Oxygen in otoliths (a calcium carbonate structure in fish ears) is used to determine fish age. In fish otoliths 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 life 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.
This validation method has been used by scientists at the Alaska Fisheries Science Center on Pacific cod and saffron cod. Other δ18O studies at the AFSC are providing information on fish growth rates and water temperature preference.