Antibiotics in Salmonid Aquaculture: Environmental Considerations for Marine Cage Farming

Antibiotics are typically used in aquaculture operations as a direct response to infectious diseases to ensure the health and well-being of farmed stock. However, there are several environmental issues associated with antibiotic usage that farmers need to understand and address when developing disease management strategies. Some of these can have direct negative implications for farming operations while others relate to effects on the broader environment. Negative outcomes for farming operations include the potential for the development of resistance in the target pathogens. However, the main concerns associated with antibiotic usage relate to effects on non-target organisms, environmental persistence and development of broader resistance. In salmonid aquaculture, antibiotics are generally administered in feed and as such these concerns are primarily related to the presence of waste feed and fish excreta in the sediments and water column.

Ensuring good farm management and husbandry practices to maximise the health of fish stocks is the best way to reduce the need for antibiotics and as a result any associated impacts. Rapid and accurate diagnosis of pathogens and development of targeted disease management strategies and alternative therapies, in particular vaccines, is also an important means to reduce and manage overall antibiotic usage. However, where antibiotics are necessary then careful feed management, monitoring feed input and limiting feed wastage, will minimise any impacts. In addition an understanding of the various environmental issues associated with specific antibiotics allows farm managers and veterinarians to make informed choices with respect to treatment regimes. Finally, establishing an effective monitoring approach for evaluation of potential ecological effects is a key factor in assessing the impact of antibiotics. Development of resistance and accumulation in the sediments are significant environmental concerns, consequently it is important to monitor the incidence of resistance in the environment and in fish bacteria. Accumulation in the sediments may affect natural sedimentary processes, such as biogeochemistry, and it would be prudent to confIrm this, and to determine threshold effect levels. Even where assessment of the available data does not suggest that major environmental changes have occurred, it is still important to identify and monitor suitable indicator species to ensure ongoing sustainability. In this chapter we review various antibiotics used in salmonid aquaculture globally, provide a summary of key environmental issues and compare various monitoring and assessment approaches.