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Institute of

OF Baltic Sea Fisheries


Otoliths make the difference

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© Thünen-Institut/B.Rotzoll
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DCF (Data Collection Framework): Investigations on the separation of cod stocks in the western Baltic Sea by means of stable isotope analysis of the otoliths

Where do you come from? What is your stock? If the fish had an identification card with information on the place of birth, this could be easily answered. Unfortunately, we have to apply significant effort to draw this secret from the fish – using their ear stones.

Background and Objective

In the Baltic Sea, the fisheries management of cod is based on two stocks, cod of the western Baltic in the ICES areas 22 to 24 (Kiel Bight/ Mecklenburg Bight, Arkona Sea) and cod of the ICES areas 25 to 32 (Bornholm Sea and all areas east- and northwards). The area 24 (Arkona Sea) is a mixing area between Western and Eastern cod. The question is: What is the proportion of cod from area 22 and 25 in area 24; and how much cod is actually originates from area 24? The mixing proportions in area 22 and 25 are also unclear. This ambiguity results in considerable uncertainty in the stock assessments, particularly when cod from one area “spill over” into other areas, thereby tampering with the stock assessment of the adjacent areas.

To improve the management, we have to re-sort the renegades to their original stock. Presently, this is not readily possible. But we use a number of different modern techniques to find these renegades out. The aim is to assign individual cod to specific areas of origin.


We use different methods for stock separation, such as genetic signatures, life history traits, spawning time, growth and maturity, parasite infestation etc. Some of these methods are fairly expensive and depend on the discovery of the right markers, i.e. the characteristic signal within the background noise. We are specialized in the analysis of otoliths, the ear stones of fishes, which are used in age determination. We use shape analysis and investigate chemical signatures in otoliths (and in the muscles). While a fish grows, the chemical characteristics of the surrounding aquatic environment are archived in the otoliths, located in the inner ear of the fish. The analysis of the chemical composition along the growth axis of an otolith can help to reconstruct the life history of a fish. Our institute has a micromill for high-resolution milling to recover powder for chemical and isotopic analysis. For instance, we investigate the ratios of the stable isotopes of 13C, 15N, 18O or 34S in otoliths to draw conclusions on the area of origin of a fish or movement patterns between different water bodies.


Several papers with contributions from OF staff have already been published on „stock separation of Baltic cod“ in the international literature.


7.2007 - 1.2016

More Information

Project status: finished


  1. 0

    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

    Schade FM, Weist P, Dierking J, Krumme U (2022) Living apart together: Long-term coexistence of Baltic cod stocks associated with depth-specific habitat use. PLoS One 17(9):e0274476, DOI:10.1371/journal.pone.0274476

  3. 2

    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

  4. 3

    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

  5. 4

    Schade FM, Weist P, Krumme U (2019) Evaluation of four stock discrimination methods to assign individuals from mixed-stock fisheries using genetically validated baseline samples. Mar Ecol Progr Ser 627:125-139, DOI:10.3354/meps13061

  6. 5

    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

  7. 6

    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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