Molluscan diversity caught by trawling fisheries: a case study in southern Portugal M. A. E. MALAQUIAS Department of Zoology, Natural History Museum, London, UK L. BENTES, K. ERZINI & T. C. BORGES Centro de Cie ˆncias do Mar do Algarve (CCMAR), Faculdade de Cieˆncias do Mar e do Ambiente, Universidade do Algarve, Campus de Gambelas, Faro, Portugal Abstract 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. KEYWORDS: by-catch, conservation, diversity, fisheries, molluscs. Introduction Discarding unwanted species occurs in most fisheries around the world, and is one of the major biological and political issues facing modern fisheries. Some or all of this by-catch may be discarded at sea, and includes fish, invertebrates, marine mammals, reptiles and birds. On average 27 million tonnes of fish are discarded each year in commercial fisheries (Alverson, Freeberg, Murawski & Pope 1994), and according to FAO (2001), 8.3% of the total world fishery landings are molluscs. Fishing has many impacts other than those on target species. Bottom trawls crush benthic invertebrates in the path of the net and lead to the mortality of invertebrates that are caught in the net; they are also discarded because they have little or no commercial value (Jennings & Kaiser 1998; Lindeboom & de Groot 1998; Hall 1999; Kaiser & de Groot 2000). Certain functional groups, such as large filter-feeding bivalves, are more vulnerable to trawling disturbance than others. Larger bivalves in the path of a beam trawl typically suffer mortality of 20% or more (Lindeboom & de Groot 1998; Bergman & Van Santbrink 2000). As a result, species like the bivalve Arctica islandica (L.) have almost disappeared in heavily trawled areas (Rumohr & Krost 1991; Craeymeersch, Piet, Rijns- dorp & Buijs 2000). Chemello, Scotti & Milazzo (2000) and Scotti & Chemello (2000) listed both Mediterra- nean and world molluscs that are endangered as a result of fisheries exploitation. In recent years, considerable attention has been devoted to the study of discards in the south of Portugal (Borges, Bentes, Castro, Costa, Erzini, Correspondence: Manuel Malaquias, Department of Zoology, Natural History Museum, Cromwell Road SW7 5BD, London, UK (e-mail: manm@nhm.ac.uk) Fisheries Management and Ecology, 2006, 13, 39–45 Ó 2006 Blackwell Publishing Ltd 39