Effects of commercial trawl fishing in the Strait of Sicily on the diversity of demersal resouces

The effects produced by commercial trawl fishing on the diversity of demersal resources in two zones of the Straits of Sicily subject to different fishing effort were analysed. Data were available from a total of 79 hauls, carried out annually in autumn 1997 and 1998 between 200 and 400 meter depth and pertained to the bathyal muddy bottoms biocenosis (Perès and Picard, 1964) on the whole. Data on fishing effort, recorded since 1995, come from the Harbour Office and have been verified from interviews to crews and captains of Mazara del Vallo trawl fisheries. Excluding pelagic and rare (appearing in <= 5% of hauls) species from the analysis, diversity indices, cluster analysis, K-dominance and size spectra were investigated. The cluster analysis (employing the Bray-Curtis similarity matrix among species) shows the presence of two distinct groups of hauls corresponding to the areas with different fishing effort. High values of the main diversity indexes seem to characterize the hauls subject to a more intensive fishing effort. K-dominance curves for the two areas show that, in "Zone 1" the main part of the catch is concentrated on commercial species (Merluccius merluccius, Parapenaeus longirostris), while in "Zone 2" the catch is mainly characterized by massive species, Elasmobranchs and Crustaceans with low or null commercial value.