Ba/Ca Ratios as an Upwelling Signature in Otolith of Southern Bluefin Tuna

Great Australian Bight (GAB) is characterized as the most productive coastal upwelling zone in southern Australia, which is the largest area of cool-water carbonate sedimentation in the world. Barium is poor in the open ocean but rich in the upwelling area. To test if Ba/Ca ratios in otolith can be used as a natural tag to chart the migratory history of Southern bluefin tuna (SBT Thunnus maccoyii) in the upwelling area, the elements, Na, Sr, Mg, Mn, Ba and Ca, in otoliths of the fish collected from both central Indian Ocean (CIO) and its spawning ground were measured by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) and age of the fish were determined by counting annulus in otolith. Then the relationship between the peak of Ba/Ca ratios and the deposition timing of annulus in otolith was examined. We found that Ba/Ca ratios in otolith significantly elevated at one to four years old of SBT and in summer zone of the otolith, which is corresponding to the age of the tuna entering GAB and the season of the upwelling happened in the GAB. This validates that Ba/Ca ratio in the otolith is a useful elemental signal to understand if the tuna have been migrating to the upwelling area. However, a part of SBT had not been migrating to GAB upwelling area because they did not appear peak of Ba/Ca ratios in the otolith. This study proved otolith Ba/Ca ratios as an alternative method of natural tag in reconstructing the migratory environmental history of fish particularly in the upwelling area.