In order to achieve high yields in pond fish farming, polyculture is typically followed by feeding and fertilization. This process introduces a large number of fish, along with plankton, benthos, and aerobic bacteria. Their waste and other organic materials decompose, consuming significant amounts of oxygen. As water temperatures rise and fish grow, this can lead to oxygen depletion in the pond. When oxygen levels drop severely, fish often float near the surface. If no immediate action is taken, fish suffocation can occur, resulting in mass mortality and even total pond failure, commonly referred to as "pan-pool." Oxygen depletion in ponds is not only caused by high stocking density but also by several other factors: 1. On cloudy days, reduced photosynthesis limits oxygen production. High temperatures and low atmospheric pressure further reduce the solubility of oxygen in water, leading to lower dissolved oxygen levels. 2. During hot, humid weather or after thunderstorms, sudden temperature drops on the pond's surface cause water convection. Oxygen-rich surface water is quickly consumed by organic matter in the deeper layers, leading to an oxygen deficit. 3. Excessive silt accumulation at the bottom, overfeeding, improper fertilization, or excessive use of organic fertilizers can cause large oxygen consumption and release toxic gases, worsening water quality and increasing the risk of pan-pools. 4. An imbalanced stocking ratio can also be a problem. For example, if only herbivorous fish are stocked, zooplankton may overgrow, reducing phytoplankton populations. This lowers the pond’s photosynthetic activity, decreasing oxygen levels. Additionally, increased zooplankton respiration further depletes oxygen, causing fish to float and potentially leading to pan-pools. **Early Signs of Pan-Pool:** 1. **Weather observation:** Hot, low-pressure conditions without wind or rain can lead to poor water quality and oxygen depletion. 2. **Fish appetite:** A sudden decrease in food intake among healthy fish may indicate low oxygen levels. 3. **Fish behavior:** Fish swimming near the surface in scattered groups or jumping out of the water suggest oxygen deficiency. 4. **Water appearance:** Dark, murky water with visible gas bubbles or foam on the surface indicates low dissolved oxygen. **Prevention Measures:** 1. In winter, remove excess silt from the pond, keeping it around 20 cm deep to maintain good water circulation. 2. Implement scientific feeding practices, and promptly remove leftover feed to prevent overloading the system. 3. Use properly fermented fertilizers, applying them in small amounts based on weather and water conditions. 4. During hot summer months, monitor the pond closely and add fresh water regularly while reducing feeding. 5. Maintain a balanced stocking ratio of different fish species to ensure ecological harmony. 6. In high-yield ponds, install oxygen-aeration equipment to manage oxygen levels effectively. **Rescue Actions When Pan-Pool Occurs:** 1. Immediately add fresh water or use aerators to increase oxygen levels. 2. Apply gypsum at a rate of 2.5 kg per cubic meter of water depth, evenly distributing it across the pond. 3. Add alum at 1.3 kg per acre for every meter of water depth, spreading it evenly throughout the pond. By understanding these causes, signs, and solutions, pond managers can significantly reduce the risk of pan-pool events and ensure healthier, more productive fish farming systems.

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