Feeding ecology of the sand smelt, Atherina boyeri (Risso, 1810) (Osteichthyes, Atherinidae), in the western Mediterranean: evidence for spatial variability based on stable carbon and nitrogen isotopes

The feeding ecology of the sand smelt Atherina boyeri Risso 1810 (Osteichthyes, Atherinidae) was investigated in a Mediterranean coastal basin (Stagnone di Marsala, Italy) by means of carbon and nitrogen stable isotope ratios. Sampling was carried out seasonally in 1999 in two locations characterised by different depth and vegetal coverage. Throughout the year the sand smelt showed enriched d13C and d15N values in both sampling locations (d13C about )12& and d15N about 12&). This result suggests the overall importance of the benthic pathway in the food web leading to A. boyeri. Our isotopic picture is only in partial agreement with stomach content data from the literature. Stomach contents highlighted that, while A. boyeri is benthivore in the location with high vegetal coverage, it is able to shift its diet toward zooplankton in the location with higher depth and lower complexity. In our opinion, this discrepancy can be a consequence of the different temporal scale which stomach contents and stable isotopes are referred to. Contrarily to stomach contents, stable isotope analysis provides time-integrated information on the real diet averaged over some months or more. We suggest that long-term and short-term diets of the sand smelt differ. Zooplankton, due to its low biomass in the study site seems to be exploited scarcely by A. boyeri and its trophic role is detectable only considering the short term diet with stomach contents, while the timeintegrated diet of A. boyeri seems to be based on benthic prey.