Ecosystem based assessment of the effects of shifting trawl codends in Gulf of Mersin, Eastern Mediterranean

The Poor selectivity and high discard ratios of demersal trawls are of significant concern in the Mediterranean. A large number of studies exist on codend selectivity, most of which focus on mesh size and shape. In consideration of these studies, the General Fisheries Commission for the Mediterranean recommends the use of 40 mm square-mesh (S40) or 50 mm diamond-mesh (D50) codends. However, the effects of improving codend selectivity on the ecology of the ecosystem are not well known. The goal of this study is to evaluate the potential effects of improved codend selectivity on the ecosystem and main target species, Mullus barbatus, Saurida undosquamis, Pagellus erythrinus as case species. This study is based on well-described species-specific catch and selectivity data for commercial codend (44 mm diamond mesh handmade slack knotted net, CD44), 44 mm diamond mesh codend (D44) and alternative codends (D50 and S40) in demersal trawl fisheries from Gulf of Mersin in the eastern Mediterranean. Four Ecopath models, with 48 functional groups and similar topologies were developed to represent the effect of different codends on the ecosystem. Specifically, the impact on M. barbatus S. undosquamis, P. erythrinus were described by splitting the functional group into two components according to minimum landing size (MLS). The consequences of improved trawl selectivity were assessed in the context of ecological framework approved that selective fisheries would benefit for various commercial species and ecosystem. S40 was found as the best alternative for the impacts of trawl fisheries on the ecosystem and various commercial species, particularly red mullet which is the most important commercial species in the region. M. barbatus (MLS, adult) showed the lowest trophic levels among demersal fish groups. For CD44, D44, D50 and S40 codends, the catches were calculated as 0.013 t.km-2.y-1, 0.011 t.km-2.y-1, 0.006 t.km-2.y-1 and 0.003 t.km-2.y-1 for M. barbatus (MLS) respectively. Ecological network analysis highlighted that if trawlers switched from CD44 to D50 or S40 codends, metabolic activity of M. barbatus (