Fish Health, Disease Diagnosis and Biosecurity
Categories of disease
Infectious causes are viral, bacterial, fungal or parasitic; non-infectious causes are nutritional, environmental and genetic. Recognise these specific agents: epizootic ulcerative syndrome (EUS), caused by the oomycete Aphanomyces invadans, which produces deep ulcers with grey margins and is a recurring problem in Odisha after cold, wet weather; Aeromonas hydrophila, causing red spot and haemorrhagic septicaemia; Edwardsiella and Pseudomonas septicaemias; white spot from Ichthyophthirius; Argulus, the fish louse; Lernaea, the anchor worm; Dactylogyrus and Gyrodactylus on gills; Myxobolus in muscle.
Diagnosis sequence
Observe behaviour and gross lesions, then take skin and gill scrapings and a wet mount under the microscope, and only then consider culture or PCR at a laboratory. Environmental causes should be excluded first, because gasping at dawn with dissolved oxygen at 2.5 mg/L is a water problem, not an infection.
Treatment and biosecurity
Common treatments: common salt 3-5% dip for a few minutes or 1-3 ppt in the pond as supportive therapy; potassium permanganate 2-4 mg/L; formalin about 25 mg/L; lime 200-300 kg/ha in affected ponds; CIFAX-type formulations for ulcerative disease as per label. Biosecurity means quarantining new stock, screening inlet water, disinfecting nets and boots, netting the pond against birds, removing and burying dead fish daily, never discharging diseased water into a shared channel, and observing antibiotic withdrawal periods.
Worked example
A pond of 0.05 ha with mean depth 1.0 m shows early EUS lesions. Apply potassium permanganate at 2 mg/L. Volume = 500 m2 x 1.0 m = 500 m3; mass = 2 g/m3 x 500 = 1000 g = 1 kg. Alongside this, stop manuring, remove dead fish, and apply lime to the affected margins; do not mix lime and permanganate in the same application.
Traps that cost marks
- Dosing chemicals without computing pond volume.
- Treating viral and water-quality problems with antibiotics.
- Blaming EUS for surface gasping at dawn.