Fish health and disease

Jul 6, 2020 · Food and Agriculture Organization of the United Nations

The most important way to maintain healthy fish in any aquaculture system is to monitor and observe them daily, noting their behaviour and physical appearance. Typically, this is done before, during and after feeding. Maintaining good water quality, including all of the parameters discussed above, makes the fish more resistant to parasites and disease by allowing the fishes’ natural immune system to fight off infections. This section discusses briefly key aspects of fish heath, including practical methods to identify unhealthy fish and prevent fish disease. These key aspects are:

Observe fish behaviour and appearance on a daily basis, noting any changes.
Understand the signs and symptoms of stress, disease and parasites.
Maintain a low-stress environment, with good and consistent water quality, specific to the species.
Use recommended stocking density and feeding rates.
Fish health and well-being

The main indicator of fish well-being is their behaviour. In order to maintain healthy fish, it is important to recognize the behaviour of healthy fish as well as the signs of stress, disease and parasites. The best time to observe fish is during their daily feeding, both before and after adding the feed, and noting how much feed is eaten. Healthy fish exhibit the following behaviour:

Fins are extended, tails are straight.
Swimming in normal, graceful patterns. No lethargy. However, catfish often sleep on the bottom until they wake up and begin feeding.
Strong appetite and not shying away at the presence of the feeder.
No marks along the body. No discoloured blotches, streaks or lines.
No rubbing or scraping on the sides of the tank.
No breathing air from the surface.
Clear sharp shiny eyes.
Stress

Stress has been mentioned several times throughout this publication and deserves special attention here. Generally, stress is a physiological response of the fish when they live in less than optimal conditions. Overstocking, incorrect temperatures or pH, low DO and inappropriate feeding all cause stress (Table 7.2). The fishes’ bodies have to work harder to overcome these poor conditions, resulting in a depressed immune system. With a depressed immune system, the ability of the fish to heal and ward

TABLE 7.2

Causes and symptoms of stress in fish

| Causes of stress | Symptoms of stress | |

– |

off disease is reduced. Stress can actually be measured in fish by monitoring certain hormones. Stress is an overall state of being, and stress alone does not kill the fish. However, if fish are stressed for an extended period, they will inevitably develop diseases from various bacteria, fungi and/or parasites. Avoid stress wherever possible, and realize that multiple factors can contribute to stress at the same time.

Fish disease

Disease is always the result of an imbalance between the fish, the pathogen/causative agent and the environment. Weakness in the animal and a higher incidence of the pathogen in certain conditions cause disease. Sound fish management practices that build a healthy defence system are the primary actions to secure a healthy stock. Therefore, adequate environmental control is equally essential in order to avoid stress in fish and to reduce the incidence of pathogens.

Diseases are caused from both abiotic and biotic factors. In previous chapters, water quality parameters have already been indicated as determinant factors to avoid metabolic disorders and mortality. In addition, control of climatic conditions as well as contaminants can offset many opportunistic infections and toxicity. The contained characteristics of recirculating systems make aquaponics less prone to pathogen introductions and disease outbreaks because of better control of inputs and in the management of key water and environmental parameters. In the case of incoming water from water bodies, the simple adoption of slow sand filtration can protect the aquaponic system from any possible parasite or bacteria introduction. Similarly, the elimination of snails and small crustaceans, as well as preventing the access or the contamination from animals and birds, can help offset the problems of parasites as well as possible bacterial contamination.

The three major groups of pathogens that cause fish disease are fungus, bacteria and parasites. All of these pathogens can easily enter an aquaculture system from the environment, when adding new fish or new water, or could have previously existed in the unit. Prevention is by far the best way to prevent disease in fish. Daily observation of fish and monitoring for disease allows the disease, if present, to be treated quickly to prevent more fish from being infected (Figure 7.14). Treatment options for small-scale aquaponics are limited. Prevent disease as much as possible.

Preventing disease

The list below outlines some key actions for preventing disease and summarizes major lessons for growing fish in aquaponics:

Obtain healthy fish seed from a reliable, reputable and professional hatchery.
Never add unhealthy fish to the system. Examine new fish for signs of disease.
It is advisable in some cases to quarantine new fish in an isolation tank for 45 days before adding them to the main system.
If possible and necessary treat new fish with a salt bath (described below) to remove parasites or treat some early stage infections.
Ensure that the water source is from a reliable origin and use some sterilization method if it comes from a well or water bodies. Remove chlorine from water if it is from a municipal source.
Maintain key water quality parameters at optimum levels at all times.
Avoid sharp changes in pH, ammonia, DO and temperature.
Ensure adequate biological filtration to prevent ammonia or nitrite accumulation.
Ensure adequate aeration to keep DO levels as high as possible.
Feed the fish a balanced and nutritious diet.
Keep the fish feed in a cool dry and dark place to prevent it from moulding.
Make sure that live food sources are pathogen-free and parasite-free. Feed that is not from a verifiable origin should be pasteurized or sterilized.
Remove uneaten feed and any source of organic pollution from the tank.
Make sure the fish tank is shaded from direct sunlight, but not in complete darkness.
Prevent access of birds, snails, amphibians and rodents that can be vectors of pathogens or parasites.
Do not allow pets or any domestic animals to access the production area.
Follow standard hygiene procedures by washing hands, cleaning/sterilizing gear.
Do not allow visitors to touch the water or handle fish without following proper hygiene procedures.
Use one fish net for each fish tank to prevent cross-contamination of diseases or parasites.
Avoid loud noise, flickering lights or vibration near the fish tank.

Recognizing disease

Diseases may occur even with all of the prevention techniques listed above. It is important to stay vigilant and monitor and observe fish behaviour daily to recognize the diseases early. The following lists outline common physical and behavioural symptoms of diseases. For a more detailed list of symptoms and more specific remedies please refer to Appendix 3.

External signs of disease:

ulcers on body surface, discoloured patches, white or black spots
ragged fins, exposed fin rays
gill and fin necrosis and decay
abnormal body configuration, twisted spine, deformed Kaws
extended abdomen, swollen appearance
cotton-like lesions on the body
swollen, popped-out eyes (exophthalmia)

Behavioural signs of disease:

poor appetite, changes in feeding habits
lethargy, different swimming patterns, listlessness
odd position in water, head or tail down, difficulty maintaining buoyancy
fish gasping at the surface
fish rubbing or scraping against objects

Abiotic diseases

Most of the mortalities in aquaponics are not caused by pathogens, but rather by abiotic causes mainly related to water quality or toxicity. Nevertheless, such agents can induce opportunistic infections that can easily occur in unhealthy or stressed fish. The identification of these causes can also help the aquaponic farmer to distinguish between metabolic and pathogenic diseases and lead to prompt identification of the causes and remedies. Appendix 3 contains a list of the most common abiotic diseases and their symptoms.

Biotic diseases

In general, aquaponics and recirculating systems are less affected than pond or cage aquaculture farming by pathogens. In most cases, pathogens are actually already present in the system, but disease does not occur because the fishes’ immune system is resisting infection and the environment is unfavourable for the pathogen to thrive. Healthy management, stress avoidance and quality control of water are thus necessary to minimize any disease incidence. Whenever disease occurs, it is important to isolate or eliminate the infected fish from the rest of the stock and implement strategies to prevent any transmission risk to the rest of the stock. If any cure is put into action, it is fundamental that the fish be treated in a quarantine tank, and that any products used are not introduced into the aquaponic system. This is in order to avoid any unpredictable consequences to the beneficial bacteria. Appendix 3 indicates some of the most common biotic diseases occurring in fish farming and the remedies normally adopted. More details are available from the literature and from local fishery extension services.

Treating disease

If a significant percentage of fish are showing signs of disease, it is likely that the environmental conditions are causing stress. In these cases, check levels of ammonia, nitrite, nitrate, pH and temperature, and respond accordingly. If only a few fish are affected, it is important to remove the infected fish immediately in order to prevent any spread of the disease to other fish. Once removed, inspect the fish carefully and attempt to determine the specific disease/cause. Use this publication as a starting guide and then refer to outside literature. However, it may be necessary to have a professional diagnosis carried out by a veterinarian, extension agent or other aquaculture expert. Knowing the specific disease helps to determine the treatment options. Place the affected fish in a separate tank, sometimes called a quarantine or hospital tank, for further observation. Kill and dispose of the fish, as appropriate.

Disease treatment options in small-scale aquaponics are limited. Commercial drugs can be expensive and/or difficult to procure. Moreover, antibacterial and antiparasite treatments have detrimental effects on the rest of the system, including the biofilter and plants. If treatment is absolutely necessary, it should be done in a hospital tank only; antibacterial chemicals should never be added to an aquaponic unit. One effective treatment options against some of the most common bacterial and parasite infections is a salt bath.

Salt bath treatment

Fish affected with some ectoparasites, moulds and bacterial gill contamination can benefit from salt bath treatment. Infected fish can be removed from the main fish tank and placed into a salt bath. This salt bath is toxic to the pathogens, but non-fatal to the fish. The salt concentration for the bath should be 1 kg of salt per 100 litres of water. Affected fish should be placed in this salty solution for 20-30 minutes, and then moved to a second isolation tank containing 1-2 g of salt per litre of water for another 5-7 days.

In bad white-spot infections, all fish may need to be removed from the main aquaponic system and treated this way for at least a week. During this time, any emerging parasites in the aquaponic unit will fail to find a host and eventually die. The heating of the water in the aquaponic system can also shorten the parasite life cycle and make the salt treatment more effective**.** Do not use any of the salt bath water when moving the fish back into the aquaponic system because the salt concentrations would negatively affect the cultured plants.

Source: Food and Agriculture Organization of the United Nations, 2014, Christopher Somerville, Moti Cohen, Edoardo Pantanella, Austin Stankus and Alessandro Lovatelli, Small-scale aquaponic food production, http://www.fao.org/3/a-i4021e.pdf. Reproduced with permission.

Fish health and well-being

Stress