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Age and Growth Program

18O sampling on a saggital otolith

Stable Isotopes

In fish otoliths, a calcium carbonate structure in fish ears, the ratio of oxygen isotopes, 18O/16O and measured as δ18O, is a function of ambient water temperature, and to some degree salinity. The δ18O level in the otoliths is inversely related to temperature; as the temperature goes up the δ18O goes down.

Otoliths function like a black box for the life of the fish, so they provide a way to reconstruct its temperature history. High-resolution sampling of the otoliths, from the center of the otolith (hatching) to the outer edge (capture) represents the life of the fish. Therefore, numerated seasonal cycles (i.e., the number of δ18O maxima, representing winter low temperatures) are an independent measure of fish “true age.” If the age estimated by growth zone counts agrees with the true age from the δ18O cycles, then the growth zone counts provided an accurate age.

In the Pacific cod otolith shown here, the δ18O was measured 36 times and represented temperature changes over the live of the fish. Five dark growth zones were counted, and there were five peaks in the series of δ18O. Therefore, the counted growth zones provided the correct age.

There is δ18O age validation research currently in process at the AFSC for Pacific cod and saffron cod. Other δ18O studies at the AFSC are providing information on fish growth rates and water temperature preference.


  • KASTELLE, C. R., T. E. HELSER, J. McKAY, and D. M. ANDERL. 2015. Age Validation of Pacific Cod Using Stable Oxygen Isotope (δ18O) in Otoliths. AFSC Quarterly Report Feature (April-May-June 2015). Online


  • Age Validation of Pacific Cod (Gadus macrocephalus) Using Stable Oxygen Isotopes (δ18O) (2014 poster) By: CRAIG KASTELLE, THOMAS HELSER, JENNIFER McKAY, DELSA ANDERL Conference: Alaska Marine Science Symposium, Anchorage, AK, Jan 2014 (2014 poster, .pdf, 4.04 MB) Online

  • A 200 Year Archeozoological Record of Pacific Cod Life History as Revealed Through Ion Microprobe Oxygen Isotope Ratios in Otoliths By: THOMAS HELSER, CRAIG KASTELLE, JOHN VALLEY, ARON L. CROWELL, IAN ORLAND, REINHARD KOZDON, TAKAYUKI USHIKUBO Conference: ICES Annual Science Conference, Coruna, Spain, Sept 2014 (2014 poster, .pdf, 1.4 MB) Online

18O sampling on a saggital otolith

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