Diseases Parasites and Predators

Management and Control

Occurrence of disease and parasites in farmed fish is mainly as a result of poor husbandry. Disease causing organisms are always in the environment fish live in and they cause few problems. The pathogens naturally exist in a stable “equilibrium” with their hosts until this balance is disturbed through environmental changes and anthropogenic activities.

Fish are normally stressed through inadequate dietary or environmental conditions. The water quality parameters such as pH, temperature, dissolved oxygen may lead to outbreak of disease pathogens and parasites.


Fish Stressors

Some fish stressors that lead to diseases that need to be avoided are;

  • Poor handling of fish is a major cause of both bacterial and parasitic infections.
  • Translocation of fingerlings/fry from one place to another without proper care can spread diseases and parasites.
  • Increased nutrient levels due to intensive cage culture promote proliferation of parasites.
  • Pollution due to high levels of ammonia predisposes fish to succumb to large numbers of parasites. Human faeces may be a source of gut parasites especially to common carp.
  • Damages of fish by predators lead to secondary bacterial or fungi infections. The predators especially birds and mammals play an important role in life cycles of certain parasites.
  • Disease, parasites or pathogens may enter fish through gills, penetration of egg membrane, ingestion, rupture of skin, wounds or through the digestive tract.

Note that Tilapia are more resistant to viral, bacterial and parasitic diseases than other commonly cultured fishes. Few diseases and mortalities have been reported in semi intensive tilapia farms in Kenya. This could due to low stocking densities in these systems. Lymphocystis, columnaris, whirling disease, and hemorrhagic septicemia may cause high mortality, but these problems occur most frequently at water temperatures below 110 C. The most important cause of mortalities is anoxia (lack of oxygen) resulting from blooms of algae. Sudden lowering of temperatures to below tolerance levels, which can happen during the rainy seasons, can lead to problems including mortalities.

Main causes of disease in farmed fish

When fish are not fed with the right food in the right way, nutritional diseases occur.

Some of the main causes of disease are:

  • Poor feeds and feeding
  • Poor quality or contaminated feed
  • Exposure to extreme conditions or toxic environments
  • Extremes in pH where the water is either too acidic or alkaline
  • Presence of toxic gases such as ammonia
  • Lack of dissolved oxygen
  • Overcrowding and/or behavioural stresses, for example in storage or transport
  • Improper and/or excessive handling
  • Toxins in food such as fungal toxins in stored feeds, pesticide residues etc.
  • Water pollution by agricultural or industrial effluents, sewage effluents, heavy silt loads.

Since prevention is better than cure, it is very important to:

  • Ensure quality and sufficient water supply, with adequate dissolved oxygen and free of pollution.
  • Maintain clean pond environment by controlling silting, plants and proper phytoplankton and zooplankton balance. Regular pond disinfection is recommended.
  • Keep the fish in stress free conditions by controlling stocking density, keeping different sizes separate to reduce fighting, providing proper food supply, handling the fish properly etc.
  • Prevent the entry of disease organisms by preventing the entry of wild fish by using screens and eradicating them from canals and ponds
  • Ensure that all fish got from outside to the farm are clean without parasites or diseases
  • Always using good quality feeds
  • Regular monitoring of the water entering the farm to ensure of its quality
  • Prevent the spread of disease within the farm by controlling predators, particularly birds and mammals
  • Disinfect ponds regularly to kill both the disease organisms and their intermediate hosts
  • Avoid water sharing among ponds
  • In case of disease outbreak, remove sick and dead fish from the ponds immediately
  • Bury diseased fish with quicklime away from the ponds
  • Always disinfect pond and fish handling equipment

Common disease symptoms in fish

Behavioural signs:

  • Decreased feeding
  • Weak, lazy or erratic swimming
  • Floating on water belly up
  • Roughing against hard surfaces
  • Crowding/gathering at the inlet

Physical signs:

  • Gaping mouth
  • Open sores, lesions, loss of scales, bloated belly
  • Pale, eroded, swollen, bloody or brownish gills
  • Abnormally folded or eroded fins
  • Cloudy or distended eyes
  • Presence of disease organisms on skin, gills, fins

Fish diseases may be:

  1. Bacterial – which cause diseases like fin rot and tail rot
  2. Fungal infections – woolly or cottony patches on the surface of fish, and gill rot causing asphyxia.
  3. Parasitic
    • Ectoparasites – Those that occur outside the fish body for example those that cause Black spot, white spot, fish louse and Nematode.
    • Endoparasites – Those that get into the body of the fish like Contraceacum, and Ligula intestinalis
  4. Dietary – High carbohydrate levels in trout feeds, lack of proteins and lipids will result in liver tumour.

Some common fish diseases and their prevention

Pathogen Symptom Prevention
  • Cottony grey-white or brown patches on the skin
  • Proper fish handling
  • Avoid handling fish in cold water.
  • Low organic matter in water
  • Black spots
  • Yellowish cysts on gills
  • Control snails and control predators like birds.
  • Remove infected fish.
  • Loss of appetite.
  • Fin and tail rot.
  • Pale gills.
  • Fluid in abdomen.
  • Improve water quality.
Nematode (Contracaecum)
  • Round worm in spiral shape near gills
  • Not really a problem for fish health but leads to consumer dissatisfaction
Parasitic protozoan
  • Fish try to scrap their bodies on hard surfaces (flashing)
  • Salt, Potassium Permanganate or formalin bath.
  • Keep water temperature near optimum range for that species of fish.


Nutritional Diseases

Cause Symptom Prevention
  • Lack of proteins
  • Poor growth.
  • Caudal fin erosion.
  • Loss of appetite.
  • Feed protein rich food eg. soya beans, slaughterhouse by-products, fish meal.
  • Lack of lipids.
  • Poor growth.
  • Feed with energy-rich foods.

Treatment of Infected Ponds.

Ponds with infections should be drained and badly infected fish culled.

  • Dry the pond under the sun for about seven days.
  • Dampen the pond bottom.
  • Spread Lime (Calcium carbonate) evenly over entire surface of pond bottom at the rate of 1500 kg/Ha.
  • Wait for 15 days then restock the pond with healthy stock.


Some common chemicals for use in fish farming

Agricultural lime and agro-industrial by-products eg. rice bran and molasses – Pest control in drained ponds.

Organic poisons such as rotenone can control pests in filled ponds.

Household bleach is a good disinfectant of non-metallic equipment and working areas.


Chlorine bleach liquid and powder can be used as a strong disinfectant for fish-handling equipment.

Common salt is cheap and easily available, kills several disease organisms and has positive effects on the fish by stimulating appetite and increasing mucus secretion, improving resistance to handling.

Formalin is toxic to fish particularly in soft water because it lowers dissolved oxygen levels. Make sure treatment water is well oxygenated.

Some common fish predators and their control measures

Predator Type of fish eaten Control measure
Insects and insect larvae Juvenile fish and eggs and fish just hatched.
  • Oil emulsion to prevent aerial breathing.
  • Use of fish that feed on insect larvae especially those that have gills and can remain in the bottom.
Frogs and toads juveniles of tilapia and catfish
  • Fence with frog proof wire mesh.
  • Clear bush around pond. Screen both in and outlets.
  • Use traps. Adult catfish and bass eat frogs.
Fish all types of fish
  • Use screen in the inlets and outlets.
  • Do pond draining periodically
Snakes destroy larval and juvenile fish
  • Clear bush around the pond and fence properly.
Crocodiles, alligators and large lizards. All types of fish
  • Proper fencing and keeping dense bushes cut down.
Turtles prey on catfish
  • Fencing around pond with wire mesh, trapping.

  • Wading birds eg. herons and egrets
  • Diving birds.
  • Kingfisher, fish eagle/
  • Cormorants, pelicans
All types of fish and at all stages especially in shallow waters. Cormorants feed on fish just after the fish are fed- when they are most concentrated.
  • Proper fencing all round and then above with netting material or manila ropes/strings on poles with bright coloured cloth or metal crossed over the pond.
  • cover ponds with nets or wire mesh, use flash guns, windmills that revolves and flash brilliantly and bells to scare the birds a way.
  • The birds can also be actively discouraged by destroying their nests.
Otters Prey on large fish at night killing more than they can eat. They burrow and live under the roots of trees near the water. Otters are very clever They can even open latches on gates.
  • Proper fencing around the ponds.
  • The otters can also be trapped using special otter traps set in their passages.
  • Guard by use of trained dogs.
  • Fence the pond half way across giving allowance for fish to pass through but not he otter thus providing hiding places for fish.
Man (theft) All types of fish. This is also considered among the major predators through which fish are lost. Extremely difficult to control and is most common in cage culture and other intensive fish farming.
Can however be controlled by

  • Employing security personnel
  • Use of trained dogs
  • Hidden obstruction to prevent pond seining.
  • Fence the farm and lock securely.
  • Burglar alarms or electrified fence
  • If the situation is bad, then trapping or shooting can be used as the last resort in cases of birds and otters but in consultation with the relevant authorities for example the Kenya Wildlife Service
  • Be very careful when poisoning predators. Humans and non-target animals can be affected.