Migration and spawning strategies of hilsa shad (Tenualosa ilisha, Clupeidae) in the Ayeyarwady River revealed by otolith chemistry

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Hilsa shad ( Tenualosa ilisha ) is an ecologically and economically important fish species in the Indo-Pacific region, yet its migration and spawning behaviour remains poorly in Myanmar. This study investigates hilsa migration and spawning strategies in the Ayeyarwady River using otolith chemistry, including elemental ratios (Sr:Ca, Ba:Ca, Mg:Ca, Mn:Ca) and 87 Sr: 86 Sr isotope signatures. Otoliths from 101 juvenile and adult hilsa collected across the Ayeyarwady River Basin showed that 94 % of hilsa exhibited life histories consistent with anadromy (spawning in freshwater rivers but growing up in the ocean). However, a small subset (6 %) displayed alternative reproductive strategies, likely spawning in higher salinity environments (such as estuarine or coastal areas) without entering rivers. Additionally, hilsa may migrate over 1500 km to the Upper Ayeyarwady from the sea. Additionally, otoliths of juvenile hilsa collected in the Chindwin River, exhibited consistently low Sr:Ca ratios along the core-to-edge profiles, indicating prolonged freshwater residency post-hatching (up to six months). Findings from this study illustrate the predominance of an anadromous life history and the existence of an alternative reproductive strategy, which are essential for adaptive management and conservation of hilsa in Myanmar. Given the species’ trans-boundary distribution and importance in the Indo-Pacific region, effective fishery management requires strengthening regional cooperation to promote sustainable hilsa fisheries in the region.