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¹ Ege University, Fisheries Faculty, Dept. of Fishing and Processing Technology, 35100 Bornova, İzmir, Turkey DOI : 10.4194/trjfas.2011.0307 Viewed : 4065 - Downloaded : 3198 In this study, the variation of Caulerpa racemosa density depending on different depths and its effects on the diversity of some fish species in catch composition were investigated. The quadrate method was used by divers to determine the variation of density of C. racemosa depending on depth and its possible effects on fish species diversity was determined by visual census method and combined trammel nets. The study was conducted during the periods September 2005 and March 2007 from the Eastern Aegean coast. As a result of sampling dives, density of C. racemosa was found as 135.4, 146.5, 191.3, 385.7 and 635.1 g/m² at the depth ranges of 0-5, 6-10, 11-15, 16-20 and 21-25 m, respectively. 14 fishing operations were performed to sample fish species by using combined trammel nets. 293 individuals belonging to 13 species from the area covered by C. racemosa and 322 individuals belonging to 16 species from the control area were caught. Any variation was not found in terms of fish number sampled from those areas (P>0.01). 24 individuals belonging to 5 species and 32 individuals belonging to 5 species were censused from the area covered by C. racemosa and the control site, respectively. The differences between the number of fish sampled from those sites was found insignificant (P>0.01). Consequently this study contains the first results about invasion density of the invasive alga species seen in Turkish coasts and its effects of fish composition. It is very important to monitor this species in the invasion area and to observe its effects on other species for fishing management and the ecosystem. Keywords : Caulerpa racemosa, quadrate, combined trammel net, fish species diversity, visual census technique