Guide to Recirculation Aquaculture: Chapter 7

Chapter 7: Disease

There are many examples of recirculation systems operating without any disease problems at all. In fact, it is possible to isolate a recirculation fish farm completely from unwanted fish pathogens. Most important is to make sure that eggs or fish stocked in the facility are absolutely disease free and preferably from a certified disease free strain. Make sure that the water used is disease free or sterilised before going into the system; it is far better to use water from a borehole, a well, or a similar source than to use water coming directly from the sea, river or lake. Also, make sure that no one entering the farm is bringing in any diseases, whether they are visitors or staff.

Whenever possible, a thorough disinfection of the system should be carried out. This includes any new facility ready for the very first start-up as well as for any existing system that has been emptied of fish and is ready for a new production cycle. It should be remembered, that a disease in one tank of a recirculation system will most certainly spread to all the other tanks in the system, which is why preventive measures are so important.

 

Separation of different production stages can minimise risks

In recirculation systems using eggs from wild fish, for example for the purpose of re-stocking, getting eggs from certified disease free strains is not possible. In such cases, there will always be a risk of introducing diseases living inside the egg, such as IPN (Infectious Pancreas Necrosis), BKD (Bacterial Kidney Disease) and possibly herpes virus, which cannot be eliminated by disinfecting the eggs. An example of a prevention scheme is shown in Figure 7.2.

A good way to prevent contamination with pathogens within the system is to physically separate the different stages in the production. The hatchery should therefore work as an isolated closed system, as should the fry unit and the grow-out unit. If any brood stock is kept, this should also be isolated in a unit of its own. This way, stamping out a disease becomes easier to carry out in practice.

Some farms have been constructed after the “all in all out” principle, meaning that each unit is emptied completely and disinfected before new eggs or fish are stocked. For eggs and smaller fish, which are grown over a shorter period of time before they are moved on, this is certainly good management, and should always be carried out in practice. For larger fish this is also good practice, however this kind of management easily becomes inefficient. Taking all the fish out of a grow-unit before stocking a new batch, is logistically difficult when dealing with large volumes of fish. It easily becomes uneconomical, because of inefficient utilization of the capacity of the system.

 

Efficacy of medication depends on many factors

Treating fish diseases in a recirculation system is different from treating them on a traditional fish farm. On a traditional fish farm, the water is used only once before leaving the farm. In a recirculation system, the use of biofilters and the constant recycling of water calls for a different approach. Pouring in medication will affect the whole system including fish and biofilters, and great care must be taken when treatment is carried out. It is very difficult to give exact prescriptions on the dose needed to cure a disease in a recirculation system, because the effect of the medication depends on many different parameters such as hardness of water, content of organic matter, water temperature and flow rates. A great deal of practical experience is therefore the only way forward. Concentrations must be increased carefully from each treatment to the next to avoid killing the fish or the biofilter. Always remember the term “better safe than sorry”. In any case of a disease outbreak, a local veterinarian or fish pathologist must prescribe the medication and explain how to use it. Also, the safety instructions should be read carefully as some drugs may cause severe injuries to people if used improperly.

Foot bath with 2% iodine solution for preventing the spread of disease.

Treatment against ecto-parasites, which are parasites sitting on the outside of the fish on the skin and in the gills, can be carried out by adding chemicals to the water. Any fungal infections will have to be treated in the same way as infestations with ecto-parasites. In freshwater systems the use of ordinary salt (NaCl) is an efficient way of killing most parasites including bacterial gill disease. If a cure with salt does not work, the use of formalin (HCHO) or hydrogen peroxide (H2O2) will usually be sufficient to cure any remaining parasitic infections. Bathing fish in a solution of praziquantel and flubendazol have also proven to be very efficient against ecto-parasites.

Mechanical filtration has also proven to be quite efficient against the spreading of ecto-parasites. Using a filter cloth of 70 micron will remove certain stages of Gyrodactylus, and a 40 micron cloth can remove different kinds of parasite eggs.

The safest way of carrying out a treatment is to dip the fish in a bath with a solution of the chemical. However, in practice this is not a feasible method as the volume of fish that needs to be handled is often too large. Instead fish are kept in the tank as the inlet water is switched off, and oxygenation or aeration of the tank is carried out by the use of diffusers. A solution of the chemical is added to the tank and the fish are allowed to swim in the mixture for a period of time. Later, the inlet water is opened, and the mixture slowly diluted as the water in the tank is exchanged. The water running out from the tank will be diluted by the rest of the recirculation system so that the concentration in the biofilter will be significantly lower than in the tank treated. This way a relatively high concentration of the chemical can be obtained in an individual tank with the purpose of killing the parasite, yet lowering the effect of the chemical on the biofilter system. Both fish and biofilters can adapt to treatment with salt, formalin and hydrogen peroxide by slowly increasing the concentrations from one treatment to the next. When a tank full of fish has been treated, this water can also be pumped out of the system to a separate compartment for degradation instead of being recirculated in the system.

Using the dipping technique for eggs is however a feasible way of treating millions of individuals in a short time, for example when disinfecting eggs in iodine. This method can also be used for treating eggs that have been infected with fungus (Saprolegnia) simply by dipping the eggs into a solution of salt (7 ‰) for 20 minutes.

 

Treating at the hatchery level is easier

In hatcheries, where fish are removed as soon as they are ready to feed, the efficiency of the biofilter is less important as the level of ammonia excreted from eggs and fry is very little. Treatment is therefore easier to carry out, because one only has to focus on the survival of eggs and fish. Also, it is worth noting that the total volume of water in a hatchery is small, and a complete water exchange with new water can be carried out rapidly. Therefore, a successful treatment in a hatchery by treating the whole system in one go, can be done safely.

Dissection of rainbow trout suffering from inflated swim bladder. A symptom probably due to super saturation of gases in the water.

Treatment of a complete system in larger recirculation facilities is a more sensitive operation. The basic rule is to keep concentrations low, and to carry out the treatment over a longer period of time. This requires care and experience. The concentration should be slowly increased from each treatment to the next, leaving several days in between without treatment in order to carefully monitor the effects on fish mortality, behaviour and water quality. Typically, an adaptation will take place for both fish and biofilter, so the concentration can be increased with no adverse effects and the probability of killing the parasite is enhanced. Salt is excellent for longer treatment periods, but formalin too has been successfully used for intervals of 4-6 hours. The biofilter simply adapts to the formalin and digests the substance like any other carbon coming from the organic compounds in the system.

As pointed out previously, it is not possible to give exact concentrations and recommendations on the use of chemicals in a recirculation system. Fish species, size of fish, water temperature, hardness of water, the amount of organic substances, exchange rate of water, adaptation, etc. must all be taken into consideration. The guidelines below are therefore very approximate.

Salt (NaCl): Salt is relatively safe to use, and can be used in fresh water for treating Ich (Ichthyophthirius multifilis or white spot disease) and the common fungus saprolegnia. Ich in the pelagic phase can be killed at 10 o/oo and new results suggests killing of the bottom living stages at 15 o/oo. Fish contains around 8 o/oo salt in their body fluids, and most freshwater fish will tolerate salinities in the water around this level for several weeks. In hatcheries a concentration of 3-5 o/oo will prevent infections with fungus.

Formalin (HCHO): Low concentrations of formalin (15 mg/L) for long periods of time (4-6 hours) have shown good results in the treatment of Ichthyobodo necator (Costia), Trichodina sp., Gyrodactylus sp., sessile ciliates and Ich. Formalin is degraded relatively fast in the biofilter at about 8 mg/h/m2 biofilter area at 15°C. Formalin can however reduce the bacterial nitrogen conversion rates in the biofilter.

Hydrogen peroxide (H2O2): Not widely used, but experiments have shown promising results as a substitute for formalin at concentrations between 8-15 mg/L for 4-6 hours. The biofilter performance can be inhibited for at least 24 hours after treatment, but the efficiency will return to normal within a few days.

Figure 7.2 An example of a prevention scheme.
What to remember: How is it done?
Clean source of new water

Preferably use ground water. Disinfect using UV.

In some cases use sand filter and ozone.

Disinfection of system Fill system with water and bring pH up to 11-12 by the use of sodium hydroxide NaOH.  Approximately  1 kg per m3 water volume depending on buffer capacity
Disinfection of equipment and surfaces Dip or spray with an iodine solution of 1.5% or according to instructions. Leave for 20 minutes before wash off in clean water.
Disinfection of eggs Leave egg batch in solution of 3 dl of iodine per 50 litres of water for 10 minutes.  Change solution for every 50 kg eggs disinfected.
Staff Change clothing and foot wear when entering facility. Wash or disinfect hands.
Visitors Change of foot wear or use footbath for dipping shoes (2 % iodine solution). Wash or disinfect hands. “Do not touch” policy for visitors inside the facility.

 

The impact of treatment on the biofilter must be considered

Use of other chemicals such as copper sulphate or chloramin-t is not recommended. These are very effective for the treatment of for example bacterial gill disease, however the biofilter will most probably suffer severely and the whole recirculation process and the production may be seriously damaged.

For treatment against bacterial infections, such as furunculosis, vibriosis or BKD, the use of antibiotics is the only way to cure the fish. In some cases fish can become infected with parasites living inside the fish, and the way to remove these is also with antibiotics.

Antibiotics are mixed into the fish feed and fed to the fish several times every day over, for example, 7 or 10 days. The concentration of antibiotics must be sufficient to kill the bacteria, and the prescribed concentration of medication and the length of the treatment must be carefully followed, even if the fish stop dying during the treatment. If treatment is stopped before the prescribed treatment period, there is a high risk that the infection will start all over again.

Treatment with antibiotics in a recirculation system will have a small effect on the bacteria in the biofilter. However, the concentration of antibiotics in the water, compared to that inside the fish being treated with medicated feed, is relatively low, and the effect on bacteria in the biofilter will be much lower. In any case, one should carefully monitor the water quality parameters for any changes because they may indicate an effect on the biofilter. Adjustment of the feeding rate, use of more new water or changing the flow of water in the system may be necessary.

Several antibiotics can be used, such as sulfadiazine, trimethoprim or oxolinic acid according to the prescription by the local veterinarian.

Treatment against IPN, VHS (Viral Hemorrhagic Septicemia) or any other virus is not possible. The only way to get rid of viruses is to empty the whole fish farm, disinfect the system and start all over again.