Health Challenges in Semi-Open Mountain Reservoir Aquaculture: A Case Study in Oncorhynchus mykis

Bacterial diseases remain a significant threat to salmonid aquaculture worldwide, especially in semi-open cage culture systems that are exposed daily to natural aquatic environments. This study investigates recurring bacterial disease events in Rainbow Trout (Oncorhynchus mykiss) raised in a semi-open cage culture facility in the Bicaz mountain reservoir in Romania. Longitudinal monitoring was conducted through clinical assessment of fish health, post-mortem examination of fish that died during recurrent mortality events, bacterial isolation followed by MALDI-TOF MS identification, and determination of antimicrobial susceptibility profiles of the isolated bacteria from 2023 to 2026. These investigations demonstrated the presence of classical pathogenic bacteria found in salmonids as well as a diverse range of opportunistic environmental bacteria. Recurrently identified bacteria included Aeromonas hydrophila, Yersinia ruckeri, Aeromonas salmonicida, Aeromonas veronii, Pseudomonas fluorescens, Hafnia alvei, Enterobacter aerogenes, Providencia alcalifaciens, and Flavobacterium psychrophilum. Mixed bacterial infection resulted in several of the bacterial disease outbreaks, indicating that there are likely to be complex microbial relationships within this semi-open aquaculture environment. Susceptibility profiles of antibiotics tested against the isolated bacteria indicated that some antibiotic classes had varying degrees of susceptibility between different bacterial disease outbreaks and also varied between susceptible and resistant strains of the same bacterial species. Susceptibility profiles were most commonly noted for florfenicol and fluoroquinolone, while beta-lactams, macrolides, and lincosamides displayed variable levels of resistance or diminished susceptibility patterns, particularly among Aeromonas spp. strains. Therefore, the results highlight the dynamic epidemiological characteristics of bacterial infections occurring in semi-open mountain aquaculture systems and underscores the importance of longitudinal bacteriological surveillance under commercial farming conditions. Permanent interaction between cultured fish and the aquatic microbiota may influence the recurrence and microbial complexity of bacterial outbreaks in reservoir-based salmonid aquaculture.