Credits: Biovision-Infonet
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 an unstable “equilibrium” with their hosts until this balance is disturbed through environmental changes and human activities. Fish are 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.
Prevention
It is better to try to prevent diseases than to wait to cure them once they start to cause problems. Avoid all kinds of stress (from feeding, handling, overcrowding, water quality) that affect resistance of fish
When you only cure fish without taking the cause of the disease away, the fish gets diseased again and again. To cure a fish disease is much more difficult and may require the services of a specialist. By the time proper treatment can be sought, the disease may have caused a lot of damage. Most disease problems in aquaculture result from events that can be avoided.
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 and rupture of skin, wounds or through the digestive tract.
Fish diseases may cause severe losses in fish farms because of reduced fish growth and production caused by lack of appetite, increased feeding cost because of inefficient digestion of feed and waste of uneaten feed, increased vulnerability to predation, increased susceptibility to low water quality and by death of fish
Main causes of disease in farmed fish
Disease, parasites or pathogens may enter fish through gills, penetration of egg membrane, ingestion and rupture of skin, wounds or through the digestive tract. Fish diseases may cause severe losses in fish farms through:
- Poor feeds and feeding; When fish are not provided with the right food in the right way, nutritional diseases occur.
- Exposure to extreme conditions or toxic environments
- Unsuitable water quality
- Extremes in pH towards acidic or basic conditions
- 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.
- Actual attack by disease causing organisms; Fish can be attacked by disease organisms, either externally (on the skin, gills or fins), or internally (in the blood, digestive tract, nervous system).
Preventing diseases through proper management
Prevention is better than cure, so it is very important to:
1. Ensure good water quality: sufficient supply, with adequate dissolved oxygen and free of pollution
2. Maintain clean pond environment by controlling silting, plants and proper phytoplankton and zooplankton balance. Regular pond disinfection is recommended, for instance by letting it dry in the sun for a week when empty.
3. Keep the fish in stress free conditions by controlling stocking density, keeping different sizes separate to reduce fighting, providing proper food, handling the fish properly etc.
4. Prevent the entry of wild fish by using screens so they can’t enter your farm. Ensure that all fish got from outside to the farm are clean without parasites or diseases.
5. Use good quality feed
6. 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
- Avoiding water sharing among ponds
- In case of disease outbreak, remove sick and bury diseased fish with quicklime away from the ponds; carefully treat infected ponds and disinfect all dead fish from the ponds immediately
- 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 can either be:
(i) Bacterial – which causes diseases like fin rot and tail rot
(ii) Fungal infections – wooly or cottony patches on the surface of fish, and gill rot causing asphyxia.
(iii) Parasititic
- Ectoparasites – causing Black spot, white spot, fish louse and nematode.
- Endoparasites – like the Contraceacum, and the Ligula intestinalis.
(iv) Dietary – High carbohydrate levels in trout feeds, lack of proteins and lipids will result to liver tumour.
Some common fish diseases and their prevention
| PATHOGEN | SYMPTOM | PREVENTION |
| Fungus | Cottony grey-white or brown patches on the skin | Proper fish handlingAvoid handling fish in cold water.Low organic matter in water |
| Trematodes | Black spotsYellowish cysts on gills | Control snails and control predators like birdsRemove infected fish. |
| Bacteria | Loss of appetite.Fin and tail rot.Pale gillsFluid in abdomen | Improved 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 |
| 1. Lack of proteins | Poor growth.Caudal fin erosion.Loss of appetite. | Feed protein rich food e.g. soya beans, slaughterhouse by-products, fish meal. |
| 2. Lack of lipids | Poor growth | Feed with energy-rich foods |
The following points should be followed in 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 stocks.
Some common chemicals for use in fish farming
– Limes and agro-industrial by-products e.g. rice bran and molasses: Pests 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 strong disinfectants for fish handling equipment.
– Common salt is cheap and easily available. Kills several disease organisms and have 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.