Application of otolith morphology to unveil the stock structure of Hilsa shad (Tenualosa ilisha) in the Bhagirathi-Hooghly river system and the associated Northern Bay of Bengal

Sajina, A M and Nair, Sangeetha M and Suresh, V V R and Maity, Tara and Mukherjee, J and Banik, S K and Sandhya, K M and Manna, R K and Samanta, R (2025) Application of otolith morphology to unveil the stock structure of Hilsa shad (Tenualosa ilisha) in the Bhagirathi-Hooghly river system and the associated Northern Bay of Bengal. Regional Studies in Marine Science, 90. pp. 1-10. ISSN 2352-4855

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Abstract

The Hilsa shad (Tenualosa ilisha), an anadromous clupeid fish, inhabits diverse aquatic environments, including freshwater, estuarine, and marine habitats across the Indo-Pacific region. This study investigated the stock structure of Hilsa migrating through the Bhagirathi-Hooghly River system, in the northeastern Bay of Bengal, India by analysing the shape of the sagittal otolith of the species. A total of 257 specimens were collected from four distinct habitats along the migratory route: marine (MA), brackish water (BW), freshwater-tidal (FT), and freshwater (FW). Otolith outlines were extracted from the captured images and analyzed using Discrete Wavelet Transform and Standard Elliptical Fourier Transform. Wavelet reconstruction achieved 98.5 % shape accuracy with only 5 levels compared to 12 harmonics needed by Fourier, indicating its higher efficiency in detecting shape differences. Canonical Analysis of Principal Coordinates (CAP) and cluster analysis of wavelet coefficients revealed no distinct separation among habitat groups, supporting the hypothesis of the existence of a single stock migrating through the Bhagirathi-Hooghly River system. This study highlights the efficacy of otolith shape analysis as a robust tool and cost-effective alternative to other resource-intensive approaches for studying fish populations, especially in low-investment scenarios. The study also highlights the need for coordinated regional management to sustain Hilsa stocks, support livelihoods, and maintain ecological balance.

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