A comparative study of the species composition of discards from ®ve ®sheries from the Algarve (southern Portugal) K. ERZINI, M. E. COSTA, L. BENTES & T. C. BORGES CCMAR, FCMA, Universidade do Algarve, 8000 Faro, Portugal Abstract Discards from ®ve of the most important ®sheries (crustacean trawling, ®sh trawling, demersal purse seining, pelagic purse seining and trammel netting) in southern Portugal were studied and compared. A total of 236 species of all taxa were discarded, with ®sh and cephalopods accounting for more than 90% of the discarded biomass, except trammel nets (81%). Although there was some overlapping of species, multivariate analysis using cluster analysis for classi®cation and multidimensional scaling (MDS) for ordination, showed that there were signi®cant dierences between the ®ve gears in terms of species composition and biomasses discarded, with the least similarity between crustacean trawling and all other ®shing operations. The dierences between ®sheries were probably as a result of a combination of gear selectivity and depth ®shed. The results suggest that discarding practices are likely to have dierent, yet signi®cant impacts on the marine ecosystem, warranting further studies on the fate of discards, the factors in¯uencing discarding and mitigation. KEYWORDS : by-catch, discards, ®sheries, purse seine, trammel net, trawl. Introduction The incidental capture of species towards which there is no directed eort is characteristic of commercial ®sheries and is termed by-catch (Saila 1983). Some or all of this by-catch may be discarded at sea in the form of whole ®sh or invertebrates, marine mammals, reptiles and birds. It has been estimated that in recent years between 17.9 and 39.5 million tonnes of ®sh per year ± with an average of 27 million tonnes ± were discarded in commercial ®sheries (Alverson, Freeberg, Murawski & Pope 1994). In addition to commercially valuable species, discards comprised non-commercial species as well as rare/endangered species/protected species of sea birds, marine mammals, reptiles and ®sh. Hall (1996) identi®ed seven categories of discards on the basis of biological or ecological signi®cance: critical, unsustainable, sustainable, biologically insigni®cant, by-catches of unknown level, ecosystem-level impacts and charismatic by-catches. The types and quantities of discards in dierent ®shing operations will depend to a large extent on the species and size selectivity characteristics of the gear used. Other relevant factors include target species, the ®shing grounds, the time of the year and the ®shing strategy. By-catch and discarding impacts can be considered at the population, trophic and ecosystem level. At the population level, discarding may have an important eect on the population dynamics and yield of target and non-target species; representing a loss in terms of production and future economic returns (Jensen, Reider & Kovalak 1988). It has been suggested that so many of the ®sh stocks of the world are threatened because of a lack of understanding of all of the dierent components of ®shing mortality, including discard-related mortality (Alverson & Hughes 1995). Community and ecosystem level impacts, particularly the eects of discarding on biodiversity, community structure, trophic interactions and stability are poorly known. There is, however, evidence that the contribution of recycled discards to the energy budget may be signi®cant in some marine ecosystems (Pauly & Christensen 1995), and a number of studies have demonstrated the importance of discards for sea birds (Hudson & Furness 1988; Camphuysen 1994; Camphuysen, Calvo, Durinck, Ensor, Follestad, Furness, Garthe, Leaper, Skov, Tasker & Winter Correspondence: K. Erzini, CCMAR, FCMA, Universidade do Algarve, 8000 Faro, Portugal (e-mail: kerzini@ualg.pt) Fisheries Management and Ecology, 2002, 9, 31±40 Ó 2002 Blackwell Science Ltd 31