Bacterial Isolates Associated with Mortality Events in Brown Trout (Salmo trutta) Restocking Farms in Spain: A Descriptive Field Study

This study aimed to identify bacterial isolates associated with mortality events in <i>Salmo trutta</i> rearing farms in Spain and to assess their antibiotic resistance profiles. The analysis covered five fish farms: two with a recent history of antibiotic use and three without any antibiotic application in the six months prior to sampling. Tissue samples were collected from moribund fish displaying clinical signs such as erratic swimming, ocular hemorrhages, fin hemorrhages, and skin lesions during disease outbreaks in 2022 and 2023. The samples were analyzed using real-time PCR, amplification and sequencing of the 16S rRNA gene and the ITS-1 intergenic spacer, and MALDI-TOF mass spectrometry. A total of 19 bacterial isolates were identified, with Gram-negative bacteria, particularly <i>Aeromonas</i> spp., being the most prevalent. Other identified taxa included <i>Plesiomonas</i> sp., <i>Hafnia alvei</i>, <i>Pseudomonas fulva</i>, and <i>Kluyvera intermedia</i>, as well as Gram-positive species such as <i>Carnobacterium maltaromaticum</i>, <i>Lactococcus</i> sp., and <i>Enterococcus faecium</i>. Notably, resistant strains were found in four of the five farms, even in those that had not administered antibiotics, suggesting that environmental contamination and anthropogenic factors may significantly contribute to the spread of resistance. Environmental stressors-such as sudden increases in water temperature and high turbidity caused by suspended organic matter-appeared to precede mortality peaks. The findings highlight the role of <i>Aeromonas</i> spp. as a key bacteria associated with mortality events in <i>S. trutta</i> and underscore the multifactorial nature of antibiotic resistance in aquaculture. No florfenicol-resistant isolates were detected in the farms where it is routinely used, indicating that florfenicol remains an effective antibiotic in aquaculture. However, the continuous and systematic monitoring of its use remains essential. The detection of bacteria not traditionally associated with fish pathology in samples from diseased animals suggests the need for further studies into their pathogenic potential. Overall, this descriptive study emphasizes the importance of preventive health strategies, prudent antibiotic use, and environmental monitoring to mitigate bacterial diseases and limit the spread of antimicrobial resistance in brown trout farming. These findings align with a One Health perspective, linking aquaculture practices, ecosystem integrity, and public health.