1. INTRODUCTION Reproductive protocols of European eel reproduction require numerous periodic handling procedures for relatively long time (even 4 months), during which fish are exposed to many skin damages which may result in extensive bacterial infections and in consequence lead to death. Thus, antibiotics were very often used to prevent high mortality of the spawners. However, up to date there is no data regarding the bacterial community responsible for such infections. The aim of this study was to evaluate bacterial diversity in European eel (Anguilla anguilla) spawners kept in freshwater and artificial seawater under controlled conditions. MATERIAL AND METHODS Bacteria were isolated from skin mucus, exudation fluids, peritoneal tissue and caudal fin. Bacteria were identified by classical methods (selective media, microscoping, API testing, drug resistance testing) and 16S rDNA sequencing. RESULTS AND DISCUSSION Greater bacterial diversity was recorded in freshwater than in seawater. Also, the presence of different pathological changes on fish skin for freshwater and seawater was found (Fig. 1). Analysis of 16S rDNA for identified bacteria isolated from eels kept in freshwater and seawater revealed the presence of 20 and 5 main genus of bacteria, respectively (Tab. 1). INTRODUCTION Reproductive protocols of European eel reproduction require numerous periodic handling procedures for relatively long time (even 4 months), during which fish are exposed to many skin damages which may result in extensive bacterial infections and in consequence lead to death. Thus, antibiotics were very often used to prevent high mortality of the spawners. However, up to date there is no data regarding the bacterial community responsible for such infections. The aim of this study was to evaluate bacterial diversity in European eel (Anguilla anguilla) spawners kept in freshwater and artificial seawater under controlled conditions. MATERIAL AND METHODS Bacteria were isolated from skin mucus, exudation fluids, peritoneal tissue and caudal fin. Bacteria were identified by classical methods (selective media, microscoping, API testing, drug resistance testing) and 16S rDNA sequencing. RESULTS AND DISCUSSION Greater bacterial diversity was recorded in freshwater than in seawater. Also, the presence of different pathological changes on fish skin for freshwater and seawater was found (Fig. 1). Analysis of 16S rDNA for identified bacteria isolated from eels kept in freshwater and seawater revealed the presence of 20 and 5 main genus of bacteria, respectively (Tab. 1). System Genera Freshwater Pseudomonas sp., Comamonas sp., Burkholderia sp. Acidovorax sp. Bacillus sp. Morganella sp., Providencia sp., Enterobacter sp., Pantoea sp., Yersinia sp., Citrobacter sp., Edwardsiella sp., Escherichia sp., Pectobacterium sp., Hafnia sp., Serratia sp., Methylobacterium sp., Chromobacterium sp., Bordetella sp., Edwardsiella sp. Seawater Pseudomonas sp., Aeromonas sp., Yersinia sp., Vibrio sp., Edwardsiella sp. Results indicate almost fourfold higher number of bacteria in European eels kept in freshwater than in seawater. It suggests that more attention to aseptic maintenance in all stages of handling and keeping the fish in captivity should be provided, especially in freshwater. However, further analyses are needed, including microbiology of fish stock, biofilm, filter beds, interaction between bacteria. Fig. 1. Ulcerated and milky changes with plentiful exudation fluids in freshwater (left) and bloody changes of skin in saltwater (right) at the bottom of muzzle. Conclusions The obtained results indicate, for the first time, particular bacteria responsible for infections of European eel, both in fresh and seawater. It may allow to choose the most effective antibiotics for veterinary treatment and prophylaxis and in consequence improve the effectiveness of European eel reproduction. Conclusions The obtained results indicate, for the first time, particular bacteria responsible for infections of European eel, both in fresh and seawater. It may allow to choose the most effective antibiotics for veterinary treatment and prophylaxis and in consequence improve the effectiveness of European eel reproduction. Tab. 1. Diversity of bacteria isolated from hatchery of Anguilla anguilla L. www.eng.InnovaFish.pl AKNOWLEDGMENTS This study was co-financed by the project “Innovations in finfish aquaculture with special references to reproduction” (acronym: InnovaFish), Operational Programme Sustainable Development of the Fisheries Sector and Coastal Fishing Areas 2007-2013" (OR14-61724-OR1400003/09/10/11). Presented at AQUA 2012 – Prague, 01-05 OCTOBER 2012