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Institute of

OF Baltic Sea Fisheries


Age validation studies on wild fish: basis for accurate age determination

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© Thuenen-Institut/Aisha Degen-Smyrek
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DCF (Data Collection Framework): Age validation studies on Baltic round- and flatfishes

We would like to know the age of a fish as precisely and accurately as possible. Since fish do not swim around with an identification card, we have to interpret the ring structures in their ear stones (otoliths). To verify the relationship between the lighter and darker rings in the otoliths of fish and the true fish age, we carry out age validation experiments on wild fish.

Background and Objective

Age validation is a central pillar for the understanding of the biology of fish stocks. Presently, the age determination of many fish stocks is subject to significant levels of uncertainty despite numerous efforts in the past. Accordingly, the predictions of the stock assessment are uncertain.

Therefore, we initiated catch-release experiments. Thereby we want to validate the age of abundant Baltic roundfish species like cod and whiting as well as flatfishes such as plaice, flounder or turbot. We analyse the ring structures the fish has produced between release and recapture. Thus, we can improve the age determination for these fish species and the quality of age data to populations models and of ecological studies.


In pilot studies we identified the chemical and the concentration appropriate for conducting age validation studies in the Western Baltic Sea. We are catching live fish to mark them internally (with tetracycline as a time tag inside the otoliths) and externally (with a numbered so-called spaghetti tag for the detection by fishers and anglers upon recapture). Then we hope and wait that the marked fish are recaptured and returned as whole fish for further analysis. A reward is paid for each recaptured fish when the whole fish is returned.

Preliminary Results

The ring formation of juvenile and young adult cod of the western Baltic Sea was successfully validated. This led to a significant improvement in the quality of the age data that Germany, Denmark and Sweden regularly provide to ICES for the Western Baltic cod stock. In addition, the age-validation findings formed the basis for a number of studies on the ecology of western Baltic cod.

Ring formation patterns in adult cod of the eastern Baltic Sea could also be validated. Currently we are working on the validation of juvenile and adult plaice, flounder and turbot in the Western Baltic Sea.


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    Follesa MC, Hilvarsson A, Songer S, Allegaert W, Basilone G, Beier U, Bekaert K, Berg F, Busawon D, Canha A, Carbonara P, Torres Cutillas P, Coad Davies J, Damme CJG van, Denechaud C, Dubroca L, Elleboode R, Farias I, Ferreri R, Krumme U, et al (2024) Working Group on Biological Parameters (WGBIOP; outputs from 2023 meeting). Copenhagen: ICES, iv, 80 p, ICES Sci Rep 6(2), DOI:10.17895/

  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/

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    Beier U, Coad Davies J, Adrian-Schütte V, Alvestad AH, Bekaert K, Bland B, de Vries M, Dussuel A, Gault M, Gillespie-Mules R, Hansen FI, Hefter-Siebold M, Husebo A, Jennings C, Johansson J-E, Koelemij E, Krumme U, Maertens I, Moerman M, Rasmussen H, et al (2022) Workshop 2 on Age Reading of North Sea plaice (Pleuronectes platessa) (WKARP2; outputs from 2021 meeting). Copenhagen: ICES, 189 p, ICES Sci Rep 4(64), DOI:10.17895/

  4. 3

    Plonus R, McQueen K, Günther C, Funk S, Temming A, Krumme U (2021) Is analysis of otolith microstructure a valid method for investigating early life history of Western Baltic cod? Mar Biol 168:30, DOI:10.1007/s00227-021-03834-x

  5. 4

    Follesa MC, Hilvarsson A, Songer S, Aanestad Godiksen J, Allegaert W, Basilone G, Beier U, Bekaert K, Berg F, Blass M, Canha A, Carbonara P, Coad Davies J, Dubroca L, Finnbogadottir G, Gault M, Gillespie-Mules R, Haase S, Krumme U, Ulleweit J, et al (2021) Working Group on Biological Parameters (WGBIOP). Copenhagen: ICES, 370 p, ICES Sci Rep 3(112), DOI:10.17895/

  6. 5

    Krumme U, Stötera S, McQueen K, Pahlke E (2020) Age validation of age 0-3 wild cod Gadus morhua in the western Baltic Sea through mark-recapture and tetracycline marking of otoliths. Mar Ecol Progr Ser 645:141-158, DOI:10.3354/meps13380

  7. 6

    Carbonara P, Coad Davies J, Damme CJG van, Aanestad Godiksen J, Allegaert W, Beier U, Bekaert K, Canha A, Farias I, Follesa MC, Gault M, Gillespie-Mules R, Haase S, Hilvarsson A, Hüssy K, Korta M, Krüger-Johnsen M, Krumme U, Stransky C, Ulleweit J, et al (2020) Working Group on Biological Parameters (WGBIOP). Copenhagen: ICES, 150 p, ICES Sci Rep 2(117), DOI:10.17895/

  8. 7

    McQueen K, Hrabowski J, Krumme U (2019) Age validation of juvenile cod in the western Baltic Sea. ICES J Mar Sci 76(2):430–441, DOI:10.1093/icesjms/fsy175

  9. 8

    McQueen K, Paige Eveson J, Dolk B, Lorenz T, Mohr T, Schade FM, Krumme U (2019) Growth of cod (Gadus morhua) in the western Baltic Sea: estimating improved growth parameters from tag-recapture data. Can J Fish Aquat Sci 76(8):1326-1337, DOI:10.1139/cjfas-2018-0081

  10. 9

    Stötera S, Degen-Smyrek AK, Krumme U, Stepputtis D, Bauer Robert, Limmer B, Hammer C (2019) Marking otoliths of Baltic cod (Gadus morhua Linnaeus, 1758) with tetracycline and strontium chloride. J Appl Ichthyol 35(2):427-435, DOI:10.1111/jai.13829

  11. 10

    Krumme U, Bingel F (2016) Tetracycline marks visible in Baltic cod Gadus morhua otoliths stored for 40 years. J Fish Biol 89(4):2189-2194, DOI:10.1111/jfb.13108

  12. 11

    Stötera S, Krumme U (2016) Use of otolith quality flags to assess distributional dynamics in Baltic cod stocks. Mar Freshwater Res 67(7):980-991, DOI:10.1071/MF15048


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