Feeding ecology and population delineation of south-eastern Atlantic and south-western Indian Ocean humpback whales using stable isotope analysis

Population delineation is vital for effectively managing and protecting populations of all at-risk species. Population boundaries of Southern Hemisphere humpback whales on their breeding and feeding grounds have not been fully resolved. A number of methods have been used to delineate breeding stocks of Southern Hemisphere humpbacks, but ecological characteristics determined via stable isotope analysis provide valuable information to contrast other data sources. In this study, stable isotope analysis is used to investigate potential separation of humpback whale populations on Southern Hemisphere feeding grounds as evidenced by carbon and nitrogen isotope values in their skin as proxies of diet. One hundred samples of whale skin obtained from biopsies in sampling localities off the coasts of Gabon, Mayotte (Mozambique Channel), and Madagascar were analysed for carbon (delta C-13) and nitrogen (delta N-15) stable isotope ratios. The results showed a statistically significant difference in the mean delta N-15 values for whales between the populations from Gabon and Madagascar (7.0 +/- 0.1 parts per thousand and 7.6 +/- 0.1 parts per thousand), and Gabon and Mayotte (7.6 +/- 0.1 parts per thousand and 7.2 +/- 0.1 parts per thousand), indicating that these breeding stocks are potentially visiting different areas of the feeding grounds outside of the breeding season. The results from this study indicate that at least some breeding stocks may show fidelity to separate feeding areas and do not widely mix with individuals from other breeding stocks while feeding.