| Summary: | This study presents the occurrence and abundance of <i>Aeromonas</i> antibiotic-resistant bacteria (ARB) and genes (ARGs) isolated from water, biofilm and fish in two commercial trout farms before and one week after flumequine treatment. Wild (WT) and non-wild (NWT) strains were determined for quinolones (flumequine, oxolinic acid and enrofloxacin), oxytetracycline (OXY), florfenicol (FFN), trimethoprim-sulfamethoxazole (TMP) and colistin (COL), and pMAR (presumptive multi-resistant) strains were classified. Forty-four ARGs for the mentioned antibiotics, β-lactams and multi-resistance were quantified for 211 isolates. <i>Bla</i>SHV-01, <i>mex</i>F and <i>tet</i>E were the dominant ARGs. A greater occurrence and abundance of <i>tet</i>A2, <i>sul</i>3, <i>floR</i>1, <i>blaSHV</i>-01 and <i>mex</i>F were observed for NWT compared to WT. The occurrence of pMAR and NWT <i>Aeromonas</i> for quinolones, OXY, FFN, TMP, COL and ARGs depended on the <i>Aeromonas</i> origin, antibiotic use and the presence of upstream activities. Our results revealed the impact of a flumequine treatment on <i>Aeromonas</i> present on a fish farm through an increase in NWT and pMAR strains. The link between fish and their environment was shown by the detection of identical ARB and ARGs in the two types of samples. There appears to be a high risk of resistance genes developing and spreading in aquatic environments.
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