General veterinary approaches in aquaculture parallel those for other production animals, such as large animals and poultry. When working with fish, disease prevention is always more rewarding than treatment. Once fish are sick, accurately identifying all present problems can be difficult, and treatment must be administered early in the course of an epizootic to be effective.
A comprehensive program of fish health management should be based on the principles of good husbandry and biosecurity. Good husbandry practices, including water quality, systems management, and nutrition, facilitate health and prevent disease. Consequences of poor husbandry are magnified in production aquaculture because of the greater size and density of populations in a production setting than in a hobbyist or aquarium display system. Population health is the focus (although valuable broodstock may warrant individual care), with preventive medicine of critical importance. Populations can vary from several individuals to millions, with many facilities averaging thousands to hundreds of thousands within any given defined population.
Biosecurity underlies critical fish health management practices for disease prevention and management (see SRAC Fact Sheets 4707, 4708, and 4712 [https://srac.tamu.edu/index.cfm/viewAllSheets/]). Good biosecurity will minimize the risk of introduction and spread of an infectious disease into and within a facility, as well as minimize the risk that diseased animals or infectious agents will leave the facility and spread to other sites. Major biosecurity goals include good animal management, obtaining and maintaining healthy stocks through good sourcing and husbandry, pathogen management, and people management through education and awareness for staff and visitors. More information is provided in these two biosecurity videos, produced in collaboration with the USDA North Central Regional Aquaculture Center, the National Aquaculture Association, and the U.S. Aquaculture Society: 1) What You Need to Know About Biosecurity and 2) What You Need To Know About Biosecurity, Part II. See also Biosecurity.
Eggs and animals should be from a reputable supplier, and animals should be screened for species-specific health parameters and diseases of concern. Previous testing and other relevant historical and husbandry information should be obtained for any lots of fish entering a facility. New fish should undergo quarantine to prevent spread of disease. Water quality, other environmental conditions (such as choice of holding units, density, water flow, lighting, and sound), and nutrition should be optimized for the facility’s goals and species.
Water source is a major risk factor for disease introduction, with protected water sources (ie, sources that do not have fish or other potential pathogen reservoirs) having low to zero risk of carrying diseases of concern. Examples of protected water sources include those from deep wells or disinfected municipal supplies. Protected water sources are preferable to unprotected sources, such as surface waters from a lake or river. Risks from unprotected sources can be mitigated (eg, through filtration, UV sterilization, chlorination, and/or ozonation) to make them safer.
Efforts to maintain as clean an environment as possible, including minimal accumulation of organic debris, proper disinfection of nets and equipment, and thorough disinfection of fish-holding units between groups of fish, help minimize disease outbreaks by optimizing water quality and reducing pathogen loads and reservoirs (see Cleaning and Disinfection below). Familiarity with common diseases for a given species, as well as potential disease reservoirs and relevant diagnostics and control are important. Some pathogens, such as the bacteria Aeromonas hydrophila and the ciliated protist Trichodina, are considered ubiquitous and opportunistic, whereas others may result in significant morbidity and mortality (eg, Streptococcus spp or Amyloodinium), and/or have regulatory consequences (eg, infectious salmon anemia or spring viremia of carp, a foreign animal disease in the USA). Quarantine of new fish at temperatures permissive for significant diseases will facilitate determination of presence of specific pathogens. Isolation of diseased populations, use of equipment dedicated to a specific group, and system and equipment sanitation and disinfection are important methods of pathogen management.
Important components of a good biosecurity program include owner and staff education, awareness, and commitment to principles of biosecurity. Written protocols and rules for workers and facility visitors help solidify understanding and accountability. Increasingly, facilities are making the effort to develop written biosecurity, health management, and disaster management plans. Interested veterinarians should be familiar with aquaculture systems and be able to evaluate these protocols, as well as staff and visitor adherence to them through general biosecurity audits.
Proper cleaning and disinfection are critical components of a good fish health management program. There are a number of good references for cleaning and disinfection in aquaculture settings (see SRAC Fact Sheet 4707: Biosecurity in Aquaculture, Part 1, Tables 1 and 2 https://srac.tamu.edu/index.cfm/getFactSheet/whichfactsheet/235/], and Guide to Using Drugs, Biologics, and Other Chemicals in Aquaculture, Table 5 [https://www.fws.gov/fisheries/AADAP/PDF/GUIDE_June_2014b.pdf]).
Cleaning and disinfection of tanks and equipment starts with general cleaning and removal of dirt and organic debris. Organic debris, including biofilms, and dirt can shield pathogens from disinfection. Liquid household bleach (3%–6% NaHClO) delivered at 35 mL/gal. of water creates a concentration of 200 mg/L. Granular bleach (HTH, calcium hypochlorite) is more stable than household bleach and is often used on commercial farms. It is available in three different concentrations: 15%, 50%, or 65% available chlorine. The target dose of chlorine for disinfection is 200 mg/L for at least 1 hr. This concentration can be achieved by mixing 1.4 g of the 15% product/L of water, 0.4 g/L of the 50% product, or 0.32 g/L of the 65% product. These products are strong oxidizers, and precautions listed on the label or relevant Material Safety Data Sheet should be followed when handling them. A 1-hr contact time at 200 mg/L will destroy most organisms of concern, including most viruses. Bleach should not be used in closed areas containing live fish, because the volatile compound may get into the solution and kill fish in nearby tanks. Mycobacteria are refractory to bleach disinfection because of their waxy cell wall. Spraying equipment and contact surfaces with alcohol after treatment with bleach should effectively eliminate mycobacteria, although access to some surfaces (eg, PVC pipework) and associated biofilms may be difficult. In these instances, it is understood that major reduction in pathogen numbers is the key goal. Another commonly used aquaculture disinfectant, Virkon® Aquatic (active ingredient 21.4% potassium peroxymonosulfate and 1.5% sodium chloride), is highly effective at 1%–2% concentrations and considered much less toxic to fish (manufacturer directions must be followed). Quaternary ammonium compounds such as Roccal® and benzalkonium chloride are also excellent disinfectants and can be used at concentrations of 500 mg/L for 1 hr, but residual films should be rinsed off completely. Both Virkon Aquatic and quaternary ammonium compounds are more suitable for net dips and net disinfection.