Food web reconstruction gives evidence of increased trophic levels in no-trawl areas: the red mullet, Mullus barbatus L. case in northern Sicily

This study was aimed at identifying the food web of the red mullet, Mullus barbatus in order to understand how it is affected by trawling disturbance. To achieve this objective: a) the main features of the red mullet habitat were investigated; b) the food web of this habitat was studied in two no-trawl areas and in two areas open to trawling. The working hypothesis is that trawling affects the biochemistry of the sediment and the trophic structure of the benthic assemblage. It was predicted: a) less biomass, smaller size and higher production rate in the benthic assemblages of protected gulfs; b) higher average trophic level for both the red mullet and its predators in protected gulfs; c) a diet shift driven by the mechanical disturbance of trawling. The results achieved confirmed our hypotheses and allowed us to characterize for the first time the trophic web structure in the red mullet habitat in the study area. Results also allowed us to compare the trophic level of red mullet of different sizes using two independent techniques, stable isotopes of nitrogen and gut contents. The average trophic level of the red mullet was higher and the energy consumption per unit of biomass larger in the protected gulfs. Species with a trophic level higher than the red mullet and therefore its potential predators were few and the most important were the white grouper, Epinephelus aeneus, the pandora, Pagellus erythrinus and the common torpedo, Torpedo torpedo.