Diseases and health problems in freshwater fish farming can occur due to factors such as pathogens, poor water quality and stress. Pathogens causing contagious fish diseases include bacteria, viruses, parasites and fungi.
Fish diseases can be classified as contagious and non-contagious and may vary between species.
Warm water fish species (e.g. tilapia and catfish) are more prone to epizootic ulcerative syndrome (EUS) while cold water species such as rainbow trout can easily contract diseases such as bacterial cold-water disease (BCWD) and rainbow trout fry syndrome (RTFS).
Rainbow trout can also contract two viral diseases; viral haemorrhagic septicaemia (VHS) and infectious pancreatic necrosis (IPN), but these do not occur in South Africa.
Signs of Diseases in Fish
High temperatures, poor water quality and stress favour the occurrence and spread of diseases in fish. Fish farmers should have an emergency plan in case of an outbreak of a health issue in their fish.
This should include early disease recognition, the support of a health inspector for confirmation of fish disease and a list with legally acceptable medication. In case of an extreme emergency, there should be an evacuation plan to move fish into another dam (with no other fish) with better water quality conditions for intermediate treatment.
Regularly (daily) check for any of these signs of disease in freshwater fish: Increased mortality (more dead fish). Fish swim slower. Fish swim near the surface. Fish swim on their side. Fins show a change of colour, ulcers or rot. Fish change colour.
Bacterial Gill Disease - BGD
Also known as ‘environmental gill disease’, this is caused by Flavobacterium and is a secondary infection of the gills that were initially damaged by irritants such as solids or high levels of ammonia in the water.
How: Occurs during poor water quality conditions and high temperatures. Symptoms: Destruction of gill tissue (white, dead patches) and visible bacterial mass growing on the gills.
Treatment: Improve the environment. Use coarse non-iodised salt (1 kg per 1000 litre of water), Chloramine-T or Hyamine.
Fin and Tail Rot
This infection of the tail and fins is caused by Flavobacterium and occurs in poor environmental conditions and in high temperatures.
Symptoms: Reddening of the fins that develops into white eroded areas on the tips of fins and tail. Treatment: Improve the environment. Use coarse non-iodised salt, Chloramine-T.
Furunculosis and Aeromonas
These bacterial diseases occur in stressed fish, usually at high densities (too many fish grouped together), in high water temperatures and poor water quality.
Symptoms: Excessive mucus on the body and areas of reddening. White patches on skin, ulcers and pop-eyes. Treatment: Disinfection for superficial infections and antibiotic treatment for systemic (internal) infections.
Costia
Costia is a small, fast-moving parasite and usually occurs when fish are stressed due to over-crowding or poor water quality.
This parasite is active in temperatures ranging from 20℃ to 29℃ and can cause severe mortality amongst small fry.
Symptoms: Milky haze (grey/white layer) over the body, especially on the sides.
Excess mucus, reddening of the skin, flashing or rubbing. (Flashing is when the fish rubs itself to rid it of parasites.) Treatment: Good husbandry, salt, Chloramine-T, Formalin.
Trichodina
Trichodina is a disease caused by the parasite found in environments with sediment build-up and high organic loads (water high in organic matter). The disease affects the skin and gills.
Symptoms: Flashing and rubbing. Excess mucus. Fin erosion and/or ulcers on the fins. Respiratory difficulty.
Treatment: Improve the fish’s environment (water quality), salt, Chloramine-T.
White Spot 'Ich'
White spot (or ‘ich’) is caused by a very common parasite, which occurs below 28℃. Often attacks stressed and weak fish and infects the skin and gills.
Symptoms: Flashing and rubbing. Lethargic movement. Excess mucus. Small white spots on the fins and body.
Treatment: Good husbandry good water quality and stable temperatures. Formalin.
Algae Blooms
Algae bloom is the excessive growth of algae as it occurs under good growing conditions - enough light and nutrients. It is very common underneath net cages and in static ponds (no running water) or dams. Some algae can secrete toxins that can cause fish deaths.
Symptoms: Water turns red, blue, green or brown. Fish struggle to breathe - gills clogged with algae. Algae can taint the flesh of fish and affect the quality of the meat.
Treatment: Manage algae levels in the water. Avoid a build-up of nutrients, especially under the net cages in trout dams. Ensure sufficient aeration of the water (increase oxygen levels) if an algae bloom occurs.
Cotton Wool Disease
Cotton wool disease is caused by the fungal-like Saprolegnia, which is always present in water. It creates problems in water, rich in organic matter or with lots of dead fish or decaying fish matter.
Trout are more susceptible during spawning season.
Symptoms: Fungus spreads over the body and causes white tufts with cotton-wool appearance.
Treatment: Improve water environment. Good husbandry - daily removal of dead fish. Limit stress and physical damage during netting and handling.
How to Treat Fish Diseases
A proper diagnosis has to be made before any fish can be treated and the benefits, as well as disadvantages of treating the fish, should be considered. Select the most appropriate treatment and treatment method.
For example, choose between bath treatments (to be used in cages) or medicated feed. When treating diseased fish with a new medicine for the first time, first test it on a small group of fish. Do not feed the fish for 24 hours before giving the medicine.
The digestion process uses oxygen and it is important to conserve as much oxygen for the fish as possible. Ensure your calculations and dosages of medicines are accurate - use accurate biomass (total weight of all the fish) and correct volume of water.
Always treat early in the morning (at low water temperatures) and monitor the response of the fish. Always adhere to the recommended withdrawal periods when using chemicals and drugs. The withdrawal period is the time it takes for the medicine to be digested so that no traces will be detected in the meat.
In addition to medicine to treat fish disease, improve the environment (water quality) and avoid any further stress such as handling or moving the fish. Keep a detailed treatment log with dates, type of treatment, reasons for treating, the reaction of fish and so on.
By Marinda Louw