Resource partitioning among early colonizing Siganus luridus and native herbivorous fish in the Mediterranean:: an integrated study based on gut-content analysis and stable isotope signatures

The present study attempts to give timely information on the resource partitioning between the lessepsian migrant Siganus luridus and two ecological native analogues, Sarpa salpa and Sparisoma cretense. Sampling was carried out in concomitance with the first record of Siganus luridus in Linosa and fish were caught simultaneously, allowing direct comparison of diets. Gut-contents analyses provided a snapshot of the feeding habits of the three species: Siganus luridus fed on 34 taxa of benthic algae (mostly represented by Dictyota dichotoma) and a total of 27 taxa was identified in the stomach of Sarpa salpa, with the predominance of Sagassum vulgare. For the first time, a detailed picture of Sparisoma cretense diet was given (N=22 taxa of identified algae) albeit, due to the high percentage of digested food, this species was excluded from gut-content comparisons. A certain resource Partitioning between Siganus luridus and Sarpa salpa was resolved on the basis of trophic indices and multivariate analyses, these latter also highlighting a more dispersed diet for Siganus luridus with respect to Sarpa salpa. Isotopic signatures were important towards defining the trophic level of the three species and particularly of Sparisoma cretense, whose gut-contents analysis was only partially informative. The values of 615 delta N-15 confirmed a strictly vegetal diet for Siganus luridus and Sparisoma cretense while Sarpa salpa was significantly more enriched. According to delta C-13. observed values matched the predicted ones for S Siganus luridus and Sarpa salpa while both species presented less enriched delta N-15 values than expected.