Epidemiological surveillance of bacterial pathogens in cultured European sea bass (Dicentrarchus labrax) along the Aegean coast of Türkiye

Intensive aquaculture of European sea bass (Dicentrarchus labrax) along the Aegean coast of Türkiye faces continuous production challenges from infectious diseases, yet comprehensive multi-year baseline data remain limited. This study presents a multi-year retrospective surveillance program (2020-2025) monitoring bacterial pathogens associated with 748 distinct cage-level disease outbreaks in fish across 19 discrete offshore commercial sea farms. To maintain industry confidentiality, diagnostic data from these separate facilities were fully aggregated and evaluated across three broad macro-regional surveillance zones. A total of 5,235 fish displaying clinical signs underwent diagnostic necropsy. Presumptive bacterial identification was conducted using phenotypic profiling through the API 20E system, and organ touch-imprint cytology. Six distinct bacterial species were isolated from positive clinical individuals, recovering a total of 2,490 isolates. Across the recorded outbreak events, etiological agents were ranked as primary drivers as follows: Photobacterium damselae subsp. piscicida (48.80%; N = 365), Tenacibaculum maritimum (35.83%; N = 268), Vibrio (Listonella) anguillarum (9.49%; N = 71), V. harveyi (3.74%; N = 28), V. ordalii (1.20%; N = 9), and V. alginolyticus (0.94%; N = 7). This hierarchy directly mirrored the overall bacterial isolation frequency, where P. damselae subsp. piscicida showed the highest relative distribution rate (46.18%; n = 1,150), followed by T. maritimum (34.10%; n = 849). Significantly lower isolation rates were recorded for V. (Listonella) anguillarum (11.41%; n = 284), V. harveyi (5.26%; n = 131), V. ordalii (1.77%; n = 44), and V. alginolyticus (1.29%; n = 32). Descriptive tracking of the purified laboratory isolates revealed localized variations in diagnostic recovery frequencies among the three sectors, while establishing a highly stable baseline dominated by P. damselae subsp. piscicida and T. maritimum across the Eastern Aegean Turkish mariculture network.