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A total of 13 026 fishes belonging to 82 species and 43 famllies were collected in a continuous transect between depths of 200 and 1800 m south of the Baleanc 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, respectivelv), middle slope (group 3, from 800 to 1400 m) and lower slope (group 4, below a depth of 1400 m) Thc differcnces 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 factol-s affecting these deep-sea fish assemblages are discussed.

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We analyze the bathymetric trends of demersal deep-sea flshes in the Catalan Sea ( N W Medlterranean) between 1000 and 2250 m depth In the first part of the study, only the pattern shown by the whole fauna ( wthin-fauna pattern ) 1s considered At 1200 m there is a significant decrease In fish mean welght, as a consequence of the replacement of the dominant large or medium-sized specles by small ones This fauna1 shift may be a consequence of the lower trophic availability at greater depths, as shown by an important decrease in megafaunal biomass between 1000 and 2250 m In the second part, the study focuses on the depth-size trends of the most common species ( within-species pattern ) Results show that the bigger-deeper phenomenon is not a well-estabhshed tendency None of the lower slope species and only 2 of the 5 middle slope species analyzed behave as bigger-deeper species Moreover some specles which display a bigger-deeper trend in North Atlantic waters do not show any significant trend In the Medlterranean Considering the recent Atlantic origin of Mediterranean ichthyofauna and the contrasting behaviour in the 2 areas, the hypothesis that mean slze increase with depth is the result of inhented ontogenic behaviour can be discarded