Preliminary microchemical analyses of North-western Persian Gulf hilsa shad otolith trace elements: Indications of complex migratory behavior and stock structure

Hilsa shad (Tenualosa ilisha) is an important anadromous fish species, supporting significant commercial and subsistence fisheries throughout south Asia. Otolith Sr:Ca and Ba:Ca profiles were used along with age data to examine the migratory history of hilsa shad from two major river systems in the northwestern Persian Gulf (Karun and Zohreh). In addition, elemental ratios (Sr:Ca, Ba:Ca, and Li:Ca) were analyzed using principal component analysis to investigate regional population structure. Variation in movement patterns was observed, though all fish analyzed had spent some time in freshwater, and most otolith signatures suggested a double downstream marine/upstream freshwater migration. There were however differences among rivers: the majority of fish from Zohreh River showed a single downstream/upstream migration through their life cycle, while Karun signatures were more complex, with multiple migrations and potential estuarine or freshwater residence. When comparing stock structure during the marine phase, the otolith signatures of Zohreh River fish showed greater separation while others tended to overlap, indicating that Zohreh fish were likely to have reared in a different location than the others. Although comprising a relatively small sample size, our results provide some preliminary insights into the life-history and stock structure of Persian Gulf hilsa shad, informing the management needs of this important fishery, and suggesting further more detailed investigations of migration and stock structure are warranted.