Photobacteriosis in farmed Atlantic bluefin tuna (Thunnus thynnus): The first comprehensive study of the Aegean Sea

Photobacterium damselae subsp. damselae is a significant pathogen that cause severe bacterial septicemia in a broad range of hosts. This study presents the first comprehensive analysis of Photobacterium damselae subsp. damselae affecting farmed Atlantic bluefin tuna (Thunnus thynnus) stocks in Aegean Sea, Türkiye. The total mortality rate of the farm was calculated as 18% exhibiting clinical signs of the disease, characterised by lethargic swimming and loss of equilibrium in bluefin tuna cages. Although external clinical signs were not determined, hemorrhages were observed in liver, swim bladder and heart. Microbiological examinations resulted in the isolation of a total of four bacterial strains, including two isolates from the summer sampling period and two isolates from the autumn sampling period. Among these, representative strains designated as THAS1 and THAS2 were selected for further biochemical and molecular characterization, through which Photobacterium damselae subsp. damselae was identified. Histopathological examinations of liver, heart, spleen, and kidney tissues associated with the infection revealed the presence of melanomacrophage centers in histological sections of the spleen and kidney. Consequently, this study reveals the epizootic and impact of P. d. subsp. damselae infection in farmed Atlantic bluefin tuna with clinical and microbiological results associated with pathological changes. Further research on the epidemiology and epizootiology of this disease is essential for sustainable aquaculture of tuna fish stocks, especially in Mediterranean.