Community structure and diversity of demersal fish assemblages: the role of fishery

Abstract

Seasonal experimental trawl surveys were carried out in the Northern Aegean and Thracian seas (NE Mediterranean, Greece), from summer 1990 to autumn 1993, during which a total of 172 fish species were caught. In these areas, fishing pressure is very high, since approximately 50% of the Greek otter trawl fleet operates there, producing more than 57% of the total demersal landings. Different statistics were used to assess spatial structure, seasonal changes and diversity of the demersal fish assemblages on the continental shelf and upper slope. The following measures were applied to the species abundance matrix: species diversity, species richness, evenness and dominance. The analysis of 501 bottom trawls revealed that, in general, species diversity, richness and evenness decreased with water depth, with the highest values at depths <100 m, whereas dominance increased with depth, with its maximum at depths >200 m. The effect of depth on the diversity patterns observed was always significant, while seasonal trends were similar with those described for the overall diversity characteristics in each area. Classification and ordination methods showed the existence of 4 groups associated with the continental shelf and upper slope in each area. Classification of the top ranking species at each group and area revealed that commercially important species were dominant in the shallowest zone (<30 m), while non-commercial species predominated at depths below 200 m. At intermediate depths (30-200 m) almost 50% of the total catches were composed of non-commercially important fish species. However, since most species had a wide distribution range these differences were rather quantitative than qualitative. The results indicated that although spatial changes in abundance could be detected, temporal changes were not obvious. Environmental variability and overexploitation, as well as differences in species´ life-history strategies, have both influenced the structure of the demersal fish assemblages found along the depth gradient studied.