Antimicrobial Use in European Aquaculture and the Aquatic Environment: From Regulatory Transformation to Sustainable Disease Management

European aquaculture is increasingly shaped by regulatory efforts to reduce unnecessary antimicrobial use and limit antimicrobial resistance risks across the human–animal–environment interface. These efforts are necessary, especially where antimicrobials are used routinely or as a substitute for husbandry or a remedy for biosecurity failures. However, stewardship in aquaculture cannot be reduced to legal restriction or lower antimicrobial input alone. Fish diseases occur in heterogeneous systems, including recirculating systems, flow-through farms, ponds, hatcheries and marine cages, with different patterns of pathogen entry, treatment delivery and environmental exposure. This review examines antimicrobial use in European aquaculture through the conditions required to make reduction clinically and environmentally workable. Its central argument is that regulatory transformation depends on implementation capacity. Aquatic veterinarians are expected to justify and reduce antimicrobial use while responding to bacterial disease events that may progress rapidly, affect populations and threaten welfare. This decision space becomes difficult where diagnostic access, authorised medicinal products, vaccines, records, surveillance systems or aquatic veterinary expertise are limited. Prevention, biosecurity, diagnostics and surveillance are discussed as decision-enabling infrastructure rather than generic sustainability measures. They reduce preventable disease pressure, help distinguish bacterial from non-bacterial morbidity, support justified treatment and provide feedback on health and environmental outcomes. The aquatic environment is treated not only as a recipient of residues, resistant bacteria and resistance genes but also as a source of risk, exposure and governance information. Responsible antimicrobial reduction is, therefore, not simple minimisation. Sustainable disease management depends on connecting regulatory ambition with veterinary decision-making, therapeutic feasibility, diagnostic support and environmental feedback.