Fish diversity associated with gas platforms: Evaluation of two underwater visual census techniques

Fish assemblages associated with offshore platforms were surveyed using two underwater visual census techniques: striptransects and mobile point counts. Data were collected from four depth ranges at three platforms off Crotone, Italy (Ionian Sea).A total of 25 fish species belonging to 12 families were recorded for the three platforms. Significant differences betweensampling methods and depth were observed in the fish assemblage structure, species richness and fish abundance. The meannumber of species recorded with the strip transect method was always greater than that obtained with mobile point counts, whilstthe opposite pattern was observed for mean abundances. Gregarious species such as Boops boops, Chromis chromis, Anthiasanthias and Oblada melanura were responsible for most of the differences in the assemblage structure. According to our results,both methods allowed us to observe species that are not usually caught by means of fishing gear. When both census techniquesare used, they provide complementary information about the fish assemblages strictly associated with the platform structures.