الفهرس 
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Abstract
European sea bass (Dicentrarchus labrax) and gilthead sea bream (Sparus aurata) are marine species of great economic importance and these species are vulnerable to seasonal serious mortality episodes causing severe economic losses, particularly in Mediterranean aquaculture. Seasonal mortality episodes targeting both seabream and seabass cultured in semi-intensive earthen pond and intensive concrete pond, Damietta province, Egypt was conducted during 2016 and 2017. During summer mortality episode, fifty freshly dead and moribund sea bream fish were collected while thirty freshly dead and moribund sea bass were collected during winter mortalities. Clinically, the collected fish revealed hemorrhages around the mouth, operculum, at the base of pectoral and anal fins, inflammation of anal opening, tail fin erosion and ulceration at caudal peduncle area. Morpho-chemical tests were performed to describe the phenotypic picture and biochemical properties of pure colonies using API®20NE and API®20E system. Definitive confirmation of bacterial isolates was performed by PCR targeting 16SrRNA, collagenase, toxR, aerolysin and 16SrDNA genes revealed 39 isolates (Vibrio species), 31 (Vibrio alginolyticus), 8 (Vibrio parahemolyticus), 8 (Aeromonas hydrophila) and 5 (Pseudomonas fluorescens), respectively. Vibrio alginolytcus was the most prevalent bacterial pathogen for sea bass and sea bream 56.7% and 28%, respectively followed by Vibrio parahemolyticus 18% in sea bream then Aeromonas hydrophila, and Pseudomonas fluorescens 26.7% and 16.7% in sea bass. Antibiogram of isolates showed sensitivity to oxytetracycline, doxycycline, flumequine, and trimethoprim-sulfamethoxazole. sharp increase in ammonia and nitrite, decreased dissolved oxygen and ability to scavenge iron from iron-binding proteins are the most possible triggering factors for initiation, establishment and spread of infections. Infectivity test of isolates revealed that the LD50 of Vibrio alginolyticus (VAS7) and Vibrio parahemolyticus (VP3) for sea bream was 3 × 105 CFU/g fish and 1 × 106 CFU/g fish, respectively while the LD50 of Vibrio alginolyticus (VAK16), Aeromonas hydrophila (AHK6) and Pseudomonas fluorescens (PFK12) for sea bass was 5 × 104 CFU/g fish, 7.5 × 105 CFU/g fish and 1.7 × 106 CFU/g fish. Experimental immunization of sea bass by formalized polyvalent bacterin applied by different routes for control of some bacterial diseases showed RPS% of 88.9%, 50%, 33.3% and 25% for intraperitoneal, prolonged immersion, dip immersion and oral routes, respectively.