Molluscan diversity caught by trawling fisheries: a case study in southern Portugal

The effects of commercial trawling on the malacological communities (except for the Cephalopoda) were examined, based on a study undertaken between 1996 and 2000 on the continental shelf and slope of southern Portugal. More than 50% of species caught by trawling in southern Portugal were discarded, with molluscs representing about 19%. Forty-four species of molluscs (15 bivalves, 28 gastropods and one polyplacophoran) were identified from the discarded specimens. Crustacean trawlers accounted for 34 molluscs species, and fish trawlers for 24. Twenty species were only caught by the crustacean trawl, compared with 10 species by the fish trawl, and 14 species were common to both trawls. The bivalve Venus nux Gmelin was the most numerous species discarded in the fish trawl, accounting for 42.0% of the total number of individuals, followed by the gastropods Ampulla priamus (Meuschen) (7.8%) and Ranella olearium (L.) (7.3%). In the crustacean trawl, the most numerous species discarded were the bivalve Anadara diluvii (Lamarck) (19.4%), the gastropod Calliostoma granulatum (Born) (15.5%), and the bivalve V. nux (15.1%). The third most discarded species from fish trawls in Algarve waters, the gastropod species R. olearium, is a species listed in Annex II of the Bern Convention. The difficulties of managing the real impact of fisheries on the molluscan populations and in defining a conservation strategy are discussed.