Cross-validation of δ15N and FishBase estimates of fish trophic position in a Mediterranean lagoon: The importance of the isotopic baseline

FishBase, a relational database freely available on the Internet, is to date widely used as a source of quantitative information on the trophic position of marine fish species. Here, we compared FishBase estimates for an assemblage of 30 fish species sampled in a Mediterranean lagoon (Acquatina lagoon, SE Italy) with their trophic positions calculated using nitrogen stable isotopes. To assess the influence of the trophic level used to compute the baseline indicator on the robustness of isotopic estimations, we compared the trophic position of fish calculated using the average delta N-15 signature of either basal resource or primary consumer taxa measured at three stations located in the lagoon in July and November 2007. In general, basal resources showed negligible among-station and inter-season variations in their delta N-15 values; however, they were characterized by a high inter-specific heterogeneity, with signatures varying by approximately 10 per mil units. In contrast, whereas primary consumer signatures showed significant spatial and temporal variations, they were characterized by a lower inter-specific variability. Fish trophic positions estimated using primary consumers as the isotopic baseline were highly correlated with values provided by FishBase, independently from whether the latter were calculated on either diet data or individual prey items. Conversely, estimations using a basal resources as the baseline indicator were significantly less correlated with FishBase estimates. The present study emphasized the crucial importance played by inter-specific variability in baseline taxa signatures for a robust assessment of fish trophic position, and confirmed primary consumers as the best candidate for baseline estimation. In addition, our results indicate that, notwithstanding the limitations characterizing the data provided in FishBase, they represent an adequate source of information on the trophic ecology of fish