Fish, eggs and fry can be affected by fungi, bacteria, viruses and parasites. Risk often rises with stress, high density, poor water quality, oxygen shortage, sharp temperature changes or handling. Many outbreaks do not start with the pathogen itself but with fish first weakened by the environment.
Early signs are often non-specific: reduced appetite, slower growth, rising mortality, increased mucus, darkening, gasping or changed behaviour. Fish that isolate from the group or crowd at inlets, oxygenation points or the surface often signal that something is wrong.
Fungal diseases
- Saprolegnia (water mould) is one of the most common diseases in freshwater culture. It often attacks damaged or stressed fish and appears as white or grey cotton-like growth on skin, fins or gills.
- Egg fungus is a major hatchery problem. Infection often starts on dead eggs and spreads to live eggs if they are not removed quickly.
- Fungal attacks are often secondary problems after handling, transport, gill damage, parasites or other disease.
- Good hygiene, removal of dead eggs, stable water quality and low stress are the main preventive measures.
Bacterial diseases
- Furunculosis (Aeromonas salmonicida) — classic salmonid disease with skin ulcers, haemorrhage, darkening and acute mortality outbreaks.
- BKD (Bacterial Kidney Disease, Renibacterium salmoninarum) — chronic salmonid disease attacking the kidneys and causing lasting production losses.
- Yersiniosis (ERM / enteric redmouth, Yersinia ruckeri) — common in rainbow trout; haemorrhage around mouth, gills and eyes.
- Columnaris (Flavobacterium columnare) — skin and gills; grey-white skin patches, fin rot or gill inflammation.
- Bacterial gill disease (BGD) — severely impaired oxygen uptake; fish gasp or crowd at oxygen points.
- Vibriosis — marine bacterial diseases with haemorrhage, ulcers and systemic infections in many marine species.
- Tenacibaculosis — mainly marine fish; severe skin ulcers around mouth, head and fin bases.
- Streptococcosis — especially important in tilapia and other warm-water species; disorientation, eye damage and high mortality.
- Opportunists such as Aeromonas, Pseudomonas and some Vibrio species occur naturally in water and mainly cause problems when fish are already stressed.
Viral diseases
- IPN (Infectious Pancreatic Necrosis) — mainly fry and juveniles in salmonids; can cause extensive mortality.
- IHN (Infectious Hematopoietic Necrosis) — serious salmonid virus with very high losses.
- VHS (Viral Hemorrhagic Septicemia) — haemorrhage in internal organs and tissues plus high mortality in susceptible species.
- ISA (Infectious Salmon Anaemia) — one of the most economically significant diseases in international salmon farming.
- KHV (Koi Herpes Virus) — serious in koi and common carp; can cause near-total mortality in susceptible populations.
- TiLV (Tilapia Lake Virus) — a growing problem in global tilapia culture.
- VNN/VER (Viral Nervous Necrosis) — nervous system of many marine species; especially serious for larvae and fry.
Viral diseases often lack effective treatment. Preventive biosecurity, health control and infection protection are therefore critical.
Parasites
- Ich (Ichthyophthirius multifiliis, white spot) — common freshwater parasite; white spots on skin and gills, strong irritation.
- Cryptocaryon irritans — marine counterpart of white spot.
- Amyloodinium (marine velvet) — mainly gills; very rapid mortality in marine systems.
- Gyrodactylus — skin fluke; extensive damage to skin and fins, especially in salmonids.
- Dactylogyrus — gill fluke; impaired respiration and growth.
- Costia (Ichthyobodo) — microscopic parasite; major problems in fry and stressed fish.
- Trichodina — irritation of skin and gills in many culture systems.
- Tapeworms, roundworms and other gut parasites — poorer nutrient uptake, growth and FCR.
- Sea lice and other crustacean parasites — mainly marine species and wild populations.
Warning signs
The following symptoms should always be taken seriously:
- Reduced appetite.
- Suddenly poorer FCR.
- Slower growth.
- Increased mortality.
- Gasping or rapid breathing.
- Crowding at inlets or oxygenators.
- Increased mucus production.
- Skin ulcers or haemorrhage.
- Fin rot.
- Discoloured or pale gills.
- White spots or visible parasites.
- Cotton-like fungal growth.
- Abnormal swimming or balance problems.
- Darkening or pale fish.
- Swelling, protruding eyes or abdominal swelling.
Biosecurity and prevention
The best disease treatment is to avoid outbreaks from the start.
- Quarantine for new fish and new egg deliveries.
- Separate year-classes and production stages.
- Disinfect equipment between departments and tanks.
- Avoid moving fish between systems unnecessarily.
- Remove dead fish and dead eggs daily.
- Maintain stable water quality and high oxygen.
- Monitor temperature, TAN, nitrite, oxygen and carbon dioxide.
- Minimise stress during grading, vaccination and transport.
- Use vaccination where such programmes are available.
- Record mortality, appetite, biomass and abnormal behaviour.
Suspected disease outbreak
If an outbreak is suspected, isolate the affected group as far as possible. Minimise movement of fish, equipment and staff between departments. Record symptoms, mortality, water quality and recent changes on the site.
Contact a fish health veterinarian or the competent authority if a serious or notifiable disease is suspected. Several viral and parasitic diseases are covered by national and international disease-control rules.
This knowledge page does not replace professional diagnosis, laboratory analysis or veterinary advice.
See also