K, Krüger-Johnsen M, Krumme U, Carlstedt Lundgreen RB, Lövgren J, McQueen K, Plikshs M, Radtke K, Raitaniemi J, Casini M (2022) New perspectives on Eastern Baltic cod movement patterns from historical and contemporary tagging data. Mar Ecol Progr Ser 689:109-126, DOI:10.3354/meps14047

  7. 6

    Haase S, Hüssy K, Casini M, Radtke K, Krumme U (2021) Eaten by a cormorant: Unexpected return of a tagged Baltic cod. In: 2021 international workshop on metrology for the sea; learning to measure sea health parameters (MetroSea). IEEE, pp 283-287, DOI:10.1109/MetroSea52177.2021.9611573

  8. 7

    Hüssy K, Krüger-Johnsen M, Thomsen TB, Heredia BD, Naeraa T, Limburg KE, Heimbrand Y, McQueen K, Haase S, Krumme U, Casini M, Mion M, Radtke K (2021) It’s elemental, my dear Watson: validating seasonal patterns in otolith chemical chronologies. Can J Fish Aquat Sci 78(5):551-566, DOI:10.1139/cjfas-2020-0388

  9. 8

    Mion M, Haase S, Hemmer-Hansen J, Hilvarsson A, Hüssy K, Krüger-Johnsen M, Krumme U, McQueen K, Plikshs M, Radtke K, Schade FM, Vitale F, Casini M (2021) Multidecadal changes in fish growth rates estimated from tagging data: A case study from the Eastern Baltic cod (Gadus morhua, Gadidae). Fish Fisheries 22(2):413-427, DOI:10.1111/faf.12527

  10. 9

    Haase S, Krumme U, Gräwe U, Braun CD, Temming A (2021) Validation approaches of a geolocation framework to reconstruct movements of demersal fish equipped with data storage tags in a stratified environment. Fish Res 237:105884, DOI:10.1016/j.fishres.2021.105884

  11. 10

    Mion M, Hilvarsson A, Hüssy K, Krumme U, Krüger-Johnsen M, McQueen K, Mohamed E, Motyka R, Orio A, Plikshs M, Radtke K, Casini M (2020) Historical growth of Eastern Baltic cod (Gadus morhua): Setting a baseline with international tagging data. Fish Res 223:105442, DOI:10.1016/j.fishres.2019.105442

  12. 11

    McQueen K, Casini M, Dolk B, Haase S, Hemmer-Hansen J, Hilvarsson A, Hüssy K, Mion M, Mohr T, Radtke K, Schade FM, Schulz N, Krumme U (2020) Regional and stock-specific differences in contemporary growth of Baltic cod revealed through tag-recapture data. ICES J Mar Sci 77(6):2078-2088, DOI:10.1093/icesjms/fsaa104

  13. 12

    Hüssy K, Casini M, Haase S, Hilvarsson A, Horbowy J, Krüger-Johnsen M, Krumme U, Limburg K, McQueen K, Mion M, Olesen HJ, Radtke K (2020) Tagging Baltic Cod - TABACOD : Eastern Baltic cod: Solving the ageing and stock assessment problems with combined state-of-the-art tagging methods. National Institute of Aquatic Resources, Technical University of Denmark, 92 p, DTU Aqua Rep 368-2020

  14. 13

    McQueen K, Mion M, Hilvarsson A, Casini M, Olesen HJ, Hüssy K, Radtke K, Krumme U (2019) Effects of freezing on length and mass measurements of Atlantic cod Gadus morhua in the Baltic Sea. J Fish Biol 95(6):1486-1495, DOI:10.1111/jfb.14171

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