Linking aquaculture practices to antibiotic occurrence, accumulation, and environmental risk in freshwater systems

• 23 antibiotics screened in freshwater aquaculture effluents, sediments, and biota. • Enrofloxacin, doxycycline, and oxytetracycline dominated across all sample matrices. • Levels of antibiotics in sediments and molluscs suggest high risk to the environment (RQ > 11-9000). • Juvenile salmonid farms were identified as key antibiotic pollution hotspots. Aquaculture is a rapidly expanding component of global food production, but its intensification raises environmental concerns due to the widespread use of antibiotics to control bacterial diseases. This study examined the occurrence, distribution, and ecological risks of antibiotic residues in freshwater ecosystems downstream of aquaculture facilities in Latvia. Twenty-three antibiotics were analyzed in outflows, sediments, and biota using UHPLC–MS/MS. Four compounds—enrofloxacin, doxycycline, oxytetracycline, and florfenicol—were detected, with enrofloxacin and doxycycline showing the highest detection frequencies and concentrations in the outflows (up to 1315 ng L⁻¹ and 775 ng L⁻¹, respectively). Substantial accumulation occurred in sediments (up to 13,811 µg kg⁻¹ for enrofloxacin) and molluscs (up to 97 µg kg⁻¹ for doxycycline), suggesting persistence and potential bioaccumulation. Risk quotient (RQ) analysis indicated high environmental risk (RQ ≥ 1) in 10% of outflows, mainly associated with enrofloxacin. Extremely high RQs were observed in sediments (11–9086) and molluscs (43–200) for all detected antibiotics. Antibiotic levels and occurrence patterns varied significantly among aquaculture types, with juvenile salmonid farms identified as key sources of pollution. These findings highlight freshwater aquaculture as an under-recognized source of antibiotic contamination in the Baltic region and underscore the need for enhanced monitoring, transparent reporting, and stricter regulations to mitigate antimicrobial pollution and resistance development in aquatic environments.