Aquaculture
Aquaculture questions test pond and tank management as a system: water quality, stocking, feeding, and health. You should be able to calculate stocking density, survival, growth, and feed conversion, and to explain how pH shifts the toxicity of ammonia.
Water quality rules
- Un-ionized ammonia (NH3) is the toxic form; its fraction rises with pH and temperature, so the same total ammonia is more dangerous in the afternoon in a high-pH pond.
- Nitrite toxicity is linked to chloride uptake; adding salt (chloride) can protect fish.
- Dissolved oxygen is lowest before dawn; afternoon supersaturation does not guarantee safe night-time oxygen.
- Liming raises alkalinity and is applied before fertilization; do not lime and fertilize on the same day.
Pond preparation and feeding
Dry, till, and lime the pond; eradicate predators; fertilize to build natural food; then stock. Feed according to biomass, not to a fixed daily amount, and adjust after sampling.
Worked example
Stock 1,000 tilapia at 10 g. Harvest 850 fish averaging 200 g after using 180 kg of feed. Biomass gain = 850 × (0.200 − 0.010) kg = 161.5 kg. FCR = 180 ÷ 161.5 = 1.11. Note that the calculation uses harvested, not stocked, fish; using 1,000 fish overstates gain and understates FCR.
Traps that cost marks
Reporting ammonia as total rather than un-ionized; computing FCR on stocked numbers; assuming a pond can carry unlimited biomass; and treating a single afternoon DO reading as proof of good oxygen through the night.