The main objective is to asses the herbivorous activities (Paracentrotuslividus) over prairie of Posidonia oceanic and in places with high-low pressure for sea hurcin. The project aim to empty the „loading capacity“ of the posidonia prairie and human influence.In order to asses the effect over the „ sea hurcin source“ and the consequences for the Posidonia Oceanic (which is eaten by the sea hurcin), it will be made detentions in an area where fishing is forbidden and in another one where it is allowed for all. The two zones will be distant in order to avoid general influences (sea conditions, flows, bottom conditions etc.).The two areas are Marina Protetta in the Parco Nazionale of La Maddalena and a proximal area to it.