Allozyme and morphological variation throughout the geographic range of the tropical shad, hilsa Tenualosa ilisha

The population structure of the shad, hilsa Tenualosa disha, was investigated with both allozymes and morphometric analysis. Fish samples were collected from nine sites within Bangladesh and compared with samples from four other countries that covered the entire species range. Five polymorphic loci, Idh-1, Idh-m, Mdh-1, Mdh-m and Pgm, were interpretable in liver and muscle with starch-gel electrophoresis, although levels of variation were low. No significant differences in allele frequencies were detected within Bangladesh or within the Bay of Bengal (SE India and Myanmar) samples. Significant differences in allele frequencies occurred between Kuwait, Bangladesh and Indonesia. Nine morphological measurements and two meristic counts were compared among the same fish that were collected for genetic analysis. Discriminant function analyses (DFA) of transformed measurement ratios varied widely between sites, even those separated by short distances (<100km). The results showed that hilsa vary greatly in shape and this showed little geographic concordance. Indonesian and Indian hilsa were more distinctive than the most distant sample from Kuwait. Cross-validation of the classification revealed that sites in the middle reaches of the Meghna River in Bangladesh (Chandpur, Goalando, Bhairob Bazar) and Cox's Bazar had less than half the fish correctly classified to their origin. Fish from other countries were much more distinctive. We suggest the morphological results reflect local environmental conditions rather than any population-level differences. The allozyme results suggest that there is substantial gene flow between groups of hilsa within the Bay of Bengal. This means that there is a need for Bangladesh to take into account actions and management decisions in India and Myanmar in order to sustainably manage their fishery. (C) 2003 Elsevier Science B.V. All rights reserved.