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Abstract

The southern bluefin tuna (Thunnus maccoyii) is a longlived, large, and highly migratory marine fish in the Indian Ocean. They can live up to 40 years and migrate to the Great Australian Bight in the summer when 1-4 years old. The Great Australian Bight is characterized as the most productive coastal upwelling zone in southern Australia and is the largest area of cool-water carbonate sedimentation in the world. The barium (Ba) level is poor in the open ocean but rich in the upwelling area. This study used otolith Ba/Ca ratios as a natural tag to confirm that southern bluefin tuna seasonally occupy the upwelling area. Southern bluefin tuna were collected from the central Indian Ocean and the spawning ground between the island of Java, Indonesia and northwestern Australia. The temporal variation of trace elements in otoliths of the specimen was measured by laser ablation inductively coupled plasma mass spectrometry. Otolith Ba/Ca ratios were significantly elevated in the summer growth zone, which corresponds to the upwelling season when juvenile tuna enter the Great Australian Bight at the age of 1-4 years old. Although almost all of the mature southern bluefin tuna collected in the spawning ground had previously migrated to the Great Australian Bight upwelling area as juveniles, some fish collected from the Central Indian Ocean didn’t migrate to the Great Australian Bight upwelling area, perhaps because they are a vagrant population and may contribute less to the spawning stock.

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