Movement patterns of the tropical shad hilsa (Tenualosa ilisha) inferred from transects of Sr-87/Sr-86 isotope ratios in their otoliths

We examine Sr-87/Sr-86 isotope ratios in transects across the otoliths of the diadromous tropical shad hilsa, Tenualosa toli, to assess the extent of movement of fish within the large Meghna (Ganges) River system and adjacent coastal waters. Hilsa collected from marine, estuarine, and freshwater habitats were born in all main rivers in Bangladesh. All fish moved widely, entering water of marine Sr-87/Sr-86 isotope ratio by 1 year of age. Most returned to fresh water after they reached sexual maturity, but not necessarily to their natal region. Comparison of the Sr-87/Sr-86 isotope and Sr/Ca ratios of hilsa showed that in the Meghna River, Sr-87/Sr-86 isotope ratios can only distinguish fish from waters when the salinity was less than 5%. Sr/Ca ratios were more useful for interpreting fish movements at higher salinities. To assess whether whole-otolith Sr-87/Sr-86 isotope ratios reflected the ratios of the water of natal origin, we compared the Sr-87/Sr-86 ratios of otolith cores with the mean Sr-87/Sr-86 isotope ratio of the whole transect. We found major differences between the two measurements, suggesting that Sr-87/Sr-86 isotope ratios from whole-otolith assays may not accurately reflect the natal origin of the fish.