Otoliths are tiny bone-like structures found within the ears of vertebrates that help animals sense gravity and directionality. Fish otoliths can be used as powerful tools by fisheries scientists to understand fish growth rates and life-histories. As a fish grows, calcium carbonate is layered onto the otolith. The rate of this accretion varies seasonally so that there are annual 'growth rings' in an otolith, similar to rings in a tree trunk. By counting the rings in a fish's otoliths, you can determine it's age.
Besides calcium carbonate, there are also trace elements that are incorporated into otoliths from the fish's environment. The amount of these trace elements present depends on the chemistry of water that the fish is living in, and so by studying otoliths, scientists can infer where fish have been at various life-stages. In Atlantic cod, for example, examining the chemical composition of otoliths from wild fish might be able to tell us the location of important nursery areas for juvenile fish and the ages of fish when they occupied these areas.
An underlying assumption when using otoliths to inform the locational history of fishes is that there are predictable relationships between environmental variables and otolith chemistry. In our recent paper, myself and some colleagues tested the effects of these environmental variables on the chemical composition of Atlantic cod otoliths. Specifically, we reared juvenile Atlantic cod under various levels of salinity and at different temperatures, then tested the resulting effect on the chemcial composition of their ototliths. We found that both temperature and salinity have strong effects on the chemical signatures present within the otoliths, highlighting the need for species-specific laboratory 'calibrations' of the environmental effects on ototlith composition in young fish. Read more here:
Stanley, R. R., Bradbury, I. R., DiBacco, C., Snelgrove, P. V., Thorrold, S. R., & Killen, S. S. (2015). Environmentally mediated trends in otolith composition of juvenile Atlantic cod (Gadus morhua). ICES Journal of Marine Science: Journal du Conseil, fsv070.