Spatial distribution of recruits and adults of Octopus vulgaris in GSAs 15 and 16 (central Mediterranean Sea)

The focus of this study is the population spatial structure of the common octopus, Octopus vulgaris (Cuvier, 1797), in the GSAs 16 and 15 of Strait of Sicily (central Mediterranean). Specifically, the depth distribution of the species and the geographic position of nursery and spawning grounds were investigated. Abundance indices by life-phases (recruits and spawners) were obtained from two series of scientific trawl surveys carried out in the Strait of Sicily in the period 1994-2007 within the framework of international and Italian national programmes of experimental trawl surveys. Distribution maps of density indices were generated for each life stage and year by Inverse Distance Weighted (IDW) deterministic interpolation. Specifically, 14 distribution maps of recruit density indices were produced for the Autumn season (main recruitment period), whereas nine density maps of mature individuals were obtained for the spring/early summer season (peak of reproduction). Moreover 4 years of data were available to produce distribution maps of recruit density indices in the African platform of GSAs 13 and 14. Nursery and spawning areas were identified, by means of GIS techniques, applying criteria based on the persistent presence of high values of recruits and adult females respectively, throughout the period of study. A stable spawning ground was identified in the north-west sector of GSA16. Several nurseries were located along the south coast of Sicily and north of Malta. An important stable nursery was found in the south-east area of GSA 13.