FISH HEALTH AND DISEASE
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 lowstress environment, with good and consistent water quality, specific to the species.
• Use recommended stocking density and feeding rates.
• Observe fish behaviour and appearance on a daily basis, noting any changes.
• Understand the signs and symptoms of stress, disease and parasites.
• Maintain a lowstress 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 behavior:
• 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.
• 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.
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 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. 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.
• Deep the fish feed in a cool dry and dark place to prevent it from moulding.
• Make sure that live food sources are pathogenfree and parasitefree. 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 crosscontamination of diseases or parasites.
• Avoid loud noise, flickering lights or vibration near the fish tank.
• 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.
• Deep the fish feed in a cool dry and dark place to prevent it from moulding.
• Make sure that live food sources are pathogenfree and parasitefree. 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 crosscontamination 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.
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
• cottonlike lesions on the body
• swollen, poppedout eyes (exophthalmia)
Behavioral 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
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
• cottonlike lesions on the body
• swollen, poppedout eyes (exophthalmia)
Behavioral 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.
Biotic disease
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.
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.