DEPTH-SIZE TRENDS IN WESTERN MEDITERRANEAN DEMERSAL DEEP-SEA FISHES

We analyze the bathymetric trends of demersal deep-sea fishes in the Catalan Sea (NW Mediterranean) between 1000 and 2250 m depth. In the first part of the study, only the pattern shown by the whole fauna ('within-fauna pattern') is considered. At 1200 m there is a significant decrease in fish mean weight, as a consequence of the replacement of the dominant large or medium-sized species by small ones. This faunal 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-established tendency. None of the lower slope species and only 2 of the 5 middle slope species analyzed behave as bigger-deeper species. Moreover, some species which display a bigger-deeper trend in North Atlantic waters do not show any significant trend in the Mediterranean. Considering the recent Atlantic origin of Mediterranean ichthyofauna and the contrasting behaviour in the 2 areas, the hypothesis that mean size increase with depth is the result of inherited ontogenic behaviour can be discarded.