DECIPHERING THE LIFE HISTORIES OF ATLANTIC TUNAS: THE POTENTIAL OF OTOLITH GEOCHEMISTRY

	Atlantic Bluefin

The isotope and elemental composition of fish otoliths are used as a tracking tool to link individual fishes to specific geographic locations. In this study, an in-situ, microbeam measurement technique was used to measure changes in the elemental composition of tuna otoliths at subannual resolution over the life of individual fish. Samples included otoliths from captive and wild samples of yellow and bluefin tunas, from the Mediterranean, western Atlantic and Pacific Oceans. The primary objective is to identify specific trace elements, either their absolute concentrations or variations through time, that can be used to identify the natal origins and basin-scale migration patterns of Atlantic tunas. Two elements were identified with potential to distinguish different bluefin populations. However, differences in otolith Na/Ca and Mg/Ca between the two populations are unrelated to differences in ocean chemistry or temperature. More likely, the elemental variations are rooted in kinetic effects caused by differences in otolith growth rate amongst populations from different environments. Sr/Ca ratios in the otoliths of both species are identical, both the absolute concentrations and the ontogenetic trends. Sr/Ca variability within the otolith appears to correspond with aspects of otolith microstructure and growth, and not directly to environmental conditions or ocean chemistry. It is proposed that the elemental chemistry of tuna otoliths is largely determined by processes which are internal to the fish and that the relationship to environmental conditions is indirect. Therefore, the utility of otolith chemistry as a natural tracer depends on our ability to resolve the processes that control trace element incorporation into otolith aragonite, and to relate them to the oceanic environment.