Fish Pond Management and Water Quality
What the exam tests
Pond management and water quality questions test preparation, liming, fertilisation, stocking and monitoring. You must know safe ranges and the reasons behind each practice, especially in Assam’s acidic, rain-fed and flood-prone ponds.
Core rules
- Prepare the pond by drying, removing weeds, repairing dykes and eradicating predators.
- Apply lime before fertiliser, not together; lime raises pH and alkalinity and improves pond bottom conditions.
- Test dissolved oxygen, pH, temperature, ammonia, alkalinity, hardness and transparency.
- Indicative ranges: DO above 5 mg/L, pH 6.5 to 8.5, ammonia below 0.1 mg/L, alkalinity 50 to 150 mg/L as CaCO3.
- Use a Secchi disc for transparency and a plankton net for qualitative plankton checks.
- Avoid over-fertilisation, which causes algal blooms and nighttime oxygen crashes.
Worked example
A pond is 100 m by 50 m and averages 1.5 m deep. Volume is 100 x 50 x 1.5 = 7,500 m3. If you exchange 10 percent, you replace 750 m3. For liming at 300 kg/ha, a 0.1 ha pond needs 0.1 x 300 = 30 kg. Show units; marks are often for the conversion.
Traps
Do not apply lime and manure on the same day. Do not test only surface water in a stratified pond. Do not confuse alkalinity with hardness. Do not assume clear water is always good; it may mean low plankton.
Key facts
Green water usually indicates healthy phytoplankton. Ammonia toxicity rises with high pH and temperature. Aeration helps at dawn when DO is lowest.