In order to achieve a high yield in pond fish farming, after polyculture is implemented, feeding and fertilization are carried out. This leads to a large population of fish, along with significant amounts of plankton, benthos, and aerobic bacteria. Their waste and organic matter decompose, which requires a substantial amount of oxygen. As water temperatures rise and fish grow, these ponds often face oxygen depletion. When the oxygen level drops severely, fish tend to float near the surface. If no immediate action is taken, this can result in mass fish deaths, even leading to the total collapse of the pond, commonly referred to as "pan-pool."
Oxygen deficiency in ponds can be caused by several factors, beyond just high stocking density:
1. On cloudy days, reduced photosynthesis lowers oxygen production, while high temperatures and low atmospheric pressure decrease oxygen solubility in water. Since dissolved oxygen levels are directly related to gas pressure, lower pressure results in less dissolved oxygen.
2. Hot, humid weather with thunderstorms at night can cause a sudden drop in surface water temperature, triggering convection between the upper and lower layers of the pond. The upper layer, rich in oxygen, is quickly consumed by organic matter in the lower layer, leading to oxygen depletion if not replenished.
3. Excessive silt accumulation at the bottom, overfeeding, or improper fertilization—especially excessive use of organic fertilizers—can lead to high oxygen consumption and the release of toxic gases, worsening water quality and increasing the risk of pan-pools.
4. Improper stocking ratios, such as releasing only certain species like whitefish without enough predators, can result in an imbalance. Zooplankton may overconsume phytoplankton, reducing photosynthetic activity and lowering dissolved oxygen levels. At the same time, increased zooplankton respiration further depletes oxygen, causing fish to float and potentially die.
**Early Signs of "Pan-Pool":**
1. **Weather observation:** Hot, low-pressure days with no wind or rain increase the likelihood of oxygen depletion and floating heads.
2. **Fish behavior:** A sudden drop in appetite among healthy fish indicates low oxygen levels and potential floating head issues.
3. **Fish movement:** Fish swimming near the surface in scattered groups or making sudden bursts of movement suggest oxygen deficiency in deeper waters.
4. **Water appearance:** Dark, murky water with visible gas bubbles or foam on the surface, especially covering half the pond, signals low dissolved oxygen.
**Prevention Measures:**
1. During winter, remove excess silt from the pond bottom, keeping it around 20 cm thick for optimal conditions.
2. Improve feeding practices by using scientific methods and promptly removing leftover food to avoid overloading the system.
3. Apply well-fermented fertilizer in small quantities based on weather and water quality conditions.
4. In summer and early autumn, monitor the pond closely and frequently add fresh water to reduce feeding intensity.
5. Maintain a balanced stocking ratio of different fish species to prevent ecological imbalances.
6. For high-yield ponds, install oxygen-aeration equipment to ensure sufficient oxygen supply.
**Emergency Rescue Steps:**
1. If fish are seen floating, immediately add fresh water or use aeration devices to increase oxygen levels.
2. Apply gypsum at a rate of 2.5 kg per cubic meter of water depth, evenly distributed across the pond.
3. Use alum at 1.3 kg per acre for every meter of water depth, also spread evenly throughout the pond.
By implementing these strategies, pond managers can significantly reduce the risk of pan-pool events and maintain a healthy, productive aquatic environment.
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