Dietary success of a ‘new’ key fish in an overfished ecosystem: evidence from fatty acid and stable isotope signatures

The bearded goby Sufflogobius bibarbatus has become a key component of the pelagic food web off Namibia following the crash in pelagic fish populations during the 1970s, and its biomass is increasing despite significant predation pressure and apparent life-history constraints. The integrated feeding of the bearded goby was studied from samples collected during April 2008, using stable isotope ratios (δ13C, δ15N, δ34S) and fatty acids, to resolve conflict amongst previous dietary studies based on gut-content analysis and to understand how diet could influence its success within the region. Isotopes of carbon and nitrogen suggest that the now abundant jellyfish could contribute up to 74% of the diet, and δ34S signatures indicate that the diatom- and bacteria-rich sulphidic sediments on the central shelf may contribute around 15% to the diet. Fatty acid analyses provided support for sulphur bacterial and jellyfish-feeding amongst gobies, and further suggest that small gobies fed more on zooplankton while large gobies fed more on sedimented diatoms. Both data sets suggest that ontogenetic changes in diet were linked to changes in habitat: pelagic when small, more ­demersal when large. The study highlights the value of using multiple tracers in trophic studies and indicates that the dietary flexibility of the bearded goby, in conjunction with its behaviour and ­physiology, likely contributes to its success within the northern Benguela ecosystem.