Fish in recirculation systems are quite susceptible to disease as explained at the beginning of the last chapter (See Ch.19). However, recirculation systems also have some inherent advantages for managing fish health over traditional aquaculture. One is the modular nature of recirculation units and another is the relative ease of disinfection. Large recirculation operations are almost always made of up of multiple independent units. This has advantages for general management as will be explained later, but also helps a lot in maintaining healthy fish. The fact that fish are separated makes it easy to quarantine new arrivals. All fish brought in from outside sources should be suspected of carrying disease that could spread to other fish, even if the source has supplied healthy fish in the past. There is no such thing as "certified disease free" fish. Sometimes eggs and fish are available that have tested negative for certain specific pathogens, but that does not rule out other diseases. So, as soon as fish are brought into the facility they should be reared separately for a few weeks to see if anything crops up. Even after fish pass initial quarantine, it is wise not to mix them with other fish unless there is a good reason for doing so. As fish in separate units are raised, care should be taken not to allow any water to be transferred from one system to another. This means keeping separate nets, graders, etc. for each system or at least thoroughly disinfecting such items before they are used in a different tank. This is inconvenient, but when disease occurs (and it will), it is good to be able to confine it to a single unit. Another advantage of recirculation systems, is that they are indoors and generally constructed of non-porous materials such as PVC and fiberglass. This makes it possible to completely disinfect a system. It is impossible to disinfect a pond and very difficult to completely disinfect outdoor raceways. In most facilities, it is standard operating procedure to disinfect a unit after a harvest cycle, before re-stocking. The most common disinfectant is free chlorine because it is cheap and effective. Items to be disinfected should be soaked for several hours in 200 ppt chlorine. Chlorine can be supplied as household bleach or in a more concentrated powdered form, sodium hypochlorite, which can obtained from swimming pool suppliers. Free chlorine is quite corrosive, of course, especially to uncoated ferrous metals.
Even when the precautions above are carefully followed disease outbreaks will occasionally occur. As with all animals, fish diseases may be categorized into parasites, bacterial diseases, and viral diseases. The first step in potential treatment is to correctly identify the disease. While veterinarians with competency in fish disease are more common than they were 20 years ago, they are still hard to find in most localities, so the aquaculturist often has difficulty finding assistance with disease outbreaks. Many fish diseases have common indications, so we speak of general "signs" of disease, not distinctive "symptoms". While a parasite can sometimes be positively identified by viewing them with either with the naked eye or a microscope, most bacterial and viral diseases require the service of a diagnostic lab. Labs that handle fish diseases exist, but are not common. Local veterinarians, the Agricultural Extension Service and the state veterinary college are places to look for help in diagnosing a fish disease problem. It would be prudent to find such help before the disease crisis arises and time is of the essence.
If a disease outbreak occurs in a single recirculation unit, the cheapest and easiest solution is often to accept the loss, euthanize the fish, disinfect, and start over. If economics demand that treatment be attempted, the treatment should be based on knowledge, not best guesses. To effectively treat, the aquaculturist must know the disease, be aware of special requirements of the fish, realize any possible consequences of the water quality, and finally, understand the chemical. Most products of recirculation aquaculture are intended for human consumption and that limits what can be used to treat to those chemicals approved by the FDA for use in food fish. That is a very short list, because, except for substances generally regarded as safe, such as salt, the FDA requires extensive testing before approval. Testing is expensive and manufacturers are reluctant to spend the money when the potential aquaculture market is so small when compared to other farm animals. Growers of ornamental products, like koi and tropicals have a wider variety of veterinary chemicals to choose from. A limitation of using many treatments intended for external conditions, such as formalin, in recirculation systems is that they may also kill the biofilter. Thus the fish might be saved from the pathogen, only to die of ammonia toxicity! Fish to be treated with external treatments should, in most cases, be removed from the recirculation unit and placed in a treatment tank. If the fish are not fed, water quality can be maintained with aeration and small amount of clean water flow through to flush out ammonia, though flow through will require re-dosing for long term treatments.
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