Fish community structure and depth-related trends on the continental slope of the Balearic Islands (Algerian basin, western Mediterranean)

A total of 13 026 fishes belonging to 82 species and 43 families were collected in a continuous transect between depths of 200 and 1800 m south of the Balearic Islands (Algerian basin, western Mediterranean). The analysis of 32 bottom trawls showed the existence of 4 groups associated with the upper slope (groups 1 and 2, from 200 to 400 and 400 to 800 m, respectively), middle slope (group 3, from 800 to 1400 m) and lower slope (group 4, below a depth of 1400 m). The differences in the mean values of the ecological parameters species richness, abundance, biomass and mean fish weight were also indicative of distinctive characteristics between these fish assemblages. Species richness decreased significantly with depth. The highest values of diversity corresponded to the samples from group 2. Biomass did not show any specific trend throughout the whole bathymetric range. Mean fish weight show 2 different trends along the continental slope: a bigger-deeper phenomenon at the upper 1000 to 1200 m depth, and a smaller-deeper phenomenon below this depth. Our results are compared with those obtained in the north Atlantic basin and in the western Mediterranean (Balearic basin), and the main factors affecting these deep-sea fish assemblages are discussed.