FISH SPECIES COMPOSITION AND ABUNDANCE IN THE ESTUARIES OF THE TRANSBOUNDARY RIVER STRYMON E. Gousia 1 , E. T. Koutrakis 2 , D. C. Bobori 1 1 Aristotle University of Thessaloniki, School of Biology, Department of Zoology, Laboratory of Ichthyology, UPBox 134, 541 24, Thessaloniki, Greece. 2 Fisheries Research Institute-NAGREF, Nea Peramos, GR-640 07, Kavala, Greece Ε-mail: evgousia@bio.auth.gr Abstract In the period autumn 2007 – spring 2008 fish sampling was realised at three sampling stations in the estuarine system of the transboundary river Strymon, using a bag seine net (2 mm mesh size knot to knot). A total of 5066 specimens, weighting 4745.98 g were caught. The specimens belonged to 19 species, 5 from which were caught only in Amphipoli lagoon. Atherina boyeri dominated the catches in terms of number (40.91%) and weight (76%). Differences in the catch species composition were found between the two sampling periods. Introduction Strymon is a transboundary river and its basin is sheared by FYROM (9.5 %), Serbia (4%), Bulgaria (50 %) and Greece 36.5 %). It originates from Bulgaria and outflows to Strymonikos Gulf (northern Greece) (Fig. 1) after flowing of about 360 km, 122 of which are within the territory of Greece. The river is embanked to its lower part, and close to its mouth it forms the small lagoon of Amphipolis. Both the mouth of the river and the lagoon constitutes important nursery grounds for fish and they are also very significant for fisheries and tourism. The aim of the present study was to record the composition and abundance of the fish species that occur in the estuarine system of the transboundary river Strymon. Materials and methods During October 2007 (autumn) and April 2008 (spring) three stations (station 1 located in the area where the Strymon river outflows to the Strymonikos Gulf, station 2 in Amphipoli’s fish-wharf and station 3 in Amphipoli’s lagoon, Fig. 1) were sampled using a bag seine net (2 mm mesh size knot to knot) of 10 m length and 1 m height. At each period, sampling was contacted at the same time of the day, at depths of about 0.5- 1 m. Each haul covered an area of about 600 m 2 . Fish samples were preserved in 6% formaldehyde solution and transferred to the laboratory, where they were identified to the species level, with the exception of Sparidae species that were identified to the genus level (Diplodus sp.), and measurements of total length (TL, cm±0.1) and total weight (W, g±0.1) were recorded. Results and discussion A total of 5066 specimens, representing 19 species and 10 families were collected during the sampling period (Table 1). Seven of them (36.84%) (Carassius gibelio, Chondrostoma vardarense, Lepomis gibbosus, Rhodeus amarus, Rutilus rutilus, Squalius cephalus, Vimba melanops) were freshwater, while the rest 12 species (63.16%) were estuarine and near- shore marine species. Of all species caught only five species (Atherina boyeri, Liza saliens, Pomatoschistus marmoratus, Solea solea and Syngnathus abaster were presented in Amphipoli lagoon (station 3). The most abundant species, in terms of number in the total catches was Atherina Fig. 1. Map of the studied area, showing the three sampling stations (1: station 1- Strymon river mouth, 2: station 2- Amphipoli’s fish-wharf, 3: station 3- in Amphipoli’s lagoon) (from Koutrakis et al., 2004 modified, http://earth.google.com/15/6/2008)