Morphometric variations between seasonal migrants of anadromous shad Tenualosa ilisha (Hamilton, 1822) from Hooghly Estuary, India

Abstract

The morphometric variations between two seasonal migrants (winter and monsoon) of anadromous shad Tenualosa ilisha from Hooghly Estuary were evaluated and compared using geometric morphometrics (GM). Altogether, 128 fish samples of T. ilisha, comprising 64 samples each from winter and the monsoon season, were collected for two successive years (2017 and 2018) and 14 landmarks were digitised uniformly on each individual. Relative warps (RW), principal component analysis (PCA), canonical variate analysis (CVA) and discriminant function analysis (DFA) were used to determine shape variations between seasonal runs. The deformation grid of RW showed that monsoon specimens have a deeper body profile, whereas winter specimens have a slender body profile. The PCA showed low variance (40.45% for first two principal components) and high overlap among all the groups. The CVA-extracted Mahalanobis and Procrustes distances (3.473 and 0.032 respectively) between the two groups (winter and monsoon) were highly significant (P<0.0001). The DFA also separated two groups with high cross-validated classification rates (85.94 and 95.31% of winter and monsoon specimens respectively were correctly classified). Hence, the results of RW, CVA and DFA clearly indicate the existence of two morphologically distinct units of T. ilisha in Hooghly Estuary.