Mapping the distribution of urchin barren grounds using sidescan sonar: the experience of the “Plemmirio” MPA

In the Mediterranean Sea, high densities of the sea urchins Paracentrotus lividus and Arbacia lixula may promote and maintain formation of areas overgrazed by sea urchins, usually referred to as urchin barrens, leading to drastic changes in ecosystem functioning. Urchin barrens are habitats of low primary production and habitat structure, that negatively affect local biodiversity. At Plemmirio MPA, abundance of both sea urchin species increased dramatically after the start of enforcement in 2005 consequently triggering the formation of “barren” seascape, evidence of a desertification process. This study focuses on the results of a high-resolution Side scan sonar (Klein 3900) surveying carried out in the July of 2010 to acquire sonograms along the shallow rocky coasts of the Plemmirio MPA. The track lines were performed by R/V “Antonino Borzì” using a navigation software that showed as background a morpho-bathymetric map previously acquired by a Multibeam Sonar System (Reson SeaBat 8125). Side scan sonar mapping confirmed that urchin barrens were present within the MPA, above all in the B zone. Our results put in evidence that sonar is an excellent tool for mapping and monitoring barren habitats.