Characterisation of the discards generated by bottom trawling in the northwestern Mediterranean

This study aims to advance knowledge of the discards from bottom trawling in the northwestern Mediterranean. The composition of bottom trawl landings and discards is analysed from data collected on board commercial trawlers, on a quarterly basis, from July 1995 to June 1996. Trawl discards averaged one third of the total captured biomass, and discards of species with commercial value was low. Altogether 115 species were landed for the market and 309 discarded. Discards varied greatly, not only in terms of weight, but also in the number of species. Within the commercial fraction, 71 were fishes, 20 molluscs and 24 crustaceans. As for the discarded catch, it consisted of 135 species of fishes (73 always discarded), 60 crustaceans (40 always discarded), 44 molluscs (37 always discarded) and 70 other invertebrates (67 always discarded). In the clustering of the hauls, the same pattern was observed for the hauls corresponding to landings and discards; depth was the factor determining the ordination of the hauls. The four groups identified in both landings and discards were significantly different (ANOSIM test, P < 0.05). As for the estimated values of ecological parameters for the four groups, the comparison between groups with the Kruskal-Wallis test (P < 0.01) and the pairwise t-test (P < 0.05) indicated significant differences for all the ecological parameters except in the case of the evenness index in the discard fraction. (C) 2003 Elsevier B.V. All rights reserved.