Effect of fishing on community structure of demersal fish assemblages

Seasonal experimental trawl surveys (fall 1991-winter 1993) carried out in the Thracian Sea and Thermaikos Gulf (N. E. Mediterranean, Greece). In this area, fishing pressure is very high, since approximately 50% of the Greek otter trawl fleet operates here, producing more than 57% of the total demersal landings. From a total of 285 bottom trawls sampled at depths between 16-420 m, 157 fish species were caught. Indices of diversity, richness, evenness, dominance and ABC plots were used to assess spatial structure, seasonal changes and diversity of the demersal fish assemblages. In general, species diversity, richness and evenness decreased with water depth, with the highest values at depths < 100 m. Dominance increased with depth, getting its maximum at depths >200 m. The effect of depth on the diversity patterns observed was always significant, while no seasonal trends were detected. Commercially important species were dominant in the shallowest zone, while non-commercial species predominated at depths below 200 m. At intermediate depths (30-200 in) almost 50% of the total catches comprised of non-commercially important fish species. The abundance/biomass comparison method proved a useful tool for assessing the impact of stresses on fish populations since it revealed moderate disturbance on fish communities at those depths where fishing pressure is the highest.