Deep-sea fish assemblages in the Catalan Sea (western Mediterranean) below a depth of 1000 m

The analysis of 48 bottom trawl collections from the Catalan Sea below a depth of 1000 m has allowed the identification of two distinct demersal fish assemblages, occurring on the middle and lower slope. The Middle Slope Assemblage (MSA) and Lower Slope Assemblage (LSA), restricted to the middle (1000–1425 m) and lower (1425–2250 m) slope, respectively, each consist of two groups with distinct structures. While one group is relatively homogeneous, the other can be considered as comprising a faunal transition. Each assemblage is dominated by species possessing very different characteristics. The preponderance of middle and large size fish with high energy requirements in the MSA is replaced, in the LSA, by smaller and more passive species with lower energy requirements. These differences are reflected in a series of ecological parameters such as abundance, biomass, mean fish weight, species richness and mean species richness. A pronounced decrease in the value of these parameters is generally seen with increasing depth, reflecting the increasingly oligotrophic conditions. Furthermore, the highly stable environmental conditions of the western Mediterranean (where water temperature and salinity rarely change below a depth of 150 m), suggest that the observed zonations may be determined by biological factors. It is possible to relate the observed faunistic changes with the vertical distribution of mesopelagic organisms. These organisms play a fundamental role in the feeding ecology of demersal fish species of the slope, either directly as prey or indirectly by enriching the sediments, and thus determining the distribution of sedimentivorous invertebrate species.