Fish Health and Disease Management
Common disease groups
Bacterial: Aeromonas hydrophila causes ulcers and red patches; Edwardsiella and Pseudomonas also occur. Fungal: Saprolegnia produces cotton-like growth on wounds and eggs; branchiomycosis affects gills. Parasitic: Argulus (fish louse), Lernaea (anchor worm), Dactylogyrus (gill fluke), Ichthyophthirius (white spot). Viral: white spot syndrome virus in shrimp; viral fish diseases generally have no direct cure, so biosecurity and reporting are essential.
Prophylaxis and water quality
Dry and lime pond bottoms; screen inlet water; quarantine new stock; avoid overcrowding; maintain DO and low ammonia; remove dead fish; disinfect nets and boots. Most disease outbreaks follow a water-quality or handling stressor, so fix that first.
Safe drug use
Use only approved products, diagnose before treating, dose by fish biomass for systemic drugs, and observe withdrawal periods. Never use chloramphenicol or nitrofurans, which are banned in food fish.
Worked example
A pond holds 1,000 kg fish. A product is prescribed at 10 mg active/kg biomass. Active needed = 1,000 x 10 = 10,000 mg = 10 g. If the product is 10% active, use 100 g. Reasoning: biomass dosing, not pond volume, and always convert active ingredient to product weight.
Traps
- Treating red disease with antibiotics without checking water quality.
- Dosing by water volume for a feed-medicated drug.
- Using banned antibiotics.
- Ignoring withdrawal periods.
- Confusing Saprolegnia with a bacterial ulcer.