Making the most of treatments

Optimizing fish husbandry practices and minimizing stress, instituting biosecurity protocols, and actively monitoring the health status of one’s livestock, enable fish farmers and their veterinarians to significantly improve the health and productivity of their fish. As the adage goes, “an ounce of prevention is worth a pound of cure.” 

Despite one’s best efforts however, disease outbreaks could still occur. In case of bacterial, fungal, and parasitic pathogens, treatment with drugs or pesticides can often be utilized to manage, if not treat, the disease outbreak. Most treatments relevant to aquaculture can be broadly divided into two main categories: systemic (usually in-feed) treatments and immersion or bath (surface) treatments. Although less common, particularly with food-fish, injectable treatments, which also act systemically, are sometimes used in certain situations. 

Antimicrobial resistance (AMR), wherein pests and pathogens become decreasingly responsive to certain treatments, has been identified as a significant threat to both human and animal health.  AMR highlights the importance of prudent and judicious use of antimicrobial products. Particularly in an industry like aquaculture, where we have so few antimicrobial tools and products to choose from, it is even more important to protect the tools we have and to ensure their continued efficacy. 

Although any diagnoses and treatments of your livestock should be conducted and overseen by a licensed veterinarian with whom you have an established relationship, there are several principles which will help ensure your treatments are successful, both today and in the years to come. 

In the case of in-feed treatments, provide your veterinarian with as much accurate information as possible regarding the population of fish to be treated. This information includes the number of fish, their average size (and biomass), density, mortality numbers and classifications, the past and current daily feeding rate, and environmental parameters such as water temperature, dissolved oxygen levels, presence of plankton, etc. Once an appropriate medication has been determined by your veterinarian, it is also important for them to know whether you will be mixing the medication into the feed at your facility or if this will be performed at the feed mill. 

Once you have a treatment plan in place and you have prepared your feed, it is critical that you ensure your fish achieve the prescribed minimum daily feeding rate so that they receive an adequate amount of medication; in general, it is better to underestimate than overestimate the daily feed rate for this reason (although the more accurate, the better). Furthermore, during a medicated feed treatment (typically around 10 days for most antibiotics, though this may vary), it is critical that you feed medicated feed exclusively – no topping up with non-medicated feed because feeding them even just a portion of unmedicated feed can significantly impact treatment success. If your fish have been consuming more than expected, just give them additional medicated feed. 

Another very important point is that you should do everything in your power to prevent any interruptions in a treatment once it has started. Part way through your 10-day treatment is not the time to take your fish off-feed for several days to grade them. These sorts of interruptions decrease treatment efficacy and further increase the risk of AMR developing. There will be situations where environmental conditions such as transient low dissolved oxygen levels or plankton blooms may preclude you from hitting the daily minimum feed rate on certain days of a treatment. While in many cases this is out of your control, it emphasizes the importance of planning and preparing for a treatment, including delaying the start of a treatment until environmental parameters become more favourable where possible. In these situations, we must do the best we can, and oftentimes a treatment can be successfully “salvaged” with the help of your veterinarian, but it is far from ideal.  

Regarding bath treatments, it is just as important to provide your veterinarian with as much information regarding your fish as possible. Information on density and environmental conditions, especially water temperature and dissolved oxygen, are critical. Understanding the details of an aquaculture system, including flow-rates, the ability to shut off flow for a certain period of time, and the downstream destination of water (e.g. is treated water re-used, does it flow directly to another rearing tank, or is it discharged?). 

The frequency and timing of bath treatments for certain pathogens, particularly parasites, will vary depending on the water temperature, as temperature directly influences the timing of the parasite’s life cycle. Certain bath treatment products have upper and/or lower temperature limits, wherein those treatments can become toxic or dangerous past certain temperature thresholds. Oxygen levels are also critical because certain treatments will decrease dissolved oxygen, while others will increase oxygen levels. 

Regardless of the treatment modality, it is critical to actively communicate with your veterinarian before and throughout a treatment. It is of little help to report a list of challenges and issues with the treatment after the fact; by doing so in real-time, steps or changes may be made to address issues that have arisen. By responsibly utilizing the few treatment options we have, we will help prolong their continued efficacious use.  

Dr Mykolas Kamaitis is a private practice aquaculture veterinarian and owner of Belwood Lake Veterinary Services in Ontario, Canada. He previously worked in the salmon industry in British Columbia as a vet and Fish Health Manager with Mowi Canada West. During his time at Mowi, Mykolas developed a strong background in production medicine and continues to take a preventative and production-focused approach to fish health management. He was named president of the Association of Aquaculture Veterinarians of British Columbia (AAVBC) in 2018 and continues in the role to this day.